| OBJECTID | MapUnit | Name | FullName | Age | Description | HierarchyKey | ParagraphStyle | Label | Symbol | AreaFillRGB | AreaFillPatternDescription | DescriptionSourceID | GeoMaterial | GeoMaterialConfidence | DescriptionOfMapUnits_ID |
|---|
| 1 | None | SURFICIAL DEPOSITS | SURFICIAL DEPOSITS | None | Colors are determined with the aid of Munsell soil color charts (Munsell Color, 1991). Soil-carbonate development was assessed after Machette (1985). | 01 | DMUHeading1 | None | None | None | None | DAS1 | None | None | DMU01 |
| 2 | None | HUMAN-MADE DEPOSITS | HUMAN-MADE DEPOSITS | None | None | 01.01 | DMUHeading2 | None | None | None | None | DAS1 | None | None | DMU02 |
| 3 | af | Artificial fill | Artificial fill | uppermost Holocene | Includes riprap and fill placed during the construction of the Spinney Mountain Reservoir dam and associated structures and roadways. The unit generally consists of clay, silt, sand, and rock fragments. The unit may exceed 25 m in thickness. Artificial fill may be subject to settlement, slumping, and erosion if not adequately compacted and (or) if it is placed on unstable slopes. | 01.01.01 | DMUUnit1 | af | af | 255-255-255 | None | DAS1 | "Made" or human-engineered land | High | DMU03 |
| 4 | None | ALLUVIAL DEPOSITS | ALLUVIAL DEPOSITS | None | None | 01.02 | DMUHeading2 | None | None | None | None | DAS1 | None | None | DMU04 |
| 5 | Qa | Alluvium | Alluvium | Holocene | The unit is unstratified to weakly stratified, poorly sorted silt to sand, and dark yellowish brown (10YR 3/4 and 10YR 3/6) in color. Sand grains are predominantly subround quartz. The unit underlies the modern channel, adjacent floodplains, and low-lying terraces approximately 1.5 m above the modern channel in Chase Gulch, along the South Platte River, and Three Mile Creek. The unit is interbedded with units Qaf and Qsw locally, especially in Chase Gulch and along Three Mile Creek. Epis and others (1979) mapped the unit as Piney Creek Alluvium (Qp). The Piney Creek Alluvium or unit Qa2 along the Colorado Front Range has been dated between 11,950 ± 0.24 cal years BP (bulk carbon; Berry and others, 2015) and 960 ± 30 cal years BP (bulk carbon; Lindsey and others, 2018). Unit Qa postdates unit Qasp3 and Qatc3 in the mapped area and includes Post-Piney Creek Alluvium or unit Qa1 from CGS mapping along the Colorado Front Range, which is Late Holocene in age. The unit is not a likely source of sand and gravel. The unit is hydraulically connected to the South Platte River and underlies areas that are mapped within the 1-percent annual chance flood zone (FEMA Flood Map Service Center). Unit Qa may be as much as 3 m thick. | 01.02.01 | DMUUnit1 | Qa | Qa | 255-255-222 | None | DAS1 | Alluvial sediment | High | DMU05 |
| 6 | None | ALLUVIUM OF THE SOUTH PLATTE RIVER | ALLUVIUM OF THE SOUTH PLATTE RIVER | None | None | 01.02.02 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU06 |
| 7 | Qasp3 | Alluvium three of the South Platte River | Alluvium three of the South Platte River | Lower Holocene and Upper Pleistocene | The unit consists of poorly sorted, clast-supported gravel with a matrix of dark yellowish-brown (10YR 3/4 and 10YR 3/6) silt to coarse sand. Sand grains are predominantly subround to round quartz. Gravel clasts are subround to round and are usually as much as 2.5 cm in diameter but can be as much as 75 cm in diameter. Clast compositions are approximately 60% volcanic breccia and andesite sourced from local bedrock units, 20% tan and white quartzite, 15% pinkish metamorphic rocks, and 5% vein quartz. Clasts have continuous carbonate rinds up to 1 mm thick. The unit differs from Three Mile Creek alluvium in that Three Mile Creek deposits consist almost exclusively of locally derived volcanic rocks. It differs from Chase Gulch alluvium in that clasts in Chase Gulch deposits are predominantly derived from local Precambrian bedrock. The unit is interbedded with Qaf and Qsw locally. The unit was mapped as Wisconsin outwash by Stark and others (1949). Epis and others (1979) later mapped the unit as Pinedale outwash or Pinedale fans. The Pinedale glaciation is correlative with the end of the Wisconsin glaciation in the area and culminated between ~15 and ~14 ka in the area (Young and others, 2011; Schweinsberg and others, 2020). Optically stimulated luminescence samples collected from similar material in secondary drainages in the adjacent (to the west) Guffey NW quadrangle estimate the age of the unit to be between 12,365 ± 570 (SAR) years BP and 20,835 ± 2205 (SAR) years BP. The unit is likely correlative with the Broadway Alluvium or Qa3 in CGS mapping along the Colorado Front Range. The unit is a source of sand and gravel. The unit may be hydraulically connected to the South Platte River and can be as much as 5 m thick. | 01.02.02.01 | DMUUnit2 | Qasp3 | Qasp3 | 255-255-179 | 38-115-0 ESRI 24k Geology 601 gravel, open | DAS1 | Alluvial sediment | High | DMU07 |
| 8 | Qasp4 | Alluvium four of the South Platte River | Alluvium four of the South Platte River | upper Middle and Upper Pleistocene | The unit consists of poorly sorted, clast-supported gravel and the matrix is dark-brown (7.5YR 3/3) to dark yellowish-brown (10YR 4/4), silt to coarse sand. Sand grains are predominantly subround to round quartz. Gravel clasts are subangular to subround and are 2.5 cm in diameter. A minor fraction of the clasts is as much as 50 cm in diameter. Clast types are predominantly volcanic breccia, andesite, tan quartzite, and Wall Mountain Tuff (PEwm). Vein quartz and metamorphic rocks comprise as much as 15% of the gravel clasts. Continuous carbonate rinds are present and can be as much as 1.5 mm thick, locally. The unit underlies terraces 20 m above the modern channel. Unit Qasp4 was mapped as the Como Surface, and Wisconsin outwash by Stark and others (1949). They suggest the Como Surface is pre-Illinoian in age. The Pinedale glaciation is correlative with the end of the Wisconsin glaciation in the area and culminated between ~15 and ~14 ka in Colorado (Young and others, 2011; Schweinsberg and others, 2020). Later it was mapped as Bull Lake Outwash (Qbo) by Epis and others (1979). The youngest credible boulders dated from Bull Lake age moraines are approximately 132 to 120 ka, suggesting the Bull Lake glaciation culminated some time after that (Schweinsberg and others, 2020). By virtue of the unit’s height above modern channels and soil development observed in similar material in the adjacent (to the west) Guffey NW quadrangle, the unit is likely late Middle to Late Pleistocene in age. The unit is likely correlative with the Louviers Alluvium or unit Qa4 in CGS mapping along the Colorado Front Range. Unit Qasp4 is not a likely source of sand and gravel and is not hydraulically connected to the South Platte River. Unit Qasp4 is approximately 6 m thick. | 01.02.02.02 | DMUUnit2 | Qasp4 | Qasp4 | 255-255-102 | 38-115-0 ESRI 24k Geology 601 gravel, open | DAS1 | Alluvial sediment | High | DMU08 |
| 9 | None | ALLUVIUM OF CHASE GULCH | ALLUVIUM OF CHASE GULCH | None | None | 01.02.03 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU09 |
| 10 | Qacg3 | Chase Gulch alluvium three | Chase Gulch alluvium three | Lower Holocene and Upper Pleistocene | The unit consists of moderately sorted, pebble-gravel and is brown (10YR 5/3, 4/3) in color. The matrix is fine to coarse sand, comprised predominantly of quartz and feldspar. Pebble clasts are granitic, sourced from nearby Precambrian bedrock. Although soil development is poor, unit Qacg3 effervesces throughout when exposed to dilute hydrochloric acid (HCl). This could be related to groundwater or atmospheric conditions instead of pedogenic processes. The unit underlies terraces approximately 2 m above the ephemeral channel in Chase Gulch and is locally interbedded with unit Qaf. The unit grades laterally into unit Qasp3 but differs from it in that unit Qacg3 has no quartzite clasts and is comprised almost entirely of clasts derived from local Precambrian bedrock. Stark and others (1949) mapped the unit as Wisconsin outwash. Later, Epis and others (1979) mapped it as Pinedale fans (Qpf). The Pinedale glaciation is correlative with the end of the Wisconsin glaciation in the area and culminated between ~15 and ~14 ka in Colorado (Young and others, 2011; Schweinsberg and others, 2020). Since the unit grades laterally into unit Qasp3, the unit is likely Late Pleistocene in age. Unit Qacg3 may be a potential source of sand and gravel and it is likely hydraulically connected to the South Platte River. The unit is as much as 3.5 m thick. | 01.02.03.01 | DMUUnit2 | Qacg3 | Qacg3 | 255-255-179 | 0-169-230 ESRI 24k Geology 601 gravel, open | DAS1 | Alluvial sediment | High | DMU10 |
| 11 | None | ALLUVIUM OF THREE MILE CREEK | ALLUVIUM OF THREE MILE CREEK | None | None | 01.02.04 | DMUHeading3 | None | None | None | None | DAS1 | None | None | DMU11 |
| 12 | Qatc4 | Alluvium four of Three Mile Creek | Alluvium four of Three Mile Creek | upper Middle and Upper Pleistocene | The unit underlies terraces adjacent to Three Mile Creek and adjacent to unnamed, ephemeral drainages south and southeast of Three Mile Creek. Deposits are mapped as one unit because they are very similar in clast composition and size, and underlie terraces at similar heights. The unit is predominantly clast supported gravel with interbedded sandy layers that are typically dark brown (10YR 3/3) to brown (10YR 4/3) in color. The matrix is poorly sorted silt to coarse sand. Sandier lenses are usually poorly sorted silt and sand. Clast types are subangular to subround and as much as 15 cm in diameter. Clasts are 80% dark gray to black volcanic clasts with lesser amounts of pink metamorphic rocks, and tan- and pink-volcanic clasts. The unit is interlayered locally with units Qaf and Qsw. The unit underlies terraces 10 to 15 m high adjacent to Three Mile Creek and 4 to 14 m high adjacent to unnamed channels. Unit Qatc4 was partially mapped as a part of the Como Surface by Stark and others (1949), which they estimate to be pre-Illinoian in age. Later, Epis and others (1979) mapped it as Bull Lake Alluvium (Qba) or Slocum Alluvium (Qs). The youngest credible boulders dated from Bull Lake age moraines are approximately 132 to 120 ka, suggesting the Bull Lake glaciation culminated sometime after that in the area (Schweinsberg and others, 2020). The Slocum Alluvium has been dated to approximately 160,000 ± 60,000 years BP based on U-series dating of fossil bones and soil development in the Canon City area (Szabo, 1980). Samples collected from the same site and analyzed by OSL techniques yielded age estimates of ~34 to ~38.5 ka (Colorado Geological Survey, unpublished data). These OSL samples were collected near the top of the deposit. The age of the Slocum Alluvium is unclear. The Slocum Alluvium is typically considered pre-Bull Lake, but the Bull Lake glaciation is thought have occurred as early as 200 to 170 ka (Madole, 1991; Kellogg and others, 2009). By virtue of the unit’s height above modern channels and the topographic continuity between surfaces adjacent to unnamed drainages, the unit is likely correlative with the Bull Lake glaciation and is late Middle to Late Pleistocene in age. It is not a likely source of sand and gravel and is likely not hydraulically connected to the South Platte River. The unit is as much as 4 m thick. | 01.02.04.01 | DMUUnit2 | Qatc4 | Qatc4 | 255-255-102 | 169-0-230 ESRI 24k Geology 601 gravel, open | DAS1 | Alluvial sediment | High | DMU12 |
| 13 | Qgtc2 | Gravel two of Three Mile Creek | Gravel two of Three Mile Creek | Middle Pleistocene | The unit consists of clast-supported gravel. Matrix was evaluated at the surface and is dark-brown (10YR 3/3) to brown (10YR 4/3) in color. The matrix is poorly sorted silt to sand. Clasts are subround to round and consist of locally derived volcanic clasts that can reach 60 cm in diameter. Clasts mantling the surface of the terrace did not have carbonate rinds. The unit underlies terraces approximately 65 m above modern channels. Epis and others (1979) mapped the unit as Verdos Alluvium (Qv). The Verdos Alluvium has been dated to approximately 620,000 years BP based on the presence of Lava Creek B ash present in some deposits (Madole, 1991). By virtue of the unit’s height above modern channels, the unit is likely correlative with the Verdos Alluvium or unit Qg2 from CGS mapping along the Colorado Front Range. Unit Qgtc2 is not a likely source of sand and gravel and is not hydraulically connected to the South Platte River. The unit is as much as 16 m thick. | 01.02.04.02 | DMUUnit2 | Qgtc2 | Qgtc2 | 255-235-102 | 169-0-230 ESRI 24k Geology 601 gravel, open | DAS1 | Alluvial sediment | High | DMU13 |
| 14 | None | MASS-WASTING DEPOSITS | MASS-WASTING DEPOSITS | None | None | 01.03 | DMUHeading2 | None | None | None | None | DAS1 | None | None | DMU14 |
| 17 | Qls | Landslide deposits | Landslide deposits | Holocene and Pleistocene | Landslide deposits consist of poorly sorted clay to boulder-sized clasts. Landslide deposits in the mapped area initiate exclusively at or near the contact between either the Tallahassee Creek Conglomerate (PEtc, PEtcw) or the Wall Mountain Tuff (PEwm) and the underlying South Park Formation (PEsp). Landslides are usually rotational, translational, earthflows, or some combination of the three. Younger landslide deposits have well-defined hummocky surface morphology and usually have well-defined head scarps, whereas older landslides have more subdued morphology and may not have well-defined head scarps. Variations in morphology indicate landslides have initiated periodically since at least the Middle Pleistocene. Landslide runouts can be as much as 600 m. Areas with similar geology may be prone to landslide initiation and related hazards. The unit is not a source of sand and gravel. Landslide deposits are as much as 40 m thick. | 01.03.01 | DMUUnit1 | Qls | Qls | 255-255-222 | 0-0-0 ESRI 24k Geology 502 periglacial | DAS1 | Debris flows, landslides, and other localized mass-movement sediment | High | DMU15 |
| 19 | None | ALLUVIAL AND MASS-WASTING DEPOSITS | ALLUVIAL AND MASS-WASTING DEPOSITS | None | None | 01.04 | DMUHeading2 | None | None | None | None | DAS1 | None | None | DMU16 |
| 15 | Qaf | Alluvial-fan and debris-fan deposits, undivided | Alluvial-fan and debris-fan deposits, undivided | Holocene to upper Middle Pleistocene | The unit is a mixture of alluvium, debris-flow, and alluvial-fan deposits. Debris-flow and alluvial-fan deposits are usually deposited during high seasonal precipitation. Mudflows (hyperconcentrated flows) and debris flows deposit these units in distinct facies. Mudflow facies are poorly sorted mud to medium sand with little to no gravel clasts. Debris-flow facies consist of matrix-supported gravels. The matrix is usually poorly sorted mud to sand, and gravels are angular to subangular clasts sourced from local bedrock. Unit color varies across the mapped area depending on source area geology; deposits sourced from volcanic bedrock are dark brown (10YR 3/3) to brown (10YR 4/3) in color whereas deposits sourced from Precambrian bedrock are brown (10YR 5/3, 4/3) in color. Rainfall from above-average seasonal precipitation entrains sediment from drainage headwalls, valley floors, and, in some cases, landslides. Mudflows and debris flows deposit the unit in fan-shaped lobes on low slopes at the base of relatively narrow, steep, and short ephemeral streams. Predominantly, unit Qaf is a sequence of several distinct deposits emplaced by separate events. An individual event deposit is usually 1 m thick or less. Debris-flow, mudflow, and alluvial-fan deposits are interlayered with alluvium locally. Alluvium is usually homogenous or weakly stratified silt and sand with pebble-gravel lenses less than 5 cm thick. Epis and others (1979) mapped some deposits as Bull Lake Alluvium (Qba). Owing to the height of these deposits above the modern South Platte River, they are likely correlative with the Bull Lake glaciation and are late Middle to Late Pleistocene in age. The youngest credible boulders dated from Bull Lake age moraines are approximately 132 to 120 ka, suggesting the Bull Lake glaciation culminated some time after that (Schweinsberg and others, 2020). Unit Qaf is likely interbedded locally with fluvial gravels but is distinguished from them in that clasts in Qaf deposits are more angular compared to fluvial gravel clasts. The unit also underlies lobate, undulating surfaces compared to fluvial terraces which usually have elongated, relatively flat morphology. Unit Qaf is not a likely source of sand and gravel. Certain deposits close to the South Platte River and Spinney Mountain Reservoir may be hydraulically connected. Areas underlain by the unit may be prone to hazards related to future debris flow and mudflow events. Deposits in the map area are as much as 4 m thick. | 01.04.01 | DMUUnit1 | Qaf | Qaf | 255-255-222 | 255-0-0 ESRI 24k Geology 601 gravel, open | DAS1 | Sediment | High | DMU17 |
| 18 | Qsw | Sheetwash alluvium | Sheetwash alluvium | Holocene and Upper Pleistocene | The unit consists of unstratified, homogenous silt to sand with dispersed pebble-sized clasts. Clasts and grains are angular to subround. The unit is derived from the Wall Mountain Tuff (PEwm), the South Park Formation (PEsp), and volcanic bedrock units. Overland flow deposited unit Qsw in apron- and fan-shaped sheets on relatively moderate to low slopes sporadically throughout the Holocene and Late Pleistocene. The unit is interbedded with unit Qaf and Qatc4 locally. Unit Qsw is not a source of sand and gravel and is not hydraulically connected to the South Platte River. Areas underlain by the unit may be prone to flooding and debris flows during years with high seasonal precipitation. The unit is probably as much as 2 m thick. | 01.04.02 | DMUUnit1 | Qsw | Qsw | 255-255-222 | 0-0-0 ESRI 24k Geology 607 sand | DAS1 | Alluvial sediment | None | DMU18 |
| 20 | Qac | Alluvium and colluvium, undivided | Alluvium and colluvium, unidived | Holocene and Pleistocene | The unit consists of poorly sorted sand and gravel. Gravel clasts are locally derived, angular to subangular clasts as much as 0.5 m in diameter. Gravity deposits colluvium as a wedge-shaped apron on steep to moderate slopes below steep bedrock exposures. Areas underlain by unit Qac may be subject to rockfall and associated hazards. The unit is as much as 2 m thick. | 01.04.03 | DMUUnit1 | Qac | Qac | 255-255-222 | 0-0-0 ESRI 24k Geology 605 breccia, open | DAS1 | Sediment | High | DMU19 |
| 21 | Qau | Alluvium, sheetwash, and debris-flow deposits, undivided | Alluvium, sheetwash, and debris-flow deposits, unidivided | Holocene and Upper Pleistocene | The unit consists of interbedded debris-flow, sheetwash, and alluvial deposits of varying ages underlying ephemeral channels. Where Precambrian clasts crop out in the source area, the unit is brown (10YR 5/3, 4/3) in color, compared to dark brown (10YR 3/3) to brown (10YR 4/3) in areas the unit is derived from volcanic bedrock. Debris-flow deposits are matrix-supported gravel deposits. Clasts are angular to subangular and as much as 0.3 m in diameter. Debris-flow deposits are as much as 1 m thick. Sheetwash alluvium is homogenous to stratified silty sand to pebble gravel. Clasts are angular to subangular. Alluvial deposits are usually silty sand to coarse sand and range from homogenous to well-stratified. Soil development ranges widely across the mapped area, from no soil development to A/Bt/Bk sequences with Stage II carbonate development. Sediments are usually deposited during high-energy, high-volume storm events (Barkmann and others, 2018). The unit is not a source of sand and gravel. The unit is locally hydraulically connected to the South Platte River. Areas underlain by the unit may be prone to flooding and debris flows, and their related hazards. The unit is typically 1.5 m thick. | 01.04.04 | DMUUnit1 | Qau | Qau | 255-255-222 | 0-0-0 ESRI 24k Geology 601 gravel, open | DAS1 | Sediment | High | DMU20 |
| 16 | Qafo | Old alluvial-fan and debris-fan deposits, undivided | Old alluvial-fan and debris-fan deposits, undivided | upper Middle to Middle Pleistocene | The unit consists of poorly sorted gravel with a clay and sand matrix and color varies across the mapped area depending on source area geology. In the north part of the mapped area, Precambrian bedrock crops out in source areas for Qafo deposits. The unit is usually brown (10YR 5/3, 4/3) in color and Precambrian clasts in these deposits are angular to subround and generally pebble-sized. Thin (<2 m) Qafo deposits mantle surfaces with lobate, undulating morphology approximately 8 to 10 m above modern channels. In many locations, bedrock is within 1 m of the surface and crops out along the edges of the deposits. In the southeastern part of the mapped area, volcanic bedrock is the predominant unit that crops out in source areas for Qafo deposits, and the unit is usually dark brown (10YR 3/3) to brown (10YR 4/3) in color. Angular to subround brown to gray volcanic clasts 15 cm in diameter mantle surfaces 2.5 to 6 m high. The unit is topographically higher than unit Qaf. The unit differs from fluvial gravel in that clasts in unit Qafo are more angular and unit Qafo underlies lobate, undulating surfaces compared to fluvial terraces which usually have elongated and relatively flat morphology. Epis and others (1979) mapped these deposits as Bull Lake Alluvium (Qba). The youngest credible boulders dated from Bull Lake age moraines are approximately 132 to 120 ka, suggesting the Bull Lake glaciation culminated some time after that (Schweinsberg and others, 2020). By virtue of heights above stream channels and the level of erosion of the surfaces, deposits in the north part of the map area are likely older than those in the southeastern part of the map area. They may be as old as early Middle Pleistocene. Deposits in the southeastern part of the mapped area are likely late Middle to Late Pleistocene in age. Unit Qaf is likely interbedded with fluvial gravel locally. The unit is as much as 4 m thick. | 01.04.05 | DMUUnit1 | Qafo | Qafo | 255-255-179 | 255-0-0 ESRI 24k Geology 601 gravel, open | DAS1 | Sediment | High | DMU21 |
| 22 | None | BEDROCK GEOLOGY | BEDROCK GEOLOGY | None | Bedrock unit classifications follow those of Carpenter and others (2016). Chemical classification of extrusive igneous rocks is that of Le Bas and others (1986). Chemical classification of intrusive igneous rocks is that of Middlemost (1994) and Frost and Frost (2008). Geologic terms follow the definitions of Neuendorf and others (2005). | 02 | DMUHeading1 | None | None | None | None | DAS1 | None | None | DMU22 |
| 23 | Na | Arkosic sandstone and conglomerate | Arkosic sandstone and conglomerate | Neogene? | Poorly lithified, arkosic sandstone and conglomerate with a minimum thickness of 500 m. Shown only on cross-section. | 02.01 | DMUUnit1 | Na | Na | 254-232-187 | None | DAS1 | Clastic sedimentary rock | High | DMU23 |
| 24 | PEtc | Tallahassee Creek Conglomerate | Tallahassee Creek Conglomerate | Eocene | Orangish-tan to greenish-gray boulder conglomerate. Though boulders are the most noticeable constituent, all sizes of gravel are present in a matrix of muddy, tuffaceous sandstone. Some of the boulders are up to 2 m in diameter. West of County Rd. 59, they include local rock types such as granitic gneiss, granite, pegmatite, Thirtynine Mile Andesite, and Wall Mountain Tuff. In some places, the uppermost part of the formation consists entirely of boulders of Wall Mountain Tuff (tuff boulder lithofacies, described separately below). East of County Rd. 59, only granitic boulders are present. The Tallahassee Creek Conglomerate lacks bedding, and the gravel clasts are matrix-supported. The conglomerate forms low, boulder-strewn hills north and east of the Thirtynine Mile Andesite, and south of Spinney Mountain. It appears to have filled paleovalleys on a landscape with considerable topographic relief. The Tallahassee Creek Conglomerate overlies the Balfour and South Park formations, and the Thirtynine Mile Andesite with angular unconformity. Rockfall is common around the sides of conglomerate-capped hills. The formation is approximately 2 to 36 m thick. | 02.02 | DMUUnit1 | :tc | PEtc | 255-179-102 | None | DAS1 | Clastic sedimentary rock | High | DMU24 |
| 25 | PEtcw | Tallahassee Creek Conglomerate, tuff boulder lithofacies | Tallahassee Creek Conglomerate, tuff boulder lithofacies | Eocene | This lithofacies is similar to the rest of the Tallahassee Creek Conglomerate, except that the boulders consist almost entirely of blocks of Wall Mountain Tuff up to 2 m in diameter. The lithofacies weathers to form fields of tuff boulders that resist weathering and cap ridges. On the quadrangle, this lithofacies occurs in two places near outcrops of Wall Mountain Tuff. The outcrop on the boundary between sec. 6 and 7, T. 13 S., R. 73 W., is especially striking because the tuff blocks extend from the tuff outcrop in a narrow band and appear to fill a meandering paleochannel. The tuff boulder lithofacies is conformable with the remainder of the Tallahassee Creek Conglomerate. Approximately 20 miles southwest of the mapped area, Epis and Chapin (1974) described a similar unit, which they called the Stirrup Ranch Tuff. Lipman and Bachman (2015) reinterpreted the Stirrup Ranch Tuff as flood deposits derived from the Wall Mountain Tuff. The boulder tuff lithofacies on the Spinney Mountain quadrangle appears to be very similar to the Stirrup Ranch Tuff, if not the same unit. However, further mapping is needed to confirm or disprove this hypothesis. The thickness of the lithofacies is variable, ranging from 6 to 36 m. | 02.02.01 | DMUUnit2 | :tcw | PEtcw | 235-153-0 | None | DAS1 | Clastic sedimentary rock | High | DMU25 |
| 26 | PEal | Thirtynine Mile Andesite, lower member | Thirtynine Mile Andesite, lower member | Eocene | Gray to very dark-gray and purplish-gray volcanic breccia and massive volcanic rocks of mostly andesitic composition. Matrix-supported conglomerate is a minor constituent. Breccias compose >80% of the unit, and the remaining <20% is composed of massive volcanic rock and conglomerate. Unit PEal is correlative with the middle andesite series of Barkmann and others (2018) and appears to be correlative with the lower member of the Thirtynine Mile Andesite of Epis and Chapin (1974). Four units within the lower member are mapped separately, and are described below. These include a unit with thicker and more abundant lava flows (unit PEalf) in the upper part of the member, a hornblende andesite porphyry flow (PEalh), a diorite plug (PEald), and some small rhyodacite flows, dikes, and plugs (PEalr). The breccias are composed of sand- to cobble-sized angular fragments of aphanitic to porphyroaphanitic igneous rocks in a matrix of similar appearance. Some breccias are matrix-supported, and some are clast-supported. Pyroxene is the most common phenocryst, though plagioclase and rarely hornblende were also observed. These breccias are interpreted to be autobreccias where matrix lithology and monomict clast lithology are identical. Unbrecciated volcanic rocks are also present in a myriad of small flows, dikes, and plugs. The lithology is similar to that of the volcanic breccias. Dikes are approximately 2 to 5 m wide, and flows are 1 to 2 m thick. Compositionally these rocks are mostly andesite and basaltic andesite, though minor basalt is also present. Geochemical analysis of a representative sample from the Antero NE quadrangle yielded a trachyandesite composition. Sanidine from that sample yielded an 40Ar/39Ar single crystal fusion mini-plateau age of 34.44 ± 0.05 Ma (Barkman and others, 2018). Volcanic layering in the flows is about 15 cm to 1 m thick. Both the brecciated and unbrecciated volcanic rocks are magnetic to varying degrees, and some contain vesicles filled with zeolites. Two types of volcaniclastic conglomerates occur in the unit. One is composed of pebble- to cobble-sized clasts of Thirtynine Mile Andesite in a matrix of reddish-to orangish-brown muddy sand. It is clast-supported but lacks bedding. The other is composed of granule- to cobble-sized clasts of glassy Thirtynine Mile Andesite in a matrix of light-gray vitric tuff. It occurs in beds 5 to 25 cm thick. These are interpreted as volcaniclastic debris-flow and alluvial deposits. The lower member is relatively nonresistant, and crops out poorly. It underlies gentle, rolling, grassy hills, through which groundwater migrates readily. The dikes, flows, and plugs are somewhat more resistant and less permeable than the breccias. The dikes tend to be aligned with the structural grain of the map area. North-northwest is the most prevalent orientation, but east-northeast is also common. The dikes appear to have been formed by magma intruding cracks and fissures in the highly fractured footwall of the Elkhorn fault system. Unit PEal makes a gradational contact with unit PEb. The contact is defined at the base of the lowest lava flow or breccia. Thickness is about 42 m. | 02.03 | DMUUnit1 | :al | PEal | 207-191-242 | None | DAS1 | Extrusive igneous material | High | DMU26 |
| 27 | PEalf | Flow-rich facies | Thirtynine Mile Andesite, flow-rich facies | Eocene | This facies occurs in the upper part of the member. Lithologically it is similar to the rest of the lower member in that most of the rocks are andesite or basaltic andesite with minor basalt. A basalt porphyry on Rogers Mountain is 63% plagioclase, 25% pyroxene, 10% magnetite, and 2% hornblende. Geochemically it is 47.3% SiO2, 15.7% Al2O3, 11.8% Fe2O3, 8.45% CaO, 5.82% MgO, 3.24% Na2O, 2.34% K2O, 0.008% Cr2O3, 1.64%TiO2, 0.18% MnO, 0.68% P2O5, 0.13% SrO, and 0.16% BaO (trachybasalt). Unbrecciated lava flows tend to be thicker (3 to 15 m), more extensive, and more numerous in comparison to the rest of the lower member. They make up >20% of the unit. The flow-rich facies is also more resistant to erosion. It forms steeper topography, and the lava flows tend to form ledges. The flows also form barriers to groundwater infiltration, perching the water table. Trees and lush vegetation can grow in places. Marshes, bogs, and springs are common. The contact with the lower part of unit PEal is gradational and is defined where the topography becomes noticeably steeper and more ledgy. The flow-rich facies appears to be transitional between the lower part of the lower member with its sparse, thin, brecciated flows, and the upper member of the Thirtynine Mile Andesite, with its abundant thick, unbrecciated flows. Rockfall is a hazard around ledges. As much as 200 m of the facies are exposed in the map area. | 02.03.01 | DMUUnit2 | :alf | PEalf | 170-102-205 | None | DAS1 | Extrusive igneous material | High | DMU27 |
| 28 | PEalr | Rhyodacite dikes and flows | Thirtynine Mile Andesite, rhyodacite dikes and flows | Eocene | Orangish-pink rhyodacite porphyry that weathers to reddish-brown. Phenocrysts of biotite, hornblende, and plagioclase are < 1 mm in diameter. The rock is about 14% plagioclase phenocrysts, 1% biotite phenocrysts, 4% hornblende phenocrysts, and 81% microcrystalline feldspar laths. The laths and phenocrysts are flow-aligned. Geochemically it is 69.3% SiO2, 16.8% Al2O3, 2.14% Fe2O3, 2.15% CaO, 0.38% MgO, 4.13% Na2O, 5.20% K2O, 0.003% Cr2O3, 0.38% TiO2, 0.02% MnO, 0.17% P2O5, 0.07% SrO, and 0.24% BaO (borderline rhyolite/trachydacite). In places it is thinly layered (5 to 10 cm) but there are also thicker breccia layers (1 to 3 m). The unit is resistant to erosion and tends to form ridges. Contacts are sharp. The flow in the southwestern corner of the map area has a basal ash layer and a vitrophyre. Dating of biotite in this flow yielded an 40Ar/39Ar plateau age of 34.73 ± 0.08 Ma. Though its composition is significantly different from the rest of the lower andesite member, the date shows that the rhyodacite is approximately coeval with it, and thus is likely a part of it. Rockfall is a hazard on slopes. The flow is about 36 m thick, and the dikes are 5-10 m thick. | 02.03.02 | DMUUnit2 | :alr | PEalr | 223-115-255 | None | DAS1 | Igneous rock | High | DMU28 |
| 29 | PEalh | Hornblende andesite flow | Thirtynine Mile Andesite, hornblende andesite flow | Eocene | Very dark-gray hornblende andesite porphyry that weathers to dark reddish-gray. Phenocrysts of hornblende and plagioclase are <3mm in diameter. Plagioclase phenocrysts are zoned and albite-twinned. The phenocrysts occur in a matrix of flow-aligned, microcrystalline plagioclase. The rock is 89% microcrystalline plagioclase, 9% plagioclase phenocrysts, and 2% hornblende phenocrysts. Minor glass and magnetite are also present. Geochemically it is 59.3% SiO2, 17.6% Al2O3, 4.42% Fe2O3, 3.89% CaO, 1.17% MgO, 3.92% Na2O, 4.60% K2O, 0.003% Cr2O3, 0.54% TiO2, 0.14% MnO, 0.26% P2O5, 0.07% SrO, and 0.13% BaO (trachyandesite). Parts of the flow are vesicular; the vesicles are filled with zeolites. The unit forms ridges with a sharp lower contact. Rockfall is a hazard on slopes. A maximum thickness of 85 m is exposed on the quadrangle. | 02.03.03 | DMUUnit2 | :alh | PEalh | 212-128-194 | None | DAS1 | Intermediate-composition lava flows | High | DMU29 |
| 30 | PEald | Diorite plug | Thirtynine Mile Andesite, diorite plug | Eocene | Medium-gray, fine-grained diorite, weathering to light reddish-gray. The mineral composition of the rock is 66% plagioclase, 22% olivine, 3% pyroxene, and 9% magnetite. Geochemically it is a monzodiorite with 50.1% SiO2, 16.4% Al2O3, 11.0% Fe2O3, 6.58% CaO, 4.17% MgO, 3.68% Na2O, 3.50% K2O, 0.004% Cr2O3, 1.43% TiO2, 0.20% MnO, 1.01% P2O5, 0.15% SrO, and 0.19% BaO. The plug contains no discernable structure. It resists erosion and forms a prominent hill. Its contact with the surrounding rock is sharp. Rockfall is a hazard on slopes. Approximately 24 m of the plug are exposed in the map area. | 02.03.04 | DMUUnit2 | :ald | PEald | 168-0-132 | None | DAS1 | Intrusive igneous rock | High | DMU30 |
| 31 | PEb | Balfour Formation | Balfour Formation | Eocene | Volcaniclastic sandstone, siltstone, and shale. The sandstone is light tan, weathering to orangish tan or orangish brown. It is a fine to coarse, and commonly contains granules. It is a matrix-rich lithic sandstone with numerous volcaniclasts and rip-up clasts of white tuffaceous mudstone. Coarser beds may be up to 15 cm thick; finer beds are about 3 to 5 cm thick, and are commonly interbedded with siltstone or shale. Siltstone and mud shale are gray to tan in color and weather to orangish tan. They are commonly laminated, fissile, brittle, and tuffaceous. 40Ar/39 dating of sanidine in a tuff sample on the Guffey NW quadrangle (Houck and others, 2025) yielded an age of 34.81 +/- 0.04 Ma. This age is slightly older than that of unit PEal, which overlies it. Soft sediment deformation occurs in unit PEb, especially in S ½ sec. 12, T. 13 S., R. 74 W. The unit forms valleys with gentle slopes. It weathers into yellowish-brown silty soils that are highly prone to sheetwash, making it poorly exposed. On the quadrangle, it is present only south of the Gulch Road fault and west of the Rogers Ranch fault. Its thickness in the map area does not exceed 30 m. | 02.04 | DMUUnit1 | :b | PEb | 242-224-255 | None | DAS1 | Sedimentary and extrusive igneous material | High | DMU31 |
| 32 | PEwm | Wall Mountain Tuff | Wall Mountain Tuff | Eocene | Light orangish-pink, densely welded, crystal-vitric ash flow tuff that weathers to tan. The tuff is rhyolitic in composition. Crystals include clear sanidine and black biotite. Secondary quartz is common. Volcanic layering is about 1 cm thick and is defined by flattened pumice clasts 2 to 3 cm long. A single-crystal total fusion 40Ar/39Ar analysis of sanidine from a sample collected on the Antero Northeast quadrangle (Barkmann and others, 2018) yielded an age of 37.07 ± 0.08 Ma. The Wall Mountain Tuff is resistant to erosion and forms prominent ridges. It nonconformably overlies Proterozoic rocks and overlies the South Park Formation with angular unconformity. Rockfall is a hazard on slopes. A maximum of approximately 115 m is exposed in the map area. | 02.05 | DMUUnit1 | :wm | PEwm | 135-33-255 | None | DAS1 | Felsic-composition pyroclastic flows | High | DMU32 |
| 33 | PEec | Elkhorn conglomerate | Elkhorn conglomerate | Paleocene? | Yellowish-tan to yellowish-brown, very poorly lithified, boulder conglomerate. The conglomerate is composed of subrounded granules, pebbles, cobbles, and boulders in a sandy, silty matrix. Clasts are the detritus of Proterozoic igneous and metamorphic rocks such as granite, pegmatite, and granite gneiss, that occur in the adjacent hanging wall of the Elkhorn thrust fault. The conglomerate is matrix-supported and is weakly bedded. The unit forms a steep apron on the downthrown side of the Elkhorn fault. It lies with angular unconformity over Cretaceous rocks. Its relationship with the Proterozoic rocks is variable. It may overlie them nonconfomably, or be in fault contact with them. Examples of both occur on the Elkhorn quadrangle (Ruleman and Bohannon, 2008) and the Sulphur Mountain quadrangle (Bohannon and Ruleman, 2009). It is interpreted to be equivalent to the Elkhorn conglomerate of Barkmann and others (2017), the syntectonic conglomerate of Ruleman and others (2011), the Elkhorn thrust conglomerate of Ruleman and Bohannon (2008), and the conglomerate and breccia of Bohannon and Ruleman (2009). It is thought to be derived from a local scarp created by the hanging wall of the Elkhorn thrust fault (Bohannon and Ruleman, 2009). Detrital zircon analysis of 134 zircon grains yielded 130 grains with an average age of 1708.2 Ma, 3 grains with an average age of 1509 Ma, and one grain with an age of 63.5 Ma. Thus, the maximum depositional age of the unit is Paleocene. Though the unit could be younger, one might expect that the abundant Eocene, Oligocene, and Miocene volcanic activity in the area would have contributed younger zircons. The Elkhorn conglomerate produces numerous falling rocks. Boulders in the unit are used for landscaping at Spinney Mountain State Park. Thickness in the map area is as much as 60 m. | 02.06 | DMUUnit1 | :ec | PEec | 255-235-179 | 0-0-0 ESRI 24k Geology 605 breccia, open | DAS1 | Clastic sediment | High | DMU33 |
| 34 | PEsp | South Park Formation, undivided | South Park Formation, undivided | Paleocene | Poorly exposed volcaniclastic mudstone, sandstone, and conglomerate. The sandstones and conglomerates are light grayish tan to peach-colored. Conglomerates contain granules, pebbles, and cobbles of K-feldspar, chert, quartz, and quartzite that are subangular to subrounded. The basal part of the section also contains well-rounded cobbles of gray-green to tan intermediate volcanic rocks that are diagnostic of the South Park Formation in the mapped area. Sandstones, some of which are conglomeratic, are matrix-rich, fine to coarse, and poorly sorted. They are feldspathic and lithic. Some of the sandstones and conglomerates are massive and matrix-supported. Others are clast-supported, and occur in thin beds, or in trough cross-bed sets about 15 cm thick. Mudstones are bright red to dark reddish-brown. They lack bedding, and break into clasts about 5 mm in diameter. In comparison to quadrangles to the west and northwest, the South Park Formation on the Spinney Mountain quadrangle has a higher proportion of mudstone and sandstone that is easily eroded. It has a lower proportion of conglomerate and lacks boulder conglomerate, which tends to be more resistant. Consequently, it is a valley former in the mapped area, with sparse ridges underlain by conglomerate beds. It nonconformably overlies Proterozoic rocks east of the Rogers fault. West of the Rogers fault it presumably overlies the Cretaceous rocks with angular unconformity, as it does on the Guffey NW quadrangle. However, this isn’t known with certainty. It is prone to swelling soils. A thickness of about 45 to 60 m is exposed on the quadrangle. | 02.07 | DMUUnit1 | :sp | PEsp | 235-204-153 | None | DAS1 | Clastic sedimentary rock | High | DMU34 |
| 35 | Kp | Pierre Shale | Pierre Shale | Upper Cretaceous | Medium- to dark-gray, fissile, predominantly calcareous shale with thin (< 10 cm) beds of brownish-gray to olive-drab sandstone and siltstone. The Pierre Shale is recessive and outcrops are rare on the quadrangle. It weathers to form soft, collapsible, olive-tan soil. It may have a “popcorn” appearance from swelling and shrinking of clay. On the adjoining Guffey NW quadrangle the Pierre Shale conformably overlies the Niobrara Formation. It is prone to landslides and swelling soils. In areas where strata are dipping, the overlying soils may be prone to heaving. With the exception of sandstone beds and local fracturing, the Pierre Shale has very limited potential to yield water to wells. Sawatzky (1967) estimated that the unit may be up to 1,585 m thick in South Park, but less than 25 m are exposed on the quadrangle. | 02.08 | DMUUnit1 | Kp | Kp | 204-255-153 | None | DAS1 | Clastic sedimentary rock | High | DMU35 |
| 36 | Kn | Niobrara Formation | Niobrara Formation | Upper Cretaceous | Chalky limestone and shale, approximately 60 m thick. Shown only on cross-section. | 02.09 | DMUUnit1 | Kn | Kn | 235-255-179 | None | DAS1 | Sedimentary rock | High | DMU36 |
| 37 | Kb | Benton Group | Benton Group | Upper Cretaceous | Gray shale with minor limestone and sandstone, approximately 85 m thick. Shown only on cross-section. | 02.10 | DMUUnit1 | Kb | Kb | 153-173-128 | None | DAS1 | Sedimentary rock | High | DMU37 |
| 38 | Kd | Dakota Sandstone | Dakota Sandstone | Lower Cretaceous | Cream-colored, fine- to medium-grained quartz sandstone in thick laminae defined by normally graded beds. An outcrop is exposed in the footwall of the Elkhorn thrust fault that is highly fractured and is steeply dipping to overturned. On the adjoining Guffey NW quadrangle it is approximately 70 m thick in the McDannald #1 stratigraphic test (API 05-093-5007; sec. 32, T. 11 S., R. 75 W.). In the well it sits disconformably on the Morrison Formation. However, only a few meters of Dakota Sandstone are exposed on the Spinney Mountain quadrangle. | 02.11 | DMUUnit1 | Kd | Kd | 179-205-102 | None | DAS1 | Mostly sandstone | High | DMU38 |
| 39 | Jm | Morrison Formation | Morrison Formation | Upper Jurassic | Variegated mudstone, siltstone, and sandstone, approximately 45 m thick. Shown only on cross-section. | 02.12 | DMUUnit1 | Jm | Jm | 43-194-141 | None | DAS1 | Clastic sedimentary rock | High | DMU39 |
| 40 | Jg | Garo Sandstone | Garo Sandstone | Jurassic | Pinkish-tan quartz sandstone, approximately 40 m thick. Shown only on cross-section. | 02.13 | DMUUnit1 | Jg | Jg | 6-140-108 | None | DAS1 | Mostly sandstone | High | DMU40 |
| 41 | Yg | Equigranular medium-grained granite | Equigranular medium-grained granite | Mesoproterozoic | Light gray, light pinkish-gray, or light orangish-gray medium-grained hypidiomorphic granular biotite-bearing granite. Modal mineralogy includes anhedral to subhedral quartz with undulatory extinction, subhedral alkali feldspar, subhedral plagioclase and subhedral biotite. Locally muscovite and (or) garnet-bearing. Geochemical samples plot in the granite field of the plutonic TAS diagram of Middlemost (1994) and are ferroan, peraluminous, and alkali-calcic by the classification diagrams of Frost and Frost (2008). Unit Yg occurs as dikes cutting unit Xgd, up to ~150 m wide, that most commonly follow unit Xgd foliation but are also observed to cut foliation where the host rock is low-strain. Locally, unit Yg has a weak foliation defined by aligned biotite that parallels the fabric in unit Xgd, which could suggest that intrusion of unit Yg was late synkinematic to deformation that imparted foliation to unit Xgd. | 02.14 | DMUUnit1 | Yg | Yg | 133-155-163 | None | DAS1 | Intrusive igneous rock | High | DMU41 |
| 42 | YXd | Quartz diorite dike | Quartz diorite dike | Paleoproterozoic or Mesoproterozoic | Dark-gray, fine- to medium-grained hypidiomorphic granular biotite quartz diorite. Composed of aligned bronze-black biotite, recrystallized quartz with undulatory extinction, plagioclase, and minor secondary magnetite. Unit YXd occurs at a single locality southeast of Spinney Mountain where it intrudes unit Xgd. Unit YXd is best exposed where transected by a roadcut and outcrop is otherwise limited to hillslope subcrop. Foliation and lineation within unit YXd are defined by aligned biotite and parallel the fabric within the surrounding unit Xgd. | 02.15 | DMUUnit1 | YXd | YXd | 179-153-102 | None | DAS1 | Intrusive igneous rock | High | DMU42 |
| 43 | YXp | Pegmatite dikes | Pegmatite dikes | Proterozoic | Pink, white, and pinkish-gray pegmatitic granite and alkali feldspar granite. Composed of pink euhedral or subhedral, commonly perthitic alkali feldspar, subhedral or anhedral quartz, and subhedral myrmekitic plagioclase, with sparse euhedral muscovite and rare euhedral or subhedral tourmaline. Observed pegmatite dikes are generally not zoned. Unit YXp occurs as decimeter- to meter-scale thickness dikes intruding other Proterozoic units. Unit YXp dikes that intrude unit Xgns are most commonly parallel to the gneissic foliation in that unit and less commonly cut foliation. Unit YXp dikes that intrude unit Xgd most commonly parallel foliation in that unit, though dikes cutting foliation are also observed. Mapped dikes generally form outcrop ridges more resistant than the units they intrude. Numerous pegmatite dikes with widths smaller than mappable scale are not depicted on the map. | 02.16 | DMUUnit1 | YXp | YXp | None | 255-0-0 01.03.01 | DAS1 | Coarse-grained, felsic-composition intrusive igneous rock | High | DMU43 |
| 44 | Xgd | Porphyritic granodioritic gneiss | Porphyritic granodioritic gneiss | Paleoproterozoic | Pinkish-gray, weakly foliated, coarse-grained hypidiomorphic porphyritic biotite granodioritic and granitic augen gneiss. Modal mineralogy includes subhedral plagioclase, pink subhedral or euhedral alkali feldspar, anhedral quartz, and biotite. Locally, unit Xgd is muscovite- or hornblende-bearing. Light sericitic alteration of feldspar and chloritic alteration of mica are present in most samples, as are epidote-rimmed opaque Fe-oxide alteration products. Where unstrained, alkali feldspar phenocrysts are as large as 7 cm, and are commonly poikilitic with inclusions of biotite, quartz, or plagioclase. Centimeter- to meter-scale biotite-rich enclaves are common, as are foliated xenoliths of unit Xgbn near the contact with that unit. Geochemical samples span the granodiorite and granite fields of the plutonic TAS diagram of Middlemost (1994) and are ferroan, calcic to calc-alkalic, and peraluminous by the classification diagrams of Frost and Frost (2008). U-Pb zircon analysis yielded an age of 1712.9±6.4 Ma. Most commonly, unit Xgd has a subplanar, gently undulating, north-dipping foliation defined by aligned biotite and a weak down-dip lineation defined by elongate clots of biotite. In localized shear zones where the unit is more highly strained, the foliation and lineation are defined by recrystallized quartz and feldspar. Unit Xgd is the most aerially extensive Proterozoic unit in the mapped area and is found in both the hanging wall and footwall of the Elkhorn thrust. This unit forms cliffs near Stoll Mountain, and foliation-parallel bedrock ledges on pediment surfaces. Unit Xgd intrudes units Xa, Xbgn, and Xgns. Rockfall is a hazard on slopes. | 02.17 | DMUUnit1 | Xgd | Xgd | 128-179-179 | None | DAS1 | Metamorphic rock | High | DMU44 |
| 45 | Xgb | Gabbro dike | Gabbro dike | Paleoproterozoic | Dark greenish-gray, biotite-bearing hornblende gabbro dike. Orangish-brown on weathered outcrops and erodes easily to greenish-gray grus. Medium-grained, hypidiomorphic, and inequigranular. Composed of sericitized tabular plagioclase, serpentinized clinopyroxene, subhedral amphibole and red-brown pleochroic biotite. Unit Xgb occurs at a single locality on the southeastern slope of Spinney Mountain as a meter- to decameter-scale thick dike where it intrudes unit Xgns and is intruded by unit Xgd. A geochemical sample of unit Xgb plots in the gabbro field of the plutonic TAS diagram of Middlemost (1994) and in the tholeiitic field on the AFM diagram of Irvine and Baragar (1971). | 02.18 | DMUUnit1 | Xgb | Xgb | None | 255-0-0 01.03.03 | DAS1 | Intrusive igneous rock | High | DMU45 |
| 46 | Xa | Amphibolite | Amphibolite | Paleoproterozoic | Dark-gray or dark greenish-gray, fine-grained biotite amphibolite. Granofelsic to weakly foliated, with foliation defined by aligned amphibole and biotite. Contains prismatic subhedral hornblende, equant to tabular subhedral and anhedral plagioclase, anhedral quartz, and minor chloritized biotite. Sparse clots of recrystallized plagioclase as large as 12 mm and hornblende as large as 6 mm may be relict phenocrysts and could suggest an originally porphyritic texture. A geochemical sample of unit Xa plots at the boundary of the basalt and basaltic andesite fields of a TAS diagram (Le Bas and others, 1986) and in the calc-alkaline field of an AFM diagram (Irvine and Baragar 1971). Unit Xa occurs in a single locality northeast of Elevenmile Reservoir, where it is intruded by units Xgd and Yg. | 02.19 | DMUUnit1 | Xa | Xa | 179-179-102 | None | DAS1 | Metamorphic rock | High | DMU46 |
| 47 | Xbgn | Biotite Gneiss | Biotite Gneiss | Paleoproterozoic | Light-gray to gray, fine to medium-grained biotite granodioritic gneiss. Most outcrops have foliation defined by gneissic layering or stromatic migmatitic texture, but are gradational into low-strain zones of fine- to medium-grained hypidiomorphic granular biotite granite and granodiorite. Low-strain zones have equant subhedral alkali feldspar, zoned tabular subhedral plagioclase, anhedral quartz, fine-grained anhedral biotite, and are locally muscovite-bearing. Gneissic foliation within unit Xbgn is either parallel or slightly oblique to the foliation within unit Xgd. Folding of foliation is common, typically as decimeter-scale open folds but also as cm-scale ptygmatic folding. A single geochemical sample plots in the granite field of the plutonic TAS diagram of Middlemost (1994) and is ferroan, calc-alkalic, and peraluminous by the classification diagrams of Frost and Frost (2008). Unit Xbgn is exposed in rounded topographic knobs on Stoll Mountain where it is intruded by unit Xgd. Xenoliths of unit Xbgn are present within unit Xgd; most are only a few meters in diameter but one is sufficiently large to be depicted on the map. | 02.20 | DMUUnit1 | Xbgn | Xbgn | 222-204-179 | None | DAS1 | Metamorphic rock | High | DMU47 |
| 48 | Xgns | Spinney Mountain gneiss and schist, undivided | Spinney Mountain gneiss and schist, undivided | Paleoproterozoic | Unit Xgns includes a lithologically complex suite of gneiss and schist. The dominant lithologies are banded quartzofeldspathic biotite gneiss, light-gray, weakly foliated, fine-grained biotite (± muscovite) granitic gneiss, and silvery-gray, garnet-bearing biotite muscovite quartz schist, with lesser proportions of dark-gray, fine- to medium-grained granofelsic amphibolite, and stromatic migmatite with granitic leucosome and biotite gneiss melanosome. A feldspathic subunit (Xgnsf) is mapped separately (see below). Unit Xgns is intruded by numerous pegmatitic, granodioritic, and aplitic dikes most commonly parallel to foliation. Map-scale pegmatite dikes are depicted separately as unit YXp but smaller dikes are not separated. Gneissic and schistose foliation dips steeply to shallowly west-southwest. Lineation defined by aligned biotite or recrystallized quartz and feldspar plunges shallowly south on average. Rockfall is a hazard on slopes. | 02.21 | DMUUnit1 | Xgns | Xgns | 179-204-204 | None | DAS1 | Metamorphic rock | High | DMU48 |
| 49 | Xgnsf | Spinney Mountain gneiss, feldspathic subunit | Spinney Mountain gneiss, feldspathic subunit | Paleoproterozoic | Dominated by dark-gray, medium-grained biotite granodiorite and granodioritic gneiss. Where low strain, hypidomorphic granular granodiorite includes subhedral alkali feldspar, subhedral plagioclase, anhedral or subhedral biotite, anhedral quartz, and locally trace amounts of muscovite. Higher strain domains have gneissic banding but lack the lithologic diversity of unit Xgns. Weakly to strongly foliated with foliation defined by aligned platy biotite that wraps around feldspar clasts. Lineation is defined by aligned elongate axes of biotite and recrystallized tails of quartz and feldspars. A single geochemical sample plots in the granodiorite field of the plutonic TAS diagram of Middlemost (1994), and is ferroan, calcic to calc-alkalic, and peraluminous by the classification diagrams of Frost and Frost (2008). U-Pb zircon analysis yielded an age of 1708.7±6.6 Ma. Unit Xgnsf is separated from unit Xgns where biotite granodiorite is the dominant lithology; the contact is gradational and approximately located. Unit Xgnsf occurs on the west side of Spinney Mountain and as foliation-parallel bodies within the undivided unit Xgns. Rockfall is a hazard on slopes. | 02.21.01 | DMUUnit2 | Xgnsf | Xgnsf | 222-235-222 | None | DAS1 | Metamorphic rock | High | DMU49 |
| 50 | w | water | water | Holocene | None | 03 | DMUUnit1 | None | w | 151-219-242 | None | DAS1 | Water or ice | High | DMU50 |