OF-03-08 Geologic Map of Falcon NW Quadrangle, El Paso County, Colorado


Geologic Map of the Falcon NW Quadrangle, El Paso County, Colorado. The map shows distribution of various rock types and their configuration and provides information about potential resources and hazards. Includes cross section, map unit correlation, shaded relief map with geology overlay, booklet of extended descriptions of map units, economic geology and selected references. Digital ZIP download. OF-03-08D

From the Author’s Notes:

The Falcon NW 7.5-minute quadrangle is in the Colorado Piedmont, a region that is distinguished primarily by the fact that is it has been stripped of the Miocene fluvial rocks that cap the adjoining High Plains Section of the Great Plains physiographic province. Older Cenozoic strata also have been eroded from most of the Colorado Piedmont, except at the higher, western part of the interfluve between the South Platte and Arkansas Rivers, which is the location of the Falcon NW quadrangle. Lower Cenozoic (Paleocene) rocks are at the surface or underlie surficial deposits everywhere in the quadrangle, except perhaps in the southeastern-most part of the area.

Sand is conspicuously abundant in the Falcon NW quadrangle because the bedrock (Dawson Formation) at and near the surface consists chiefly of arkosic sandstone and pebble conglomerate, much of which is weakly cemented. Sandy alluvial and eolian deposits blanket nearly half of the quadrangle, and thick (5 ft or more), sandy residuum (unconsolidated material derived from weathering of the underlying bedrock) is widespread. Bedrock outcrops are relatively small and few in number. Most are on valley sides 1) in the Cottonwood Creek drainage basin (northwestern part of the map area), 2) in the Jimmy Camp Creek drainage basin (southeast corner of the map area), and 3) along Sand Creek (central and southwestern parts of the map area).

Most Quaternary alluvium in the Falcon NW quadrangle is related to six streams, which today are ephemeral but at times in the past were probably perennial. The streams head in or near the Black Forest just south (2–5 mi) of the Palmer Divide, the local name for the drainage divide between the South Platte and Arkansas rivers. Initially, most streams in the quadrangle drained from north to south across the map area. However, during the late Pleistocene, Cottonwood Creek, which drains to Monument Creek about 3.5 mi west of the map area, captured the small south-trending valleys in the northwestern part of the quadrangle. The valley of Cottonwood Creek and the lower reaches of the captured streams are more deeply incised than the other valleys in the quadrangle and, thus, have the best exposures of bedrock and Quaternary alluvial strata. Because upper Cottonwood Creek is deeply incised, all alluvial units there, except the youngest one, are notably higher above stream level than their equivalents in the other drainage basins in the map area.

Most deposits of Pleistocene alluvium are broad, relatively thin, and consist chiefly of coarse sand and very fine to medium pebble (about 0.1–0.6 in. in maximum dimension) gravel. In the upper reaches of the larger drainage basins, fluvial deposits are typically 7–16 ft thick and as much as a mile wide. The extent and nature of the alluvium indicate that it was deposited during times when climate was effectively much wetter than today. Braided streams appear to have spread broadly near valley heads, and then narrowed southward into well-defined valleys.

The windblown sand that mantles parts of the Falcon NW quadrangle was derived chiefly from alluvial sand on valley floors. Because moisture and vegetation greatly increase the threshold velocity required to entrain sand, it is likely that winds mobilized valley-floor alluvium during times when the alluvium was dry and relatively free of vegetation. Most eolian sand is in sheets rather than dunes and is most extensive on the eastern sides of the larger valleys because the dominant sand-moving winds were northwesterly. Conditions favorable for sustained wind erosion and deposition probably occurred many times during the Quaternary, but in the absence of deep exposures and evidence of unconformities within these deposits, only one unit of eolian sand is shown on the map.