The primary uses of sand and gravel is for concrete aggregates, road base and coverings, road stabilization, construction fill, asphaltic concrete and other bituminous mixtures, and other concrete products. Other uses include plaster and gunite sands, snow and ice control, filtration, railroad ballast, and roofing granules. Crushed stone is primarily used for construction material especially in road construction/maintenance and cement manufacturing. For a description of the geology of sand, gravel, and quarry aggregates, the CGS published ON-007-05 Colorado Aggregate Resources that shows the locations, geology, and descriptions of these natural deposits.
Colorado uses a large amount of aggregate to build and maintain infrastructure. Colorado quarry operators produced 51.81 million tons of aggregate (sand, gravel, and crushed stone) in 2018. The estimated 2018 production value was $284 million for sand and gravel and $141 million for crushed stone (see IS-82 for more information). The cost of aggregate to the user is highly dependent on aggregate transportation costs. Locating quarries close to population centers helps lower overall costs. However, residential and commercial development near an aggregate source can make permitting a new or expanding quarry a challenge.
Special Publication 5A and 5B, Sand Gravel and Quarry Aggregate Resources, Colorado Front Range Counties will assist local Front Range governments identify potential sources of sand, gravel and quarry aggregates. Digital versions of the aggregate resource maps may be found in OF-00-09 Atlas of Sand, Gravel, and Quarry Aggregate Resources, Colorado Front Range.
Sand, Gravel, and Quarry Aggregates
Naturally occurring aggregates, such as sand, gravel, and crushed stone are produced in Colorado. Aggregates are used in several applications including construction fill, concrete, asphaltic mix/concrete, road base, other concrete and asphalt products. Other uses of aggregate include rip-rap, plaster and gunite sands, snow and ice control, filtration systems, railroad ballast, hydraulic fracturing, and roofing granules. Aggregates are generally defined as granular materials that are: 1) used with a cementing medium to form concrete or mortar or 2) used as a construction material, meeting the requirements of road, paving, or other construction applications (modified from ASTM C125-18 and D8-18, 2018).
Sand, gravel, and crushed rock are all quarried for use as aggregate in the state. Crushed rock can include igneous, sedimentary, and metamorphic rocks. This natural rock material must be blasted and crushed to the desired aggregate size. Igneous rocks form from molten rock or magma and include intrusive rocks (magmas that cooled at depth) and extrusive rocks (rocks produced by eruptions onto the earth’s surface). Igneous rocks that are typically quarried for aggregate include granite and basalt.
Sedimentary rocks are formed by deposits of eroded igneous and metamorphic rocks as well as material produced by living organisms. Limestone, sandstone, and other sedimentary rock types are mined in Colorado for aggregate. Limestone is primarily mined and crushed for use as concrete aggregate while well-indurated sandstones (difficult to break with a hammer) can also be a good source for crushed stone.
Metamorphic rocks include rocks that were subsequently transformed after being exposed to heat and pressure. Metamorphic rock types that are typically mined for aggregate include gneiss and schist. Several quarries in the Front Range mine gneiss and provide crushed rock to the Denver metropolitan area.
Sand and gravel deposits are typically unconsolidated or weakly cemented sedimentary deposits formed by several different natural processes including:
- fluvial (streams, also known as alluvial deposits)
- lacustrine (lakes)
- marine (oceans)
- glacial (glaciers)
- eolian (wind) processes.
Most sand and gravel quarried in the state occurs within the floodplain and underlies low terraces of modern streams that may contain less weathered aggregates. Terraces are remnants of former floodplains that formed when a stream was at a higher elevation prior to down cutting and forming a new lower floodplain. Other deposits include upland gravels and alluvial fans which are fan-shaped wedges of sediment that form where a stream emerges onto an open plain from an area of higher elevation. Upland gravel deposits are associated with ancient stream courses and may or may not occur near modern streams and their tributaries. Both upland gravels and alluvial fans can be quarried for aggregate but upland gravel deposits typically are more weathered and may not be suitable for specific uses. Eolian sand deposits cover a large portion of eastern Colorado and are dominantly comprised of sand and silt. Glacial sand and gravel deposits occur in former glaciated areas at higher elevations in the Rocky Mountains.
Many factors determine what constitutes a viable sand, gravel, or crushed rock aggregate resource including:
- material composition, size, size distribution, color, shape, texture, and weight
- vertical and horizontal extent of the deposit
- overburden thickness
- material strength, durability, soundness, porosity, water adsorption, reactivity, specific gravity
- the presence of fines, sulfates, calcium carbonate, and other minerals
- environmental and mine permitting
- community relationships
- local supply and demand
- distance and transportation to the source
- available land for lease/purchase.
Although a natural aggregate deposit may meet many of these factors, developing these resources can be limited by issues associated with one or more of these elements. Also, one sand and gravel deposit may meet construction specifications for one type of application but may not meet the requirements for another. For example, sandstones and other sedimentary rock types may not meet engineering specifications for use in asphalt and concrete but could be used for road base or fill.
Colorado uses a large amount of aggregate to build and maintain needed infrastructure. The cost of aggregate to the end user is highly dependent on the cost of transporting aggregate. Locating quarries close to population centers helps lower overall costs. However, residential and commercial development near an aggregate source can make permitting a new or expanding mine a challenge.
The demand for aggregate is regional but land-use planning and permitting for aggregate supplies is done on a local level. Balancing the need for readily available and reasonably priced aggregate with other regional, social, and environmental priorities is not an easy task for local governments. A 1973 state law recognized the need to balance competing interests in urban areas and counties with a population of least 65,000 residents, which must now develop a master plan for the extraction of industrial mineral deposits. The intent of this state law is to allow for the extraction of aggregates—as well as other commercial mineral deposits—while protecting the environment and public safety.
To help local Front Range governments identify potential sources of sand, gravel and quarry aggregates, the CGS created “Special Publication 5A and 5B, Sand Gravel and Quarry Aggregate Resources, Colorado Front Range Counties”. Digital versions—GIS datasets—of the aggregate resource maps are available in OF-00-09 Atlas of Sand, Gravel, and Quarry Aggregate Resources, Colorado Front Range. The contents of these reports are also available as an online map.
Projected population growth in other counties throughout Colorado have prompted local governments in these areas to create mineral extraction plans in conjunction with future land use planning. Mineral extraction plans may prevent quality aggregate resource areas from being lost to other land uses.
The following are GIS maps showing specific state-wide industrial minerals resources:
ON-007-01 — Aggregate Resources of Colorado — Naturally occurring aggregates, such as sand, gravel, and crushed stone are quarried throughout Colorado. This interactive map includes the digital versions of our sand, gravel, and quarry aggregate publications for most of the Front Range counties, Garfield County, and includes a statewide map of older quarry locations. This map shows the locations and ratings of these potential resources to assist with land use planning and resource development in the Front Range and beyond.
ON-007-03 — Mineral Resource Potential Derivative Map — The CGS compiled mineral resource derivative maps using 7.5-minute quadrangles published under the current CGS STATEMAP geologic mapping program. These maps show the general location and mineral potential rating of select aggregate and industrial mineral deposits by geological unit as mapped during the STATEMAP program. The maps are created from these geological maps and historic mining/quarry information to provide a general rating as to their potential for containing select mineral resources. Potential mineral resources include sand and gravel, decomposed granite, crushed stone, clay/claystone/shale, fluorspar, gypsum, limestone and dolomite, dimension stone, and others. Derivative maps are generalizations of detailed geological information that are used to assist non-geologists with evaluating complex geological information.
ON-007-05 — StoryMap: Colorado Aggregate Resources – Geology and Industry Overview — An interactive website that integrates maps, text, and photos about the location and geology of sand, gravel, and quarry aggregates in the state. Includes a description of aggregates, what they are used for, where they are found, how and why the location of these deposits are important to land-use planning, and the general geology of sand, gravel, and rock quarry aggregates.
ON-007-07 — Reconnaissance of Potential Sand Sources in Colorado for Hydraulic Fracturing — An online map of the data provided on the two plates included in this report: RS-47 Reconnaissance of Potential Sand Sources in Colorado for Hydraulic Fracturing. This online map includes over 800 sample locations across the state, photos of each sample location, photomicrographs of samples, geologic descriptions, estimates of rounding and sphericity, estimates of quartz content and friability, mineralogy, sieve results, and resource ratings.
See the Reference tab for both CGS and other third-party publications on industrial minerals. Recently, the CGS worked with the USGS on a comprehensive publication Development of Industrial Minerals in Colorado.
Colorado Geological Survey. “RockTalk V05N3, July 2002 – Industrial Minerals in Colorado.” RockTalk, July 2002.
Colorado Stone, Sand and Gravel Association — Information about the state’s aggregate mining business.
National Stone, Sand and Gravel Association — Nation-wide information about the aggregate mining business.