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<br />Related
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<br />Bulletin 36
<br />Geologic Control of Supply and Quality
<br />of Water in the Mountainous Part of
<br />Jefferson County, Colorado $2.00
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<br />Bulletin 42
<br />Water Resources of Boulder Countyl
<br />Colorado $4.00
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<br />Information Series 37
<br />Water Resources Beneath State Lands
<br />in Port of T. 16 S., R. 63 W., Block
<br />Squirrel Creek Basinl EI Paso County,
<br />Colorado $10.00
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<br />Informatian Series 48
<br />C%rado Water Quo/ity Database
<br />from the Environmental Protection
<br />Agency's STOREr Database $20.00
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<br />publications continued on p. 14
<br />
<br />6
<br />
<br />-
<br />
<br />listed below, are illustrated on the
<br />map on page 4: South Platte,
<br />Republican, Arkansas, Colorado,
<br />Yampa, While, Gunnison, San
<br />Juan, Dolores, and Rio Grande.
<br />Sedimentary rock aquifers are
<br />composed of consolidated clastic
<br />and carbonate deposits. Ground
<br />water in sedimentary rock aquifers
<br />can be either confined or uncon-
<br />fined. The major sedimentary rock
<br />aquifers in Colorado consist pre-
<br />dominantly of sandstones and
<br />limestones of varying age. Many
<br />of these aquifers are located in
<br />structural basins that contain mul-
<br />tiple geologic units/aquifers.
<br />Large quantities of ground water
<br />occur in deep basins such as the
<br />Denver Basin and in the flat-lying
<br />High Plains Aquifer of eastern
<br />Colorado. Basin-wide aquifer sys-
<br />tems, illustrated on the map on
<br />page 5, include the: Denver, Pic-
<br />eance, Paradox, San Juan, Eagle,
<br />Raton, and Sand Wash Basins; and
<br />North and Middle Park, South
<br />Park, and H uerfano Park.
<br />The intermontane valleys of
<br />central Colorado contain a net-
<br />work of hydraulically intercon-
<br />nected aquifers within valley-fill
<br />deposits. These unconsolidated to
<br />
<br />RockTalk
<br />is published by the
<br />Colorado Geological Survey
<br />1313 Sherman Street,
<br />Room 715, Denver, CO 80203
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<br />can be obtained FREE by contact-
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<br />
<br />THIS ISSUE
<br />Editor: Matt Sares
<br />Production: Cheryl Brchon
<br />Illustrations: larry Scott
<br />
<br />-
<br />
<br />poorly-consolidated aquifers con-
<br />sist of sediments that were
<br />deposited by wind, water, and
<br />gravity, such as landslides from
<br />erosion of the surrounding moun-
<br />tain ranges. Similarly, the Great
<br />Plains of eastern Colorado are
<br />underlain by a thick sequence of
<br />gravel, sand, silt, and clay that
<br />was eroded from the Rocky Moun-
<br />tains. These poorly-consolidated,
<br />often localized, sedimentary
<br />aquifers include the San Luis Val-
<br />ley, Wet Mountain Valley, and
<br />High Plains Aquifer.
<br />Lastly, Colorado's crystalline
<br />rocks are exposed at the surface in
<br />the west-central portion of the
<br />state (see map on page 4). The
<br />crystalline rocks throughout this
<br />province are Precambrian-aged
<br />igneous and metamorphic rocks;
<br />largely granites, gneisses, and
<br />schists; and geologically recent
<br />(Tertiary age) volcanic and igneous
<br />intrusive rocks. Ground water in
<br />crystalline-rock aquifers is general-
<br />ly unconfined, and occurs where
<br />joints, fractures, and faults have
<br />crosscut the rock. These rock types
<br />occupy approximately 19 percent
<br />of the state's total area, and repre-
<br />sent the fractured, crystalline-rock
<br />aquifers that supply much of the
<br />domestic water-supply needs in
<br />the mountainous portion of our
<br />state.
<br />As Colorado's population
<br />grows, the importance and use of
<br />ground water also grows. The
<br />CGS is in the process of compiling
<br />a Ground Water Atlas of Colorado
<br />tha t will describe each of these
<br />major aquifers or aquifer systems,
<br />addressing individual hydrogeo-
<br />logic units, their hydraulic charac-
<br />teristics, principal water uses and
<br />withdrawals, and a brief look at
<br />water quality. When complete in
<br />early 2003, this atlas will provide
<br />important information useful to
<br />farmers, ranchers, homeowners,
<br />businesses, and decision-makers
<br />in Colorado.
<br />
<br />~
<br />
<br />!'
<br />
<br />p
<br />
<br />-Ralf Topper
<br />
<br />Colorado Geological Survey ROCKTALKVol. 5, No.4
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