.0 INDEX MAP OF COLORADO SHOWING PRINCIPAL SOURCES OF GEOLOGIC DATA PRINCIPAL SOURCES OF GEOLOGIC DATA Geologic data for the Colorado map were derived principally from compilations of 1 X 2 quadrangles, references to which are given below. The individual 1 X 2 quadrangle maps in turn incorporate many other maps, references to which may be found on the 1 X 2 quadrangle maps. All quadrangle maps have been generalized as necessary to fit requirements of scale, and some have been modified or amplified from sources indicated under the quadrangle headings. 1. Vernal quadrangle Tweto, Ogden, 1975, Preliminary geologic map of east half of Vernal 1 X 2 quadrangle, Colorado: U.S. Geol. Survey Open-file Rept. 75-588. Supplementary data from: Rowley, P.D., and Hansen, W.R., U.S. Geological Survey unpub. maps of parts of quadrangle. 2. Craig quadrangle Tweto, Ogden, 1976, Geologic map of the Craig 1 X 2 quadrangle, northwestern Colorado: U.S. Geol. Survey Misc. Inv. Series Map I-972. Supplementary data from: Snyder, G.L., U.S. Geol. Survey unpub. map of Steamboat Springs-Oak Creek-Rabbit Ears Peak area. 3. Greeley quadrangle Interim compilation by Ogden Tweto from: Abbott, J.T., 1976, Geologic map of the Big Narrows quadrangle, Larimer County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-1323. Beckwith, R.H., 1942, Structure of the upper Laramie River valley, Colorado-Wyoming: Geol. Soc. America Bull., v. 53, no. 10, p. 1491-1532. Birch, Alvin, Colorado State Univ., unpublished map of parts of Deadman and South Bald Mountain quadrangles, Larimer County, Colorado. Bjorklund, L.J., and Brown, R.F., 1957, Geology and ground-water resources of the lower South Platte River Valley between Hardin, Colorado, and Paxton, Nebraska, with a section on Chemical quality of the ground water, by H.A. Swenson: U.S. Geol. Survey Water-Supply Paper 1378, 431 p. Braddock, W.A., Calvert, R.H., Gawarecki, S.J., and Nutalaya, Prinya, 1970, Geologic map of the Masonville quadrangle, Larimer County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-832. Braddock, W.A., Calvert, R.H., O'Connor, J.T., and Swann, G.A., 1973, Geologic map and sections of the Horsetooth Reservoir quadrangle, Larimer County, Colorado: U.S. Geol. Survey open-file map. Braddock, W.A., Connor, J.J., Swann, G.A., and Wohlford, D.D., 1973, Geologic map and sections of the Laporte quadrangle, Larimer County, Colorado: U.S. Geol. Survey open-file map. Braddock, W.A., Nutalaya, Prinya, Gawarecki, S.J., and Curtin, G.C., 1970, Geologic map of the Drake quadrangle, Larimer County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-829. Colton, R.B., and Lowrie, R.L., 1973, Map showing mined areas of the Boulder-Weld coal field, Colorado: U.S. Geol. Survey Misc. Field Studies Map MF-513. Eggler, D.H., 1968, Virginia Dale ring-dike complex, Colorado-Wyoming: Geol. Soc. America Bull., v. 79, no. 11, p. 1545-1564. Gable, D.J., and Madole, R.F., 1976, Geologic map of the Ward quadrangle, Boulder County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-1277. Hershey, L.A., and Schneider, P.A., Jr., 1972, Geologic map of the lower Cache la Poudre River basin, north-central Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map I-687. Hunter, Z.M., 1955, Geology of the foothills of the Front Range in northern Colorado: Rocky Mtn. Assoc. Geologists, Denver, Colorado, 2 map sheets. Izett, G.A., 1974, Geologic map of the Trail Mountain quadrangle, Grand County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-1156. Lovering, T.S., and Goddard, E.N., 1950, Geology and ore deposits of the Front Range, Colorado: U.S. Geol. Survey Prof. Paper 223, 319 p. [1951]. O'Neil, J.M., Colorado Univ., unpublished map of Mount Richthofen quadrangle, Grand and Jackson Counties, Colorado. Pearson, R.C., and Speltz, C.N., 1975, Mineral resources of the Indian Peaks study area, Boulder and Grand Counties, Colorado, with a section on Interpretation of aeromagnetic data, by Gordon Johnson: U.S. Geol. Survey Open-File Rept. 75-500. Pipiringos, G.N., and O'Sullivan, R.B., 1976, Stratigraphic sections of some Triassic and Jurassic rocks, from Douglas, Wyoming, to Boulder, Colorado: U.S. Geol. Survey Oil and Gas Inv. Chart OC-69. Scott, G.R., and Cobban, W.A., 1965, Geologic and biostratigraphic map of the Pierre Shale between Jarre Creek and Loveland, Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map I-439. Smith, R.O., Schneider, P.A., Jr., and Petrie, L.R., 1964, Ground-water resources of the South Platte River basin in western Adams and south- western Weld Counties, Colorado: U.S. Geol. Survey Water-Supply Paper 1658, 132 p. Soister, P.E., 1965, Geologic map of the Fort Lupton quadrangle, Weld and Adams Counties, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-397. Soister, P.E., 1965, Geologic map of the Hudson quadrangle, Weld and Adams Counties, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-398. Soister, P.E., 1965, Geologic map of the Platteville quadrangle, Weld County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-399. Trimble, D.E., 1975, Geologic map of the Niwot quadrangle, Boulder County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-1229. Weist, W.G., Jr., 1965, Reconnaissance of the ground-water resources in parts of Larimer, Logan, Morgan, Sedgwick, and Weld Counties, Colorado, with a section on The chemical quality of the water, by Robert Brennan: U.S. Geol. Survey Water-Supply Paper 1809-L, 24 p. Wrucke, C.T., and Wilson, R.F., 1964, Geologic map of the Boulder quadrangle, Boulder County, Colorado: U.S. Geol. Survey open-file map. U.S. Geol. Survey unpub. maps by W.A. Braddock, R.B. Colton, D.J. Gable, N.M. Denson, D.M. Kinney, M.E. McCallum, R.C. Pearson, G.R. Scott, and Ogden Tweto. 4. Sterling quadrangle Scott, G.R., 1978, Geologic map of the Sterling 1 X 2 quadrangle, north- eastern Colorado: U.S. Geol. Survey Misc. Inv. Series Map I-1092 [1979]. 5. Grand Junction quadrangle Cashion, W.B., 1973, Geologic and structure map of the Grand Junction quadrangle, Colorado and Utah: U.S. Geol. Survey Misc. Geol. Inv. Map I-736.