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Fluvioglacial features of the Animas River Valley, and Charles A. Bandoian, 1968, pp. 28-32 in: San Juan, San Miguel, La Plata Region (New Mexico and Colorado), Shomaker, J. W.; [ed.], New Mexico Geological Society 19th Annual Fall Field Conference Guidebook, 212 p.

This is one of many related papers that were included in the 1968 NMGS Fall Field Conference Guidebook.

Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico.

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By CHARLES A. BANDOIAN The University of New Mexico, Albuquerque, New Mexico

INTRODUCTION In spite of the amount of glacial erosion, the The Animas River heads in the , contour of the canyon still displays at most local- a broad domal uplift approximately 90 miles from east to ities the dual nature of the gorge as represented west and 70 miles from north to south (Thornbury, 1965). The river leaves the San Juan Mountains at their southern in the cross-section between Howardsville and boundary, the large hogback ridge immediately south of Middleton [fig. 2]. There is a pronounced shoul- Durango, Colorado (fig. 1 ). The total distance from the der on the canyon's wall midway between the head of the Animas at Animas Forks to the southern modern stream channel and the higher summits boundary of the San Juan Mountains is about 50 miles, on each side. although the river flows a considerably greater distance to reach this point. The river continues nearly due south from Durango across the New Mexico-Colorado state line to Aztec, New Mexico where it swings southwest and continues on to Farmington to join the San Juan River. In the valley of the Animas River, Tertiary volcanic rocks, the most important rocks in the San Juan Moun- tains, are exposed. Volcanism began in Miocene time and continued at intervals into the Quaternary. Six distinct volcanic episodes have been recognized (Larsen and Cross, 1956). Precambrian, Paleozoic, and Mesozoic rocks are also exposed along the river. The effects of Pleistocene glaciation can be seen in the Animas Valley of the San Juan Mountains and deposits of at least five glacial advances have been identified. South of the San Juan Mountains, Tertiary sedimentary rocks are exposed along the valley and glacially derived Pleistocene sediments are abundant.

PHYSIOGRAPHIC AND GEOLOGIC SETTING The valley of the Animas River heads near Animas Forks about ten miles north-northeast of Silverton. It is the largest valley in the San Juan Mountains and during the Pleistocene was occupied by the largest glacier in the range. The Animas Glacier extended 50 miles down the valley to Animas City and Durango at various times during the Pleistocene. The glacier was fed by over 75 catchment basins and the ice was as much as 2000 feet thick (Atwood and Mather, 1932). The drainage area of the river extends to the northern margins of the mountain range. The Animas Valley inner gorge is 2000 feet deep at Animas Forks where several tributaries join to form the main channel of the Animas River (Atwood and Mather, 1932). The valley has been eroded by alternating glacial and fluvial action aided by mass-wasting which is very common in some areas. At- wood and Mather state that:

32 NEW MEXICO GEOLOGICAL SOCIETY—NINETEENTH FIELD CONFERENCE

Dickinson, R. G., 1965, Landslide origin of the type Cerro Till, Richmond, G. M., 1954, Modification of the glacial chronology of southwestern Colorado, in Geological Research 1965: U. S. Geol. the San Juan Mountains, Colorado: Science, v. 119, p. 117. Survey Prof. Paper 525-C, p. C147-C151. 1962, Three pre-Bull Lake tills in the Wind River Moun- Fenneman, N. M., 1931, Physiography of the western United States: tains, Wyoming: in U. S. Geol. Survey Prof. Paper 450-D, p. New York, McGraw-Hill, 534 p. D132-D136. Holmes, G. W., and Moss, J. H., 1955, Pleistocene geology of the ______1964, Three pre-Bull Lake tills in the Wind River Moun- southwestern Wind River Mountains, Wyoming: Geol. Soc. tains, Wyoming—a reinterpretation, in Geological Survey Re- America Bull., v. 66, p. 629-654. search 1964: U. S. Geol. Survey Prof. Paper 501-D, p. D104- Larsen, E. S., Jr., and Cross, Whitman, 1956, Geology and petrology D-109. of the San Juan region, southwestern Colorado: U. S. Geol. Sur- ______, 1965, Quaternary stratigraphy of the Durango area, San vey Prof. Paper 258, 303 p. Juan Mountains, Colorado, in Geological Survey Research 1965: Moss, J. 11., 1951, Early man in the Eden Valley, Wyoming: Penn- U. S. Geol. Survey Prof. Paper 525-C, p. C137-C143. sylvania Univ. Mus. Mon. 6, p. 9-92. Thornbury, William D., 1965, Regional Geomorphology of the Richmond, G. M., 1948, Modification of Blackwelder's sequence of United States: New York, John Wiley and Sons, p. 346-347. Pleistocene glaciation in the Wind River Mountains, Wyoming [abs.]: Geol. Soc. America Bull., v. 59, p. 1400-1401.