4 Heitmeyer and Aloia THE HISTORICAL MONTE VISTA ECOSYSTEM
GeoLoGy aNd GeoMoRPhoLoGy Monte Vista NWR. The entry of the Rio Grande into The SLV is the largest of a series of high-altitude, the SLV created a large, low elevation, alluvial fan intermontane basins located in the Southern Rocky that extends south of Monte Vista, Colorado (Fig. 2). Mountains (Jodry and Stanford 1996). The SLV is This fan is characterized as Quaternary-age younger part of the much larger Rio Grande Rift Zone that alluvium with surficial deposits (Fig. 5) that overlie extends from southern New Mexico north through older Pleistocene Alamosa Formation coalescing the SLV to its northern terminus near Leadville, alluvial fans and moderately well-sorted fluvial Colorado (Chapin 1971, Bachman and Mehnart deposits near the valley margins that adjoin the 1978). The SLV Basin is a compound graben San Juan Mountains (Fig. 3). Drainages including depression that was down-faulted along the base Rock, Spring, and Cat Creeks that originate of the Sangre de Cristo Mountains, which resulted from the San Juan Mountains historically flowed from extensive block faulting during the Laramide across Monte Vista NWR and deposited erosional Orogeny. The western side of the SLV, close to Monte sediments throughout their narrow floodplains (Fig. Vista NWR, is bounded by the San Juan Mountains, 6). A small alluvial fan created by the entry of Rock which was created by extensive Tertiary volcanism Creek onto the larger Rio Grande alluvial fan covers about 22 to 28 million years before the present (BP) the western boundary of the Monte Vista NWR (Fig. (McCalpin 1996). The Oligocene volcanic rocks of 2). Another alluvial fan along the Alamosa River the San Juan Mountains slope gradually to the SLV is present immediately to the south of Monte Vista floor where they are interbedded with alluvial-fill NWR (MWH et al. 2005). deposits (BLM 1991). From the Pliocene to middle Pleistocene time, a large, high altitude lake, Lake Alamosa, occupied SoiLS most of the SLV (Machette et al. 2007). This ancient lake accumulated sediments that are designated About 30 distinct soil types are present on as the Alamosa Formation (Siebenthal 1906, 1910). Monte Vista (Fig. 7). The distribution of soil series Lake Alamosa existed for about three million years on Monte Vista NWR reflects the three major before it overtopped a low wall of Oligocene volcanic landforms of the region: the San Juan Mountain rocks in the San Luis Hills and carved a deep gorge foothills, the large Rio Grande alluvial fan, and that flowed south into the Rio Grande, entering at Spring, Rock, and Cat Creeks and associated flood- what is now the mouth of the Red River. Monte Vista plains. Soils generally are dominated by loamy NWR apparently was never within the ancient Lake sands, which cover much of the former salt desert Alamosa basin proper, but was near its western edge shrub areas present on the Rio Grande alluvial fan (Fig. 3). Santa Fe Pliocene and Miocene formations (U.S. Department of Agriculture Soil Conservation underlie the Alamosa Formation, which is in turn Service) (SCS 1980). Some heavy loam and clay loam underlain by Echo Park alluvium and then Precam- soils are present on Monte Vista NWR and indicate brian rocks (Fig. 4). the presence of former wetland areas (SCS 1980). The Rio Grande enters the SLV near Del Norte, Cobbled and gravelly loams are present along relict Colorado and flows to the southeast just northeast of stream courses and terrace edges. 5 6 Heitmeyer and Aloia
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