THE LAS ANIMAS GLACIER. 



One of the largest of the extinct glaciers of the Rocky 

 Mountains was that which occupied the valley of the Las Ani- 

 mas river. This stream originates in the San Juan mountains 

 in southwestern Colorado, and flows nearly south to its junction 

 with the San Juan river in New Mexico. The San Juan moun- 

 tains, with their outlying spur, the La Platas, are the first high 

 mountains encountered by the moist winds from the direction of 

 the Gulf of California on their way northeastward ; and although 

 so far south, this region has perhaps the heaviest snow fall in 

 Colorado, as Fremont found to his cost. His expedition up the Rio 

 Grande attempted to penetrate the snowiest part of the mountains. 



Silverton -is situated about fifteen miles from the head of the 

 valley, and Durango about sixty. About one mile north of Du- 

 rango, near Animas City, two well defined morainal ridges extend 

 across the valley of the Las Animas, and from thence a plain or 

 series of terraces of water -washed morainal matter extends for 

 several miles down the river. I have not explored far below 

 Durango, and do not know the extreme limit of the ice. At 

 Durango the ice rose to about the same height as the mesa lying 

 east of the city, on which is the reservoir of the water -works, 300 

 or more feet above the valley terrace. This is proved by the fact 

 that a thin sheet of morainal matter covers the slopes of the bluff 

 and extends back for a short distance on top of the mesa (up to 100 

 feet); whereas, beyond that the top of the mesa is a base level 

 of erosion in the sedimentary rock, with none of the far -traveled 

 bowlders that abound in the moraine stuff. The glaciated 

 bowlders are largely composed of rocks found only near the 

 head of the valley, such as volcanic rocks, Archean schists and 

 granites, Paleozoic quartzites, etc. Most of these must have 

 traveled thirty to sixty miles. 



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