SOILS OF THE SAN LUIS VALLEY, COLORADO. 



LOCATION AND BOUNDARIES. 



The San Luis Valley lies principally in south-central Colorado. 

 Although sometimes designated as San Luis Park, it is not to be 

 confused in physiographic features and geologic history with the in- 

 termountain parks forming extensive land features in other parts of 

 the State. 



An extension of the valley continues for some distance southward 

 into New Mexico. This portion is separated from the more northern 

 part, which lies entirely within the State of Colorado and which is 

 the subject of this report, by a series of flat-topped, lava-capped 

 mesas or tablelands of moderate elevation as compared with regional 

 mountain features, known as the San Luis Hills and extending in a 

 northeasterly direction from a point near the Colorado-New Mexico 

 boundary line. The valley extends in a north-and-south direction 

 for a distance of about 80 miles and has a maximum width of 45 

 miles. It reaches an elevation of slightly more than 7,500 feet above 

 sea level and is one of the most extensively cultivated irrigated dis- 

 tricts of its altitude in the United States. 



It is inclosed on the east by the bold and serrated Sangre de Cristo 

 Mountains of abrupt slope, culminating in the peak of Sierra Blanca, 

 reaching an altitude of more than 14,000 feet. Upon the west it is 

 bounded by the Saguache and Conejos Ranges, of less abrupt char- 

 acter. These mountains gradually approach each other toward the 

 north and unite at Poncho Pass, at the head of the valley. The 

 valley is drained by the Rio Grande del Norte, which enters from 

 the west at Del Norte and crosses the southwestern portion. A num- 

 ber of minor tributary streams debouching from the mountains find 

 their way into the valley. Few of these are of perennial flow, but 

 lose their waters by seepage through underlying sands and gravels. 

 Of these, the more important having their source in the west side 

 ranges, enumerated in order of occurrence from south to north, are 

 the Conejos River, and Alamosa, Piedra Pintada, La Jarita, and- 

 Saguache Creeks. The last mentioned constitutes the most important 

 stream of the northern part of the valley. Of the numerous streams 

 entering the valley from the east only a few possess any degree of 



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