Irrigated Agriculture in the San Luis Valley 5 
The mean annual temperature is 42 degrees, the coldest month 
being January, with a mean temperature of t 8 degrees, and the 
warmest month July, with a mean temperature of 64 degrees. The 
minimum temperature recorded was 34 degrees below zero, and the 
maximum 98 degrees above. Frosts are liable to occur any mionth 
of the year. Owing to the high altitude and rare air, these ex¬ 
tremes of temperature are not keenly felt. 
The most disagreeable feature of the climate of the Valley is 
the high wind, the general direction of which is toward the north¬ 
east. In some of the lightest soils the wind does some damage to 
newly planted crops, since it blows steadily for days at a time. 
SOILS. 
In appearance the San Luis Valley is a vast plain. To the 
eye it appears to be level and devoid of hills and irregular slopes. 
Actually, it is the basin-shaped bottom of an ancient lake, which 
at present appears as a treeless plain surrounded by high and rugged 
mountains. The surface slopes are unusually uniform and gradual. 
Close to the eastern edge, which is the low portion of the Valley, 
or Valley trough, the slopes are so slight that drainage is very poor, 
causing considerable areas of seeped land, some swampy portions, 
and shallow basins filled with alkali lakes. 
•A large proportion of the soils of the Valley is sand, sandy 
loams, and gravels, underlaid at from two to four feet of the sur¬ 
face with gravel more or less porous in nature. The soil materials 
were deposited in large part at the geologic time when the San 
Luis Valley was occupied by a lake. These materials were carried 
from the slopes of the adjacent mountains by the torrential streams 
which flowed into the ancient lake. The so-called recent soils have 
been deposited by the streams since the drainage of the lake,^ which 
occurred when the canon of the Rio Grande was cut thru in 
past geologic time. This ancient soil material was laid down in 
alternating layers of clay, sand and gravel. In many places at 
the present time the surface has been worked over and new ma¬ 
terial deposited by the streams. Some of this material is known 
as adobe, being of very fine texture and extremely difficult to work. 
These adobe or clay soils are classed as “heavy.”^ As a total 
area they are comparatively unimportant in the Valley acreage, 
since they are usually confined to the bottoms and flood plains of 
the larger streams. Away from the streams, towards the slopes in 
the foothills, the soils are a little lig'hter in texture. Most of the 
heavy soils are comparatively recent stream deposits—that is, the}^ 
have been deposited by present streams since the drainage of the 
lake which once occupied the present San Luis Valley. 
