14 FIELD OPERATIONS OF THE BUEEAU OF SOILS, 1916. 



mulation of a great deal of organic matter in the soil, notwithstand- 

 ing the fact that its native vegetation consists of grasses. The rain- 

 fall is too low also to allow the leaching of the whole soil layer. 

 Thus the carbonates are leached from the surface layer but are found 

 in abundance in the subsoil. On the other hand, the county lies 

 where the moisture supply is too great to permit the growth of a 

 desert shrub vegetation and the development of the gray soils rich in 

 carbonates characteristic of such regions. In development the well- 

 drained soil on the ujjland of the county, regardless of its texture, 

 has reached a stage of maturity, . and because of uniformity both of 

 climatic forces and of parent material it is uniform in its charac- 

 teristics. It belongs to the important group of chestnut-colored 

 soils marked by their brown surface color, their lighter colored and 

 rather compact subsurface, and their highly calcareous subsoil. To 

 jJl intents and purposes the upland of the county is covered with a 

 single soil with minor variations due to the fineness or coarseness of 

 the material. The Sidney loam and Sidney silt loam are the varie- 

 ties, or, as they are designated in the nomenclature of the Bureau of 

 Soils, the types that represent the most complete stage of de- 

 velopment. 



The terrace soils of the county, represented typically by the Tripp 

 series, have reached practically the same stage in soil development 

 as the upland soil, while the recent alluvium has just begun its de- 

 velopment. Its profile is not determined by the soil-making forces 

 characteristic of the region but by the geological processes operating 

 during its deposition. It is a geological formation in its stage of 

 development and is not a soil in any other sense than that it is ca- 

 pable of supporting a growth of plants. 



The soils along the steep slopes are in a sense nothing but the dis- 

 integrated debris of the geological formations, and have not yet 

 reached the mature stage of soil development. 



The soils have been derived almost entirely from a single geologic 

 formation. In this area this consists of a great thickness of sand, 

 gravel, and silt containing a high percentage of lime, in the form 

 of very finely divided calcium carbonate disseminated throughout 

 the material and in coarse fragments of calcareous rocks. 



The beds have generally been cemented through the action of cal- 

 cium carbonate, forming a white calcareous grit and conglomerate 

 of varying degrees of hardness, to which the name " mortar beds " 

 has been applied. The formation is heterogeneous, containing frag- 

 ments of practically all the great variety of igneous and sedimentary 

 rocks of the Rocky Mountain region to the west. Quartz, feldspar, 

 and pebbles of granite probably predominate. 



The formation is of late Tertiary age (Pliocene) and probably of 

 fimial and lacustrine origin. In the general geologic mapping 



