or "basin rim" surrounding the "trough" of the valley floor. 

 Soils which occupy these different physiographic positions 

 have developed certain cheiracteristics which vary, depending 

 upon the Influence of one or more of the soil -forming 

 factors. These factors aj?e climate, organic matter, topo- 

 graphy, and time. 



The present valley floor has been built up with recent 

 Quaternary stream deposits or with sediments of fluctuating 

 Pliocene lakes. These deposits were composed of eroded 

 materials transported from the drainage areas of the 

 surrounding mountains. 



Because of the arid conditions on the west side, the composi- 

 tion of parent material prevails as the most important single 

 factor in the process of soil genesis. Soils that occupy 

 recent alluvial, older alluvial, and basin rim positions 

 have been derived from rocks of the Diablo Range. This 

 mountain range is composed of a series of calcareous and 

 gypsiferous sandstones, shales, and conglomerates of the 

 Cretaceous and earliest Tertiary (Eocene) periods. A minor 

 influent from relatively small areas of metamorphic rocks of 

 the Franciscan formation (Jurassic) is expressed in the red 

 coloring and basic tendencies of some of the alluvial soils. 

 The basin soils reflect the genetic influence of mixed acid- 

 igneous parent materials of the Sierra Nevada. 



Description of Soils 



Within the area of investigation, several distinctly differ- 

 ent soil series are associated with tile drain systems. A 

 soil series is a group of soils having horizons similar in 

 differentiating characteristics and arrangement in the soil 

 profile, except for texture of the surface portion, or, if 

 genetic horizons are thin or absent, a group of soils that, 

 within defined depth limits, is uniform in all soil charac- 

 teristics diagnostic for series (7). Variations of surface 

 texture give rise to a "soil type", the latter being a 

 combination of the soil series neune and the soil surface 

 texture, for example, "Panoche fine sandy loam". Different 

 soil series may develop from the same parent material 

 depending on the degree of influence of one or more soil- 

 forming factors. Soils occupying different physiographic 

 positions may have formed under similar environmental condi- 

 tions except for parent material, which can significantly 

 affect the chemical and, in many cases, the physical charac- 

 teristics of a soil . 



Generalized descriptions condensed f rom U . S. Department of 

 Agriculture soil surveys are given for individual soil 

 series in this report. Soil characteristics which could 



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