SOIL SURVEY OF THE EED BLUFF AREA, CALIFORNIA, 



51 



The following table gives the average results of mechanical 

 analyses of the soil and subsoil of this type : 



Mechanical analyses of Vina clay loam. 



VINA CL.VY ADOBE. 



The Vina clay adobe is a very dark-gray or black, heavy textured 

 clay of pronounced adobe structure. It is extremely hard and com- 

 pact when diy and cracks into the typical rough cubes. When wet 

 it is almost impassable because of its sticky character. Altogether 

 it is a refractory, poorly structured soil, although carrying quantities 

 of small volcanic gravel throughout its principal bodies. It has 

 many small drainage ways down its slopes, sometimes defined as 

 water channels, but usually occurring as ill-defined meandering de- 

 pressions without abrupt banks. These depressions contain great 

 quantities of roughly shaped volcanic bowlders from 3 to 8 inches 

 in diameter and give the surface a netted appearance. This gravelly 

 adobe material usually does not have a depth of more than 20 to 48 

 inches where it grades sharply into the indurated mass of gravel and 

 volcanic tuff which underlies much of the east side of the valley. 



Boundaries of the areas of the Vina clay adobe are rather indis- 

 tinct and transitions rather than sharp lines characterize its contact 

 with other types. It includes fi-equent small bodies of Tuscan soils. 

 There are a few minor areas of the type occurring below the terrace 

 and associated wdth the alluvial plains soils and here a greater depth 

 of soil, absence of the surface accumulations of bowlders, and a flatter 

 surface prevail. This latter phase, which is indicated upon the map 

 by ruling, covers several poorly drained depressions occurring wdthin 

 the Vina cla}^ loam. The main bodies extend irregularly down the 

 slopes within bodies of Tuscan stony sandy loam, being associated 

 with that type in position and elevation, and so properly classed as 

 an upland soil. 



Its general surface is that of a sloping plain of good average 

 drainage, except along its lower borders wdiere the fall decreases on 

 contact with lower-lying types. The greatest variation in surface 

 condition is caused by the parallel or winding stone-laden waterways. 



In origin the type is probably coUuvial and alluvial. It is formed 

 mainly of material l)rought from beyond the eastern boundary of the 

 survey by intermittent streams. 



