22 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1910. 



more shallow. Rarely is it ever over G inches thick and in all but 

 very limited bodies underlain by a hardpan. This is somewhat 

 thicker and harder than that underlying a portion of the Redding 

 gravelly sandy loam. The line of contact between the layer of heavy 

 subsoil and the great bodies of underlying impervious beds is quite 

 clear, but that portion defined as a true hardpan occurs as the harder 

 portion directly in contact with the subsoil. It is co'mposed largely 

 of clay particles firmly cemented into an imj^enetrable layer carrying 

 little gi'avel. This true hardpan is a secondary development, occur- 

 ring after the entire mass was deposited. It is explained as the 

 result of the downward movement of chi}^ particles accumulating in 

 the presence of iron compounds as layers on the surfaces of the 

 underlying material. The true hardpan, averaging about 6 inches 

 in thickness, well defined on its upper surface, can be seen to lose 

 its clay structure gradually on its lower surface. It then becomes 

 more gravelly and so grades rather indefinitely into the ever-present 

 masses of gravel, coarse sand, and silt. These latter materials occur 

 either as mixtures of varjang proportions or in roughly assorted 

 beds, all sufficiently cemented to be practically impervious to plant 

 roots even when the overlying hardpan is shattered. It will be noted 

 that the total depth available to plant roots, consisting of the loam 

 with gravel and the small layer of heavy subsoil, is seldom more 

 than 24 inches. 



This type is comparatively extensive in the west-central portion 

 of the area and is practically confined to the west side of the valley. 

 At the time when the valley trough had a higher general elevation 

 than at present the material of which this type is composed ex- 

 tended as a vast plain over the section of the Red Bluff area west of 

 the Sacramento River. Present remnants of this old plain are to- 

 day in evidence in the sections west of the Red Bluff' area and south 

 of it in the Colusa area. Partial erosion of this ancient material, 

 with deposition of transported particles to form soil types of a lower 

 level, has left portions now occurring as ridges or elevated rolling 

 jihiins. These are separated by the minor drainage ways of the 

 valley slopes and mapped as Redding loam. 



Its general surface, while rolling and marked by drainage depres- 

 sions, has detailed features of topography making it, during the 

 rainy season, a poorly-drained soil. Numerous hummocks or " hog 

 wallows " confine much surface water or permit it to escape slowly 

 by meandering courses, little water escaping through the subsurface 

 layers. The type is usually bounded by terrace lines or rather 

 iibrupt slopes. Along the courses of present streams these small 

 bhifi's are evidence (if active erosion. A change in the stream course 

 which allows the liliilT to wealhei- into a slope accounts for most of 

 the sharj) lines -eparat iiig lliis type fiom the Tehama, Elder, and 



