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'.'.-.' .- ,-~", '," .', ':~";' ~:':~.-, <, I'V <Xl ...., N :,..;5'~~;.,:c.:<t 1--;--:,:.' ;' _~..<)-:' .-<:.::'" - 3 - Although only the high, moderate and low sediment yield levels are shown on the attached table, interpolation between these levels may be made. Surface GeolOllY Over much of the southwest area, the effect of surface geology on erosion is readily apparent. The' weaker and softer rocks are more easily eroded and generally yield more sediment than do the harder more resistant types. Sandstones and similar coarse-textured rocks that disintegrate to form per- m~able soils erode 'less than shales and related mudstones and siltstones under the same conditions of precipitation. On the other hand, because of the absence of cementing agents in 'some soils derived from sandstone, large storms may produce some of the highest sediment yields known. The widely distributed marine shales, such as the ~lanccs and shale members of the Moenkopi Formation, constitute a group of highly erodible formations. The very large areal extent of the shales and their outwash deposits 'gives, them a rank of special im~ortance in relation to erosion. Few.of the shale areas are free from erosion. Occasionally, because of slope or cover con- ditions, metamorphic rocks and highly fractured and deeply weathered granites and granodiorites produce high sediment yield. Limestone and volcanic out- crop areas are among the most stable found within the western lands. The principal reason for this appears to be the excellent infiltration character- istics, which allow most precipitation to percolate into the underlying rock. In some areas all geologic formations are covered with alluvial or colluvial material which may have no relation to the underlying geology. In such areas the geologic factor would have no influence and should be assigned a value of o in the rating. Soils Soil formation in the Pacific Southwest generally has not had climatic con- ditions conducive to rapid development. Therefore, the soils are'in an immature stage of development and consist essentially of physically weathered rock materials. The presence of sodium carbonate (black alkali) in a soil tends to cause the soil particles to disperse and renders such a soil sus- ceptible to erosion, There are essentially three inorganic properties--sand, silt, and clay-- which may in any combination give soil its physical characteristics. Organic substances plus clay provide the binding material which tends to hold the soil separates together and form aggregates. Aggregate fonnation and stability of these aggregates are the resistant properties of soil against erosion. Un- stable aggregates or single grain soil materials can be very erodible.