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".~::. ,:"," ::,.,.~. .~": :,'" Geolo~ and Soils (;) o w C.? Soil genesis and the quality of groundwater and surface water in the lower basin are influenced primarily by the underlying Mancos Shale formation, a massive, salt-bearing bed of strictly deep water marine shale containing but minor quantities of limestone and sandstone in its lowermost portions. The effects of Mancos Shale on the salinity of the surface and groundwaters of the San Juan River Valley are manyfold. The San Juan River, its tributaries, and irrigation delivery and return systems all flow over terrain composed of rocks of the Mancos Shale or of soils derived from that type of rock. Soluble salts in the rocks and soils are removed by surface water or are transported into the groundwater by deep percolating water. Also, the groundwater of the lower basin occurs primarily in the Mancos Shale and is more saline than that of the upper basin. The Mancos Shale forms the uppermost geologic unit in the rangelands of the watershed. Soils derived from the shsle in this region are thin, sparsely vegetated, and erosive. When eroded, the soils are either transported to the flat-lying areas of the watershed or flood plain and deposited, or are carried into the river or its tributaries. Thus the salts associated with the soils are "::,.. ';~":;~': delivered to the surface waters. Soils in the cultivated areas are of alluvial origin, and are deep and fertile with potential rooting depths of at least 60 inches. Internal drainage ranges from somewhat poorly in wet soils subject to periodic flooding or seasonally high water tables to somewhat excessively in the droughty loamy sands and sandy loams. The poorly drained soils are more likely, although not necessarily, to have salinity or sodicity hazards when irrigated and cropped than the well drained soils. The primary hazard to irrigation agriculture in the non-wet soils is their tendency to be eroded by wind. 10