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w o tv o daytime and around 15 degrees C (low 60's degrees F) at night. Relative humidity is usually low during the growing season, which is common in all of the semi-arid Colorado River Basin. The average annual relative humidity is 58.8 percent. The prevailing wind direction is east-southeast with an average velocity of about 13.4 kilometers per hour (8.3 mph). GEOLOGY The plateaus and mountains in the Colorado River Basin are the products of a series of land masses deeply eroded by wind and water. However, long before the earth movements which created the uplifted land masses, the region was the scene of alternate encroachment and retreat of great inland seas. The sediment rock formations underlying large portions of the basin are the result of material accumulated at the bottom of these seas. In the Grand Valley, the primary geologic formation is the Mancos Shale. Mancos Shale is a very thick sequence of drab, gray, fissile, late Cretaceous marine shale that lies between the underlying Dakota sandstones and the overlying Mesa Verde formation. The thickness of the Mancos Shale usually varies from between 900 to 1,500 meters (3,000 and 5,000 feet). Due to its great thickness and its ability to be easily eroded, this shale forms most of the large valleys of western Colorado and eastern Utah. A general geologic cross-section of the Valley can be seen in Figure 7. Because of the marine or1g1n of the shale, it contains a very high percentage of water soluble salts which can be readily seen in the many patches of alkali (white and black) in both irrigated and non irrigated areas. The types of salts which are present in the shale are mostly calcium sulfate with smaller amounts of sodium chloride, sodium sulfate, magnesium sulfate, and calcium and magnesium carbonates. In fact, the minerals gypsum and calcite (calcium sulfates) are commonly found in crystaline form in open joints and fractures of the Mancos Shale, as well as in the soil profile. This can be seen in Figure 8. Due to the compactness of the clay and silt particles making up the shale, the formation is not considered water- bearing at depth. However, the weathered zone near the surface does transmit small quantities of water along joints, fractures, and open bedding planes. In this zone, the percolating water, which primarily originates from the overirrigation of cropland, dissolves the salts directly out of the shale. The soils of the Valley are also quite saline because they have been derived from the Mancos Shale. 22