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~ta~nin~ Unit Number 1 -Shallow (less than 20 inches to bedrock), light colored, loamy to claye}', • undeveloped soils has~ng no soil horizonation other than a slightly darkened surface horizon, or inconsis- tent weak horizons of secondary carbonate accumulation, occurring in parts of the study area where mean summer soil temper:tturc (June, July, and August mean temperature measured at 20 inches or at bedrock, sshichever is more shallow) is 59°F or warmer. Classification of Maior Components -The components listed below comprise the major part of the mapping unit: A. Ustic Torriorthents (Lithosols end Rcgosols of semi-arid areas). 1. Loamy, mixed, (calcareous), mesic, shallow family. (Shingle Series) (T3SSCI SCTICS) ?. Loam, mixed, (calcareous), frigid, shallow lamily. (Blazon Scrics) (Shinbara Series) 3. Clayey, montmorillonitic, (calcareous), frigid, shallow family. (Samsil Series) 4. Clayey, montmorillonitic, (calcareous), frigid, shallow family. (Movcrson Scrics) Lithic Ustic Torriorthents (Lithosols of semi-arid rceions). 1. Loamy, mixed, (calcareous), mesic family (Lazcar Series) (Travessilla Series) • Extent and Maior Areas of Occurrence - 1,4apping unit number 1 is an extensive unit in the study area. li occurs at elevations of less than 7,800 feet. Greatest acreages are to be found in the western and nonhcrn points of the study area. Characteristic Toeoeranhv -Typically the soils of mapping unit number 1 occur on ridge crests and shoulders, adjacent to upland escarpments, on upper footslopes, or on the edges of deeply incised drainage channels. They may occur at times on less steeply sloping mesa or ridge tops or at the margins of young alluvial L+ns, where the sedimentary bedrocks are close to the surface of the ground. Generally these soils arc associated with the points of erosion where normal wearing away of the ]and surface has prevented soil formation. Slopes range about 5 to 60 percent. Physical Properties - Soils of mapping unit number 1 are young soils in tehich natural soil structure is onie n•cakh• developed, and what structure is present can be easily destroyed. P. majority of the compo- nents of this mapping unit, therefore, arc not well adapted to disturbance and development of new seed beds. hlost of these soils are subject to excessive compaction if traversed by heavy machinery and to irreversible puddling if manipulated when the moisture of the soils are near field capacity. Fine textured components (soils with more than 35 percent clay) are particularly susceptible to these effects. Soils of mapping unit number 1 are ven• susceptible to water erosion and moderately susceptible to wind erosion. They contribute runoff and sift during rains that can be dctrimenta{ to lower lying areas.