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non~o7 . CHAPTER II DESCRIPTION OF STUDY AREA forces attack not only from the front but also from the flanks and rear. A common result is an isolated mass left standing for a while in front of the line. In their long-range erosion cycle the colorful canyons of this area are about at their maximum of grandeur. Tne escarpments continue to recede while intricate canyons continue to increase in depth and number. The Great Sage Plain in the southeast portion is one of the study area's distinctive features. In distant views from the Abajo Mountains, the plain appears as a broad expanse of level land, sage covered except for parts that have been cleared and cultivated. Although there are oc- casional canyons hundreds of feet deep in the plain, these escape notice from high level views unless they are in the immedlate foreground. From the bottom of these interlaced canyons the country seems to be made up of cliffs and gorges that would justify the term "mountains." The substruc- ture of this wide flat is the strong Dakota sandstone from which most of the higher and weaker cretaceous beds have been stripped. . The Dakota sandstone is a poor soil maker so that the Great Sage Plain is mostly barren. Exceptions are small areas, as near the Abajo Mountains, where the overlying Mancos shale has not been wholly carried away, and larger areas farther east where deep red soils have been car- ried in by winds from the southwest. In these places the topography ,is faintly rolling and the soils are fertile. Irrigation farming is prac- ticed on fertile lands that can be served from the small streams that flow from the mountains. Beans and wheat are grown without irrigation on the deep aeolian soils. West of the Great Sage Plain, astride the Utah-Arizona border, is the Monument Uplift 100 miles long from north to south and 40 to 50 Iniles wide. Structurally this large area of horizontal rocks is raised several thousand feet. The landscape on the part of this uplift that is south of the San Juan River is characterized by great steep-sided mesas and buttes of red Triassic rocks. These are the "monuments" that give name to the district. Extending southward from the base of the Abajo Mountains into Ari- zona is a sharply defined fault known as Comb Ridge. The portion of the ridge north of the San Juan River forms a demarcation line between areas of different topography. The area west of the ridge is characterized by deep, colorful gorges while the east side has rolling hills, numerous washes and gullies, and is less scenic. Within the study area are a number of volcanic and orographic fea- tures in which the country deviates from a typical plateau character. The La Sal and Abajo Mountains are a group of laccoliths rising above the pla- teau surface. Of similar character are mountains near the border of the study area, including Ute Peak near Mesa Verde, the Carrizo Mountains in the northeast corner of Arizona, and ,Navajo Mountain on the Utah-Arizona . 12