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The area described, in San Juan County, Utah, is approximately 7 miles wide and extends from the San Juan River northward for about 20 miles. It includes the northern part of Comb Ridge and a narrow belt on each side. Altitudes range from 4,170 feet on San Juan River at the southwest to 6,000 feet on Grand Gulch Plateau at the northwest, where its eastern rim forms a high escarpment. Comb Ridge, a conspicuous hogback of eastward-dipping rocks extending through the area and far to the southwest, is the most striking topographic feature and a nearly impassable barrier. San Juan River is the major drainage course and the only perennial stream. It cuts across Comb Ridge, follows the strike in softer beds for 2 miles, and then flows in a deep, tortuous canyon through steeply dipping hard rocks. Comb and Butler Washes are the two principal tributaries, flowing in strike valleys. These streams and their numerous branches show close adjustment to the nature and structure of the formations over which they run. This remote area is about 140 miles by road from the railroad to the north and about 215 miles from that to the south. No one is now living within its boundaries. The nearest communities are Blanding and Bluff to the east and Mexican Hat to the west. Only one main road, the newly improved State Highway 47, reaches the area, and it crosses the southern part. A new, similar rocksurfaced road extending from Blanding to the Natural Bridges comes within a few miles of the area's northern edge. The only other roads within the area are of dirt; after the abnormal rains and floods in the summer of 1953 they were nearly unusable. The exposed sedimentary formations range in age from Pennsylvanian to Recent and aggregate 5,000 feet or more in thickness. The oldest rocks the upper part of the Hermosa and the lower part of the Rico are exposed in the deep canyon of San Juan River. Hard upper limestones of the Rico form the surface of a structural uplift and topographic upland near the river. Successively younger formations crop out toward the east, the youngest being the Bluff sandstone of Late Jurassic age, which forms a westward-facing escarpment along the east margin of the area. Most of the formations show approximately the same appearance and thickness that is characteristic of them over a wide region. A few show more variation or unusual features. The Cedar Mesa sandstone member of the Cutler formation, usually made up chiefly of a thick series of massive gray to buff sandstones, has a soft zone about 130 feet thick at its base in the northern part of the area. Farther south, the entire

Geology of Comb Ridge and vicinity north of San Juan River, San Juan County, Utah