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Show have a small orchard on top and should attract considerable interest. Verde Valley.- The walled- in Verde Valley differs in physiographic character from other large valleys in Arizona, yet it is one of the most delightful because of the large permanent stream flowing down its length. On three sides the valley is enclosed by cliffs forming the margins of upraised blocks of the Colorado Plateau. Details of geologic structure have not yet been determined in this area, but a series of great vertical faults apparently controlled its early development. Erosion has since greatly modified the escarpments on each side and partly obliterated evidence of the crustal movements involved. Lavas formed a dam across the river in one place and extensive deposits of limestone are the result of deposition in a very large lake that once covered much of the valley. Because of this complex history, many geologic features of unusual interest are represented in the Verde Valley. Unlike the dry plateau country of northern Arizona or the mountain ranges to the south, Verde Valley is destined, both because of its character and location, to be developed as an important source of water and of agricultural produce. Among the best- known features of this area deserving consideration and more study are Verde Hot Springs, Soda Springs, fossil tracks of lions and other prehistoric mammals, salt deposits and prehistoric salt mines, and lake deposits with mollusks. RECENT VOLCANIC AREAS Within and along the borders of the Colorado Plateau are several areas of geologically recent volcanic activity which have so modified both the appearance and the character of the country as to merit special treatment in this report. Although underlain by normal strata of the plateau type, lavas, cinder cones, and strata volcanoes have covered and masked the sedimentary rocks, thus obscuring the original topographic forms. Outstanding examples of such areas are the San Francisco Mountains and the White Mountains. A lesser example is found in the Mount Trumbull region. San Franciscan volcanic field.- One of the largest, most spectacular, and probably the most interesting geologically, of the volcanic fields within the Colorado River watershed is that developed about San Francisco Mountain in the central part of Northern Arizona ( PL 3, in pocket). This lava field covers an area of approximately 3,000 square miles. In addition, several hundred small cinder cones are scattered over this surface, and a series of five large strato- volcanoes form a row from east to west, dominating the landscape. San Francisco Mountain, the largest of the volcanoes, reaches an elevation of 12,611 feet above sea level or about 5,000 feet above the plateau surface. The age of the various volcanic features in this area varies considerably, for the results of three distinct periods of activity can be recognized. The oldest of these, which was a time when very extensive basic lava flows poured forth on the surface of the plateau, dates back at least a million years ( Pliocene). During the middle period the high peaks were formed of more viscous lavas. The last period, like the first, was featured by vast sheets of basalt spreading over the surface and by the development of small cinder cones, but it is most notable because of its recency. One cone, Sunset Crater, is less than a thousand years old. The diversity of features represented and the excellent state of their preservation due to recency of origin combine to make this an outstanding area for illustrating volcanic phenomena. Most of these features are well known to scientists through the works of Robinson and Colton; some of them are familiar to the general public through the educational programs of the National Park Service and U. S. Forest Service, the agencies which control a large part of the area. Still, much remains to be learned about the various features and only a beginning has been made in utilization of the recreational possibilities of this area. Features of special geological interest in the San Franciscan volcanic field, having potential value from an educational and inspirational standpoint, range from entire mountains that illustrate the results of remarkable or unusual processes to small detailed structures or products incidental to the volcanism. In the former category are Slate Mountain and Marble Mountain, which are excellent examples of semi- laccoliths- small hills due to the partial doming up of once flat- lying layers of sedimentary rocks through the force of molten material 48 |