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Show and equally thick Mesozoic formations. The character of this region is described in the section of this report concerning the basin and range province. ARIZONA MOUNTAIN PROVINCE This province, as described in this report, refers to the area along the south margin of the Colorado Plateau occupied by certain outliers which structurally resemble the plateau but which appear very different because they are eroded into rugged mountains. Like the Grand Canyon province, this province is formed of large upraised crustal blocks or segments. Here, however, the rocks forming the surface are not flat- lying sedimentary types of Paleozoic age as at Grand Canyon, but granite and meta- morphic rocks of earlier age. The result is that erosion has sculptured them into forms very different from the characteristic mesas and buttes of the plateau. Outstanding examples of these outliers are the granite masses around Prescott, Ariz., the Maza- tzal and the Sierra Ancha Mountains. Between these are some large valleys, including those of the Verde River and Tonto Creek, which have distinctive character and are of exceptional interest from a geologic standpoint. Prescott Area.- The mountainous region about the town of Prescott, in central Arizona, is largely composed of granite. It weathers into rugged hills and irregular surfaces that are in marked contrast to topographic features of the plateau area farther north. This granitic area extends eastward to the Verde Valley, north to Chino Valley, west to the Aquarius Cliffs, and south as far as Yarnall Hill. Lavas cover and conceal the granite across some rather extensive portions of the region, locally modifying the general appearance of the area. For the most part, however, the vast granite mass dominates the landscape. Along the eastern borders of the Prescott Mountain area, faulted blocks, formed of sedimentary strata resting on granite, are preserved, indicating that this entire region once was covered with strata such as now form the plateau province. An enormous amount of erosion, therefore, must have stripped away the overlying sediments here, leaving the great granite core to form the present land surface. Peculiar rock forms resulting from this erosion, as illustrated by the Granite Dells, are among the most interesting features of the area. From a geologic standpoint, the most significant elements of the Prescott area are those that have to do with ore deposits. Great ore bodies have been brought up by faulting, especially along the border areas, and many mines have been developed in this region as a consequence. Mazatzal and Sierra Ancha Mountains.- South of the Mogollon Plateau and east of the Verde Valley is a portion of central Arizona that differs geologically from other parts of the State. It consists of two great mountain blocks- the Sierra Anchas on the east and the Mazatzals on the west- separated by the Tonto Basin. The ranges which extend for many miles along north- south axes are eroded into jagged, irregular skylines, resembling the mountains of the basin and range province to the south. Structurally, however, they appear to be more closely related to the plateau, having been raised vertically as great blocks, the upper parts of which were long ago removed by erosion. Very ancient quartzites ( Algonkian) form the present mountain summits, whereas in the plateau to the north, rocks of corresponding age, if they occur at all, lie buried at great depths. The height of the mountains ( Aztec Peak in the Sierra Anchas, 7,400 feet), the abrupt rise above the surrounding valley floors, and the ruggedness of the terrain combine to make the Sierra Ancha- Mazatzal Mountain area one of the wildest, least accessible parts of Arizona. Vegetation is dense over wide areas and wildlife is abundant. In general, this area is one of the most favorable places for recreational development of the type that involves hunting, camping, and hiking. Among features of outstanding geologic interest in this area are the asbestos deposits high in the Sierra Ancha Mountains. They appear as a conspicuous white line when seen from a distance. Similar asbestos deposits are found in other parts of this area, notably in Salt River Canyon. Between the Sierra Ancha and Mazatzal Mountains, near the town of Payson, Ariz., is a natural bridge which has been developed in limestone through the work of solution. It is large enough to 47 |
