"Pegleg" country roughly encompasses the deserts and mountains surrounding the Salton Sea and extending eastward to the Arizona border. This vast expanse of desert wilderness consists of uplifted, block- faulted mountain ranges separated by alluvium-filled valleys, all trending northwest-southeast. The mountain ranges have been uplifted along fault lines that have formed above small granite batholiths and plutons. Masses of younger volcanic rocks overlie many of the uplifted granites. The mountains generally consist of a core of ancient Precambrian basement rock intruded by younger Mesozoic granites and younger still Tertiary volcanics. The basins lying between the mountain ranges are comprised of Quaternary alluvium deposits, sand dunes, and occasional lake (or lacustrine) deposits.
The Chuckwalla Mountains consist of an uplifted block of ancient Precambrian metamorphic basement rock (mostly gneisses) intruded by younger Mesozoic granites. The younger granitic intrusives comprise most of the eastern part of the range. The contact between the younger granite and older country rock is generally sharp; in the center of the range the contact is fault-bounded. The Chuckwalla Mountains are highly faulted. Two sets of faults are recognized, one trending northwest-southeast and the other trending northeast- southwest. Several mines occur in the western part of the range, including the Red Cloud Mine, Lost Pony Mine, and Model Mine. All of these are hosted in Precambrian basement rock. In the eastern part of the range are the Aztec mines and several smaller mines and prospect pits just north of Chuckwalla Spring. These are likewise hosted in Precambrian basement rock. The southern part of the range is also host to a fairly extensive exposure of Tertiary volcanic rocks. The exposure trends northwest-southeast along the southern flank of the Chuckwallas.
The Chocolate Mountains form the eastern boundary of the Salton Basin. The western flank of the range, about 10 miles east of Niland, has yielded black-coated gold nuggets in the past. This section of the Chocolate Mountains consists of an uplifted, block-faulted core of ancient Precambrian igneous and metamorphic basement rock intruded by younger Mesozoic granitic plutons. Some small exposures of Pre- Cretaceous metasedimentary rocks occur in the area. The Chocolate Mountains and adjacent fault zones all trend northwest-southeast.
The Superstition Hills lie just south of the Salton Sea and east of the Fish Creek Mountains. They consist almost entirely of Pleistocene non-marine sedimentary deposits. The hills are heavily weathered and eroded and are cut by several faults (including the Superstition Hills Fault). The ancient shoreline of Lake Coahuila laps along the northern and eastern flanks of the Superstition Hills. The shoreline is marked by the occurrence of Quaternary lake deposits.
The Fish Creek Mountains, which lie about 7 miles west of Superstition Mountain, consist of an uplifted core of Mesozoic granitic rocks. Upper Miocene marine sediments occur along the northwestern flank of the range. The Fish Creek Mountains are cut by a northwest-southeast trending fault which may be part of the larger San Felipe Fault system. The ancient shoreline of Lake Coahuila fringes the eastern edge of the range and is again marked by Quaternary lake deposits.
The Borrego Badlands lie just south of the Santa Rosa Mountains, on the western edge of the Salton Basin. The southern portion of the badlands consists of Tertiary lake (or lacustrine) deposits while the northern part is underlain by Pleistocene non-marine deposits.
West of the Yuma village on the Colorado River, the searing desert sands and stark mountains of the Colorado Desert present a formidable barrier to the traveler. Three days travel by horse should place one somewhere in the Salton Basin. Curiously enough, black-coated gold nuggets have been discovered in many parts of the Salton Basin. For example, the desert terrane between Fish Creek Mountain and Grave Wash has yielded a number of black, manganese-coated nuggets. The canyons and arroyos that cut the western flank of the Chocolate Mountains have also been a source