Black Mountain (Maricopa County, Arizona)
Description
Black Mountain is located in north-central Maricopa County, Arizona, between the towns of Carefree and Cave Creek.GeologyThe mountain is composed of two main rock types that are roughly divided between western and eastern halves of the mountain. The west side of the mountain is primarily composed of a ~250 m. bed of black meta-sedimentary rocks which are distinguishable as phyllite and argillite. Viewed from a distance the meta-sedimentary rock is often mistaken for basalt, which is common in the area; however basalt is not found on Black Mountain. The phyllite has a slaty cleavage and is foliated in all directions, the result of shortening due to pressure-solution volume-diffusion of as much as 50%. As a result of shortening, streambeds of nearby washes are filled with black alluvium.The western face meta-sedimentary beds strike ~120, 45 and, when fully illuminated by the afternoon sun, resemble overlapping flatirons. The meta-sedimentary beds rise 830 feet above the local topography, with an actual elevation of 3398 feet above sea level.The eastern side of the mountain is composed of a granitic intrusion into the meta-sedimentary rock. The granite is part of the granite formation that extends southward from Black Mountain and is expressed in outcrops in the McDowell Mountains, the Union Hills, and Camelback Mountain. Texturally, the granite varies from a large crystal with phenocrysts of up to 2.5 cm., to a finer grained crystal texture devoid of phenocrysts. The granite containing larger crystals has formed large joints as well, fractured both vertically and horizontally, and is weathered into spheroidal boulder columns and balanced rock formations. Jointing in the finer grained granite is smaller and proportional to grain size. The granitic half of Black Mountain tapers to a flat-topped peak that rises to a height just below that of the pointed, black phyllite peak of the western side. The two units meet as a saddle between the twin peaks of meta-argillite/phyllite and granite.
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