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Sandy Benchmark (Alvord Mountain)

Alvord Mountain (Part 4): Sandy Benchmark. Alvord Mountain is a geologic feature of localized rocky peaks, some of which are partially buried beneath sand. Alvord Peak is the highest point of Alvord Mountain. The Sandy Benchmark resides on the eastern outlying margins on the escarpment that partially encircles Alvord Mountain. Alvord Mountain is situated in the Mojave Desert of California 40 km northeast of Barstow, 8.5 km north of I-15, and just south of the training grounds of the United States Army’s National Training Center at Fort Irwin. To the immediate west of Alvord Mountain is Coyote Lake and 8 km to the east are the Cronise Hills. Alvord Mountain hosts a range of rocks from the Paleozoic to Cenozoic. Alvord Mountain can be divided into two sections based on geology, an east and west section. The eastern portion is mainly composed of Cenozoic volcanic and sedimentary rocks that overlay Mesozoic plutonic basement rock (Miller and Walker, 2002; Byers, 1960; Fillmore, 1993). The western portion is composed mainly of exposed Paleozoic metamorphic rocks, which are intruded by younger dikes and Mesozoic plutonic rocks (Miller and Walker, 2002). Metamorphic rocks exposed at Alvord Mountain are primarily marble and schist. Intrusive igneous rocks that intrude both sedimentary and metamorphic units are monzodiorite – quartz monzonite, hornblende gabbro – diorite, leucocratic quartz monzonite, and dacitic/granodiorite dikes (Miller and Walker, 2002). Extrusive igneous rocks found primarily in the eastern section are andesite, basalt, and volcanic tuffs (Wyatt, 2005; Fillmore, 1993). Sedimentary rocks found at Alvord Mountain are conglomerates, sandstones, mudstone, siltstone, and carbonate rock Encompassing Alvord Mountain on the northwest, north, and east is an inboard facing escarpment. Also, in small southern outlying areas are granitic fanglomerates and gravel units that unconformably overlay older sediments and igneous units. The Sandy Benchmark is the highest point along this escarpment on the east side. The western facing edge of the escarpment is steeply cut, highly weathered, with high relief. The slopes dipping east from the escarpment are gentle and incised by small arroyos. Much of the area contains well – sorted, light – gray quartz monzonite boulder beds and is interbedded with pebbly sandstone. Localized lower portions of the granitic fanglomerate also bear well rounded gray granodiorite, diorite, and gneissic diorite that is also interbedded with pebbly sandstone. In the areas around the Sandy Benchmark, much of these pebbles – boulders are scattered as float. The estimated age of the mentioned unit ranges from Pliocene to Quaternary (Byers, 1960). Alluvium and windblown sand that encompass and overlay the gentle sloping topography away from Alvord Mountain and overlays the granitic fanglomerate, volcanic gravel, and older units (Byers, 1960). References: Byers, F.M., 1960, Geology of the Alvord Mountain quadrangle, San Bernadino County California: U.S. Geological Survey Bulletin 1089-A, p. 71. Fillmore, R.P., 1993, Sedimentation and extensional basin evolution in a Miocene metamorphic core complex setting, Alvord Mountain, central Mojave Desert, California, USA: Sedimentology, v. 40, p. 721 – 742 Miller, J.S., and Walker, J.D., 2002, Mesozoic geologic evolution of Alvord Mountain, central Mojave Desert, California, Geologic Evolution of the Mojave Desert and Southwestern Basin and Range: Boulders, Colorado, Geologic Society of America Memoir 195, p. 59 – 77. Wyatt, B.R., 2005, Plio-Pleistocene deformation at Alvord Mountain Central Mojave Desert, California, Master’s Thesis: San Jose University, p. 66. All photos and videos were taken by Alexander F. O'Connor. Soundtrack by Alexander F. O'Connor. Music composed, performed, and recorded on a Yamaha Clavinova CVP 701 Digital Piano. Video produced using Corel VideoStudio Ultimate 2023 by 2023 Corel Corporation. I am not responsible for anyone choosing to explore the desert or mountains themselves. All persons are solely responsible for their own decision-making, whereabouts, well-being, and safety. Thank you for watching!