A Cross-Section of California

Sierra DetailIn the 1850’s, with the California Gold Rush in full swing, the United States was looking into connecting the Atlantic and Pacific coasts with a transcontinental railway; thus, the Pacific Railroad Surveys were born. After Congress passed the Pacific Railroad Survey bill in 1853, four east-west survey routes were quickly approved by the Army Corps of Topographical Engineers: within months, Corps lieutenants Robert S. Williamson and John G. Parke were given the orders to survey between the West Coast’s 32nd and 35th parallels for potential passes through the Sierra Nevada to connect the San Joaquin and Tulare Valleys with the Colorado River. Accompanying Williamson and Parke was geologist and mineralogist William P. Blake.

The party departed July 10, 1853, taking a ferryboat across the Carquinez Strait to Martinez where, skirting the flanks of Mount Diablo, the expedition headed south, reaching the vast San Joaquin Valley in a week’s time. By August 8th, the travelers had reached the foothills of the Sierra Nevada and established a camp they dubbed “Poso Depot” after the nearby and near-dry Poso Creek (also referred to as “Ocoya Creek” and “Posé Creek”) in present-day Kern County. Two days later, Williamson and Parke set out for the Sierra, leaving Blake and Heermann at the Depot to explore the region on their own. At some point during the expedition, the geologist Blake began preparing what would later become a series of panoramic fold-out geological section maps for publication in the survey’s final “Geological Report.” One of these maps, the 1853 “Geological Section of the Coast Range and Sierra Nevada,” charts the change in elevation along a cross-section of California from the Farallones (Farallon) Islands sea stacks and islands off the Pacific Coast to the Walkers (Walker) River in the Great Basin of west central Nevada.


 (click to enlarge)

Painting this 270-mile cross-section with a broad brush, the breadth of Blake’s panorama includes, from west-to-east, Point Lobos (Land’s End), Fort Point (the ruins of the old Spanish fort, the Castillo de San Joaquin and the present-day Presidio of San Francisco), San Francisco, Yerba Buena (Yerba Buena Island), the Bay of San Francisco, Mount Diablo, Livermore Pass (Livermore), Elk Horn (undetermined), the Delta of the San Joaquin River, Stockton, Knight’s Ferry, Green Spring (undetermined), and Sonora. Blake’s geological section puts to paper the elevational rise and run of California with peaks that seem to dwarf the shorelines and floodplains in orders of magnitude. Rising from the sea stacks of the 1/16-inch Farallon Islands (~312 feet; actual = 357 feet) to the 3/4-inch peak of Mount Diablo (~3,750 feet; actual = 3,849 feet) to the towering heights of the Sierra Nevada’s 2 ½-inch Tower Peak (~12,500 feet; actual = 11,755 feet), at a vertical scale of 5,000 feet per inch, Blake captures California’s vertiginous vista with surprising accuracy.

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