Archive for category Geology

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.

GeoSect_Flattened_Cleaned_Webpanorama

 (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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Book Review: Rise of the Ranges of Light

Rise of the Ranges of Light: Landscapes and Change in the Mountains of California, by David Scott Gilligan, Heyday (www.heydaybooks.com), 2011, 209 pages, $18.95

Tectonic activity isn’t exactly an ice-breaker at a cocktail party, nor is it anything you’d expect to hear expounded on at a poetry reading. But that’s exactly what David Scott Gilligan has accomplished in Rise of the Ranges of Light, the sort of fortunate result of spontaneous generation you might expect if you left a copy of Outside Magazine, a geology primer, and Henry David Thoreau’s Walden in a dark corner, which when spawned unravels in a lyric, tectonic tale.

Although billed as a work of “natural history”, Gilligan’s opus to the genesis of California’s mountains is more accurately a “geological history” of the primal forces that painstakingly crafted the landscape we know today. Take for example California’s “mutt” geologic ancestry best seen along California’s coast, an accreted jumble of Sierran granitic crusts and tropical terranes stacked like congealed grease on the coastal spatula of North America. Or California’s paper-thin crust (12-19 miles thick), the result of the North American and Pacific plates pulling at each other like taffy, leading to the San Andreas Fault and the rerouting of mountain ranges.

Centering his expose on the Sierra Nevada, the range John Muir dubbed the “Range of Light”, Gilligan introduces the Sierra as being “one of the earth’s most extensive complexes of granite ever distilled from the earth’s mantle and emplaced in the crust”. Beginning with the batter churning deep below the earth’s crust, Gilligan patiently paints the process of mountain building that shouldered granite peaks like the Sierra Nevada into the heavens, a melange of spreading sea-floors, crumbling continental crusts, and mulling magma. What makes his writing so inviting, though, is his ability to conjure up these processes as if viewing the earth like a halved orange: the settling of the Central Valley and Great Basin caused by the sheer weight of a volcanic basement of granite overburdened by continental crust, or mountains “floating” in the continental crust like so many ice cubes in a glass of tapwater. When Gilligan describes the elastic, see-saw tension between mountains and the earth’s crust, it becomes all too apparent how a gardener (for example) barely scratches the earth’s surface moving a shovelful of garden soil, an act equivalent to throwing a soggy pea at an elephant compared to the tectonic forces at work.

For better or worse, the mountains of California are both hallmarks and harbingers of change. As sobering as it is imaginative, Gilligan’s prose brings the realities of the Sierra Nevada’s present plight to bear. Writing about global warming’s increasing influence above tree line, he laments:

“Everywhere I go the glaciers are dying… White spots on the map are reduced a hundredfold, now mere trimmings of once vast snowfields. It’s like returning to a beach to find that the sand has disapepared, taken away by tidal rips to some undefinable place-not moved or changed, but altogether gone… Trees seem to march upslope and young recruits are seen on high sites where no adults have been seen for a thousand years… The land has a thirsty feel that a winter or two of heavy snowfall can no longer slake.”

Gilligan’s writing betrays his passion for the mountain landscapes he has come to know so well, not only as a professor, but as a student of the wild, an avid naturalist, and a climber. If every geologist wrote with such passion, perhaps more Californians would abandon the shackles of Silicon Valley to become Shackletons of the High Sierra.

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