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Conservation Issues of the Ventana Chapter | santa cruz county

Pogonip geology rocks

February 2010
Shawn PadillaJerry Weber displays colorful maps depicting the geology of the Pogonip.
Photo: Ventana Staff

by Sarah Rabkin

Geology,” according to Gerald “Jerry” Weber, a geological consultant and retired UCSC earth sciences lecturer, “is a four-dimensional jig saw puzzle with most of the pieces gone.”

On January 22, Weber led 47 Sierra Club hikers on a geologic ramble through the Pogonip—the 640-acre Greenbelt, owned by the City of Santa Cruz, bordered by the UCSC campus to the west and Hwy. 9 to the east. This sunny Saturday marked the 110th Pogonip outing organized by club members Peter and Celia Scott.

Introducing the group’s guide, Peter Scott quoted UCSC colleague Al Smith: “Jerry Weber knows more about the geology of Santa Cruz County than anybody else in the world.” Yet even Weber laments the amount of guesswork involved in piecing together the origins of local landforms that have been acted on by hundreds of millions of years of rock formation, earth movement, and erosion, and covered with dense vegetation.

Weber held up a series of colorful maps representing contradictory interpretations of the Pogonip’s faults and formations. Nobody yet knows which, if any, is correct. Wielding a rock hammer, he chipped at outcrops along a route shaded by live oak, bay laurel, and redwood trees. He identified a deposit of mudstone here, a block of marble there.

Scientists like Weber have gleaned invaluable knowledge of the area’s geology—information that planners and builders ignore at their peril. Parts of the Pogonip and UC Santa Cruz campus, for example, sit on a block of limestone, called a karst landscape, in which the carbonate rock has been gradually eaten away by weak carbonic acid formed when rainwater mixes with carbon dioxide. As a result, the dissolved and fractured ground is riddled with caves, tunnels, and sinkholes that turn the land into a “giant sponge.” In addition the area is also crossed by a series of faults, which fortunately are not seismically active, except for possibly the Ben Lomond fault, along which Ben Lomond Mountain has been elevated.

Weber enchanted his audience with tales of the dangers of overlooking the underlying geology. Engineers hired by the University have sited building supports atop “void spaces” (sinkholes) where truckload upon truckload of concrete simply disappeared into the underlying cave systems. Weber also divulged that campus planners in the early 70s selected well sites on the advice of dowsers, only to drill deep into dry rock.

Weber noted that the karst landscape features few visible creeks or streams; most rainwater seeps through the permeable soil and accumulates underground in the vast complex of fractures and passages. When a several-hundred-foot-deep campus well was pump tested, withdrawing one million gallons of water over the course of one week, the testers saw negligible (if any) impact on the overall volume of water emerging from the springs that lie at the base of campus. Weber said the amount of water stored under campus is about 3,000 acre-feet, or a billion gallons. In times of drought or other emergency need, this untapped underground reservoir might serve, he said, as an alternative water supply.

Stopping at a bulge of moss-covered, hole-pocked stone curving gracefully outward alongside the trail, Weber explained, “This is tufa—a limestone deposit created by a spring, similar to formations found at Mono Lake and other sites.”

Another piece of the puzzle fell in place for the attentive hikers.

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