FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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A BRIEF GEOLOGIC HISTORY OF EDGEWOOD NATURAL PRESERVE By Bill Korbholz Most of us are by now aware that last July Edgewood County Park dramatically changed from a county park to a county park and a natural preserve. And some of us can recount the history of the park as far back as 1980, when San Mateo County acquired the 467 acres of the current site, or even as long ago as 1967 when the property was originally acquired by the State of California as a possible college site. But when considered in the context of the geologic evolution of this land, these last thirty years represent only a heartbeat. Most of the land which now constitutes Edgewood Natural Preserve was at the bottom of the Pacific Ocean around 150 million years ago, and consisted of deepwater marine cherts and limestone resting on the basaltic ocean floor. This land was subjected to the pressures of the North American tectonic plate to the east and the Pacific tectonic plate to the west grinding and mashing against each other. This movement resulted in a huge land mass being squashed up against and attached to the mainland as the sea floor was being dragged under (subducted by) the westward advancing North American continent. This so-called Franciscan formation is generally believed to have been created during the Cretaceous period, from about 130 to 150 million years ago. To put this in perspective, the dinosaurs roamed the earth from around 100 to 200 million years ago. The San Andreas fault is young by comparison, having been created around 30 million years ago. Much of the terrain in Edgewood Natural Preserve is a part of this Franciscan formation, consisting of greenstone, chert, graywacke, siltstone, shale, and of course the state rock, serpentine. More on the makeup of Edgewood Natural Preserve's land in the next issue. |
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