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CSK Auto#4025 GeoTek Project No.:BC 1224025 Tracy,California 95376 Page 6 of 20 March 2,2000 mountain-making episode known as the Nevadan Orogeny. The sediments and volcanic rocks were altered to a second series of metamorphic rocks and shoved against the older Calaveras rocks. Following the Nevadan Orogeny, the site of subduction moved from the Sierran foothills 80 miles west to the present site of the Coast Ranges. By early Cretaceous time, the leading edge of the descending oceanic plate had reached depths sufficient to cause partial melting. The resulting magma rose to the surface, creating large-spread volcanic activity and a magmatic arc. The volcanic rocks formed during this time have long since been removed by erosion, but the debris is preserved in the sediments of the Great Valley. In addition, part of the sediment washed into the California arc-trench basin spilled over into the trench, and part of the trench sediment resulted from landslides and turbidity currents generated on the slopes of the trench itself. These early Cretaceous sedimentary deposits in the arc-trench basin are part of what is known to geologists as the Great Valley Sequence, possibly as much as 40,000 feet thick and including strata from Late Jurassic to Late Cretaceous in age. By late Cretaceous time, as a result of continued elevation, erosion in the Sierra had removed the cover of volcanic rocks and considerable thickness of the underlying metamorphic rocks. Large areas of the deeply buried granites, including the Cretaceous granites that provided the gold of the Mother Lode quartz veins, were now exposed. Erosion of these granites released a distinctive mineral, orthoclase, to the late Cretaceous sediments that were spread into the Great Valley. The activation of the modern San Andreas Fault System was an important event in the Cenozoic history of California. Movement within the fault system began in southern California when the East Pacific Rise, separating the Pacific and Farallon plates, reached the continental border about 29 million years ago. The rise entered the subduction zone and subduction ceased because the northward-moving Pacific plate was now against the continent and its motion began to carry along it parallel to the coast. Today, movement occurs along the San Andreas Fault Zone, an area stretching from less than a half mile to several miles wide, and hundreds of miles long. Localized glaciation during the Ice Age (Pleistocente Epoch) did not occur in the central California area. Alpine glaciers were present on the high Sierra Nevadas. The indirect effects of this alpine glaciation resulted in the floods of debris from the melting Sierran glaciers. In the Coast Ranges, on the other hand, the indirect effects were related to fluctuations in the level of sea as water was extracted to form glacial ice and later