GEOLOGIC SETTING OF LAKE MERRITT, THE EAST BAY'S STRANGE ATTRACTION

Lake Merritt stands out as a drowned stream valley that extends several kilometers into the eastern shore of San Francisco Bay, which is otherwise either steep and rocky or a gentle and marshy aggradational terrain. Geomorphic and geodetic evidence suggests the following sequence of Quaternary events in the transcurrent tectonic setting of the California Coast Range. Around 1 Ma, a 5 × 8 km block of Franciscan basement on the west side of the Hayward fault was tilted and uplifted by approximately 1 km. Orographic precipitation on the relatively high gradient of this block established the compact dendritic stream network of Pleistocene Merritt Creek. Unlike other East Bay streams, Merritt Creek was never beheaded by the fault. During glacial sea-level cycles, this stream network repeatedly incised a ravine, Merritt Canyon, that cut more deeply and filled more slowly than those incised by other streams on the East Bay alluvial plain. Lake Merritt, bordered by three different Pleistocene landforms occupies a distinctive geomorphic setting that accentuates its scenic quality and educational value.