Alhambra Creek - Geology

Geology

The geology of the Alhambra Creek watershed has shaped and continues to shape the landscape. The bedrock of the area was originally deposited as layers of sand and mud in the deep ocean. Over time, from nearly 100 million years ago to 10 million years ago, layers of sediment accumulated, were buried, compressed and turned to rock. Later, these rocks were folded and faulted and the east bay hills uplifted. As the hills rose, erosion cut away at the rocks, carving valleys in the soft shales and leaving the harder sandstones as hills. The Briones Formation and several other geologic units have common seashell fossils, telling us they began as layers of sand in a shallow sea. The Martinez formation is named for the town. The Hambre formation is named for Arroyo del Hambre Creek. Probably the most dramatic man-made geologic change is the extension of the stream floodplain into the bay, creating virtually all of the land now north of Main Street. This was built up of huge volumes of sediment coming down the Sacramento River as a result of hydraulic mining in the Sierra Nevada during the California Gold Rush, and probably increasing run-off from Alhambra Creek itself. Maps from 1850 show the shoreline meandering between Main and Escobar Streets. Along Alhambra Creek at Escobar Street in 1998, runoff eroded the stream bank revealing fresh, unweathered sand and silt about 10 feet (3 m) thick, probably reflecting gold rush era sedimentation. The entire area to the north is similarly new land deposited at the mouth of the stream.