Hercules Historical Society

Rocks and Soils

Refugio Creek’s watershed - the Refugio Valley - lies at the northern extent of a bed of shale rock that makes up the Berkeley Hills. Almost all of the surface soil in the Refugio watershed is this shale rock. Small outcrops of other rock types are found along valley bottoms and the San Pablo Bay waterfront. 

In the upper Refugio Valley the shale soil layers are thick and generally stable. Beneath the surface of lower Refugio Valley, below the old shoreline of San Pablo Bay, are thinner layers of alluvium resting on deposits of bay muds.