Hercules Historical Society

Rocks, Soils, Hills and Valleys

Natural and human and natural history is shaped by the land. Physical and chemical properties of the rocks and soils in a place are the basic conditions of plant and animal life, and human settlement. Contours and topography set boundaries and other important conditions at larger scales. 

The natural and human history of Hercules was shaped by two low ridges of hills descending about four miles northwest into San Pablo Bay, forming Refugio Valley. The hills and most of the valley floor are the surface of a thick bed of sedimentary rock that has been weathered into soil. 

In the lower elevations near the Bay shoreline, valley soils are younger; beneath are layers of unconsolidated muds. Along the shoreline between Pinole and Rodeo, you can see small deposits of volcanic rock - “tuff” - and beds of ancient sea shells. 

The geology of the Bay Area is complex and dynamic. The rock making up the Pinole and Franklin ridges of hills was elevated from beneath the Pacific Ocean. San Pablo Bay was a dry valley until land movements and sea levels filled the bay to the shoreline we know today. 

Rocks and Soils

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Refugio Creek’s watershed - the Refugio Valley - lies at the northern extent of a bed of shale rock that...

Hills and Valleys

Hercules is located at the northerly extent of the Berkeley Hills range of the Coastal Province of California. Refugio Valley is...

Since deep time, this location has been at the seam joining the Bay Area with California’s Great Valley. The natural history in Refugio Valley exemplifies the story of plant and animal life in the East Bay. 

Indigenous humans used Refugio Valley, particularly along the Bay shoreline, creek mouths and marshes. Few traces remain. 

European settlement of Refugio Valley first, briefly, was for agriculture. From 1878 the main production facilities of California Powder Works occupied the lower Refugio Valley; to the southeast in the upper Valley were small storage and shipping facilities. 

When the era of explosives manufacture ended, the upper Valley was subdivided and sold as residential lots beginning in 1972. Later, beginning in 2001, residential development occured in the lower Refugio Valley as economic values and new construction techniques overcame geologic and hydrologic challenges.