Rocks, Soils, Hills and Valleys

Hercules is located at the northerly extent of the Berkeley Hills range of the Coast Range province of California. Refugio Valley is formed by two low ridges of hills descending about four miles northwest from elevations of about 800 feet to nearly sea level at the shore of San Pablo Bay. 

The Berkeley Hills, with its three northern valleys, rose from the Pacific Ocean seafloor two million years ago. Almost all of Refugio Valley’s soils and rocks are Miocene shale, formed as sedimentary rock sixteen million years ago. In a few isolated places, rocks from other ages can be seen, such as older volcanic “Pinole tuff” and marine terraces of younger sedimentary rock. 

Refugio Valley is the watershed of Refugio Creek, located between Pinole Ridge to the southwest and Franklin Ridge to the northeast.  The valleys of Refugio, Pinole and Rodeo  run generally northwest - southeast, at the feet of a series of low ridges descending into San Pablo Bay.

These ridges are the northern extent of the Berkeley Hills, a defining geologic feature of the Bay Area. A few miles north of these ridges is the boundary between two major geomorphic provinces of California: the Coast Range meets the Great Valley.  

Rocks and Soils

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