Miocene Shale

Go outside and scoop up a handful of soil. In your hands is a clayey loam soil, high in nutrients. In winter it is wet, cold and crumbly; in summer it is dry and packed. 

That handful of soil is shale rock that has been weathered, decomposed and mixed with organic material. Geologists describe it as “white, cherty and diatomaceous marine shale or gray to buff shale and sandstone.” This Miocene-era shale is one of the many different types of soil that characterize the complex, dynamic geology of the Bay Area. 

Shale is a rock formed from sediments. Under your feet in most of the Refugio Valley is a bed of shale rock formed from silt or clay sediments deposited at the bottom of the Pacific Ocean. This bed of shale rock was formed in the late Miocene era, which dates from five to twenty-three million years ago, then moved and lifted by tectonic forces into its current position a few hundred feet above today’s ocean surface.  

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This bed of Miocene shale is about seven miles wide. Traveling westerly along Interstate 80,  it extends from Cummings Skyway to Hilltop Mall. The hills east of Cummings Skyway are built from much older sedimentary rocks formed on land during the earlier Cretacious period, from 65 to 100 million years ago. In the flatlands west of Point Pinole, and extending southeast along the entire East Bay, is alluvium: unconsolidated material washed down from the hills during most recent geological period.