Earth Geology Rock Types

Rock Types

The rocks that the Bryant and its tributaries have cut into to create the watershed are all of sedimentary origin. The rock layers were deposited as layers of sand, silt and mud in an ancient shallow sea. 

The most common rock is dolomite. You find layers of sandstone sandwiched between layers of dolomite. Chert, the common fieldstone and gravel rock, occurs as nodules (globs), lenses and layers within the dolomite. Chert, commonly called flint, resists weathering and remains in the soil after the dolomite has weathered and eroded away. At the very northern edge of the watershed, which is highest in elevation, you find shale and limestone. 

Beneath the surface are more layers of dolomite and sandstone. Finally, at depths of around 2,000 feet is the ancient basement, the very old granite, volcanic and metamorphic rocks that underlie the region. You can find these layers on the surface a hundred miles or so to the east in the St. Francis Mountains.