Geology of the Canyon

MILLS CANYON GEOLOGY

By Roger Taylor

Origin and Geologic History
Mills Canyon owes its origin to the downcutting of Mills Creek into the Buri Buri Ridge, the elongate hill presently being gradually pushed up along the northeast side of the San Andreas Fault, which lies just one mile west of the canyon. This ridge began rising from bay level about 3.5 million years ago, when the great North American and Pacific tectonic plates shifted slightly, causing slight compression across the predominately strike slip fault. The ridge consists mainly of old sedimentary rocks called the Franciscan formation, which were deposited in an ancient ocean about 190 to 70 million years ago, back when dinosaurs roamed the earth and swam in the sea. However, sand and highly polished pebbles from a much younger - probably 2 to 3 million years old - beach deposit called the Merced formation have been found on the upper north wall of the canyon. Movement along several thrust faults underlying the ridge have pushed rocks of the Franciscan up and to the northeast over sediments of the Merced Formation. This type of upward fault movement is the primary mechanism by which the Buri Buri ridge has grown. The flat summit of the ridge is a remnant of an old marine terrace, the Buri Buri terrace, that might be as young as 300,000 years old. This terrace was cut across both the Franciscan and Merced Formations.

Rock Types found in the Canyon
The Franciscan formation, as exposed in scattered outcroppings in the canyon, is a tectonically mixed suite of marine volcanic and sedimentary rocks that prior to hardening or weathering were basalts, deep sea oozes (later to become cherts), and continental slope sands and muds. Some of the sands and certain volcanic rocks were altered due to burial heat and pressure, to rocks known as greenstone and serpentine. While walking the trail, note the following Franciscan rocks: dark sandy greenstones on the south wall of the canyon, vari-colored chert and serpentine boulders in the creek bed, and the monolithic gray sandstone of Picnic Rock 100 yards upstream from the footbridge. Less common rock types, found mostly on the north slopes of the canyon, are (1) poorly consolidated sands and loose highly polished pebbles of the much younger Merced formation, (2) dark Franciscan volcanics with small air cavities, and (3) grayish calcite-veined quartz-rich rocks that show considerable alteration due to heating and burial.