CREATING A MOUNTAIN
by John Werminski - 1993

Although Mt. Diablo is one of California's best-known landmarks, the details of its origin have long been cloaked in mystery. To try to understand the mountain's history, we must look far back in time and far beyond the peak itself, at geologic processes operating on a global scale.

Scientists have discovered that the earth's crust is divided into large fragments or "plates" that slowly move relative to one another. In fact, California appears to be the product of a long-term collision between two such plates: one including most of North America, and the other much of the Pacific Ocean floor.

Between the melange and the continental uplands to the east a basin formed, filled with the waters of a prehistoric sea. This sea covered much of what someday would become California's Coast Ranges and Central Valley. Into the sea washed layer upon layer of sediment, over an almost unimaginable span of time: through eighty million years when giant reptiles ruled the earth, then through another sixty million years while land masses rose and eroded back into the sea, and the climate gradually became cooler and drier. As the eons passed, countless plants and animals lived and died here, some leaving their remains in the ever-thickening beds of sand and mud.

About twenty million years ago the collision process changed in this region. The great plates continued to move past each other, but laterally now, with the San Andreas fault system developing to accommodate the sideways motions. This rearrangement of geologic forces also led to the creation of the Coast Range mountains. Sea water retreated from the area for the final time.

According to current theory, within the last four million years local fault activity brought a slab of old ocean floor into contact with the layered beds of sediment. Then, during the last million years or so, powerful compressive forces caused the land to buckle, producing a massive fold. In the center of the fold, the resistant slab of old ocean floor material became exposed in bold relief as the softer surrounding strata were worn away. And so the landform that we recognize as Mt. Diablo was born.

Even now, the mountain's evolution is not complete. Geologists suspect the peak may still be rising. Landslides re-shape its contours from time to time, and the erosion carries fresh sediment each winter to the valley flats. Every jolting earthquake and every tumbling rock are new evidence of Mt. Diablo's dynamic nature and new additions to an ancient story that continues to unfold.

The above text was taken from the video presentation "Creating A Mountain"
that is part of the geology display in the Summit Museum. (ã California State Parks, 1993)
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