Frenchman Mountain

"The rocks on the right are 1.7-billion-year-old Precambrian granite and schist. These formed deep within the earth in the core of an ancient mountain range. During hundred of millions of years this old mountain range was eroded away, leaving these rocks exposed on the earth's surface about 500 million years ago. As sea-level rose, the exposed granite and schist was buried by sand on the floor of a shallow sea. The sand was cemented together to form the Tapeats Sandstone, on the left. This buried erosion surface is the "Great Uncomformity," which represents about 1.2 billion years of missing history. The earth is about 4.5 billion years old, so this surface represents more than one fourth of earth history.

The "Great Unconformity" is also exposed at the bottom of the Grand Canyon, but there the Tapeats sandstone and overlying sedimentary rock are still horizontal, as they were deposited. Here at Frenchman Mountain the rocks were tilted about 50° to the east about ten million years ago, during the Miocen Epoch."

(Geologic Interpetive Site; Stephen Rowland, UNLV)