They contain a core of granite and gneiss that date back at least 1.4 billion years. The core is overlain by a 1,500 feet (460 m) layer of Cambrian-era rhyolite that is about 525 million years old. Atop the rhyolite is about 15,000 feet (4,600 m) of folded and faulted limestones, dolomites, sandstones, and shales deposited in shallow seas from Late Cambrian through Pennsylvanian time (515 - 290 million years ago).[1]