Precambrian rocks form the basement of the Continental plates. By definition, these ancient igneous and metamorphic rocks formed between the earliest days of the Earth (4.6 billion years ago) and the onset of the Paleozoic Era (600 million years ago). In most areas, they are covered by thick deposits of sedimentary and volcanic rocks from the Paleozoic, Mesozoic and Cenozoic Eras and are only visible under one of three circumstances: where a river has cut a deep canyon through the overlying sediments (e.g. the Grand Canyon), where erosion (such as glaciation) has removed the overlying sediments (e.g. the Canadian Shield) or where these deep Precambrian rocks have been uplifted to form mountain ranges (e.g. the Rocky Mountains and the Blue Ridge Mountains).
In some areas, the uplift of the Precambrian rock has occured as a broad dome; the Adirondacks of New York, the Black Hills of South Dakota and the St. Francois Mountains of southeast Missouri are excellent examples. Following the uplift, the overlying sediments are eroded away and the Precambrian core is carved into a cluster of mountains by continued erosion. The layers of sedimentary rock that once covered the dome are still evident along its outer margin, usually forming "cuestas" that slope upward toward the mountains. The variable resistence of these sedimentary layers produces a topography of ridges and valleys that ring the dome of mountains. Satellite views of these areas illustrate the geomorphology especially well.
