The higher elevations are mostly either Archean or Paleozoic formations projecting above Tertiary deposits.
The crystallines are confined to the portion of the belt east of the Great Valley where Paleozoic rocks are always highly metamorphosed and occur for the most part in limited patches, excepting in New England and Canada, where they assume greater areal importance, and are besides very generally intruded by granites.
The Paleozoic sediments, ranging in age from Cambrian to Permian, occupy the Great Valley, the Valley Ridges and the plateaus still farther west.
Recession of the plateaux margins has exposed underlying Mesozoic strata and, in some areas, the Paleozoic basement.
The southernmost mountain system of the Anti-Atlas thus consists of crystalline basement and Paleozoic sediments.