The present San Juan Basin, a Loromide structural feature, has as its eastern boundary the Nacimiento and Son Pedro Mountains an uplift some 45 miles in length and about 15 miles wide, with a fault of several thousand feet displacement marking the division between uplift and basin. The general Nacimiento-San Pedro Mountain area was uplifted during parts of the Paleozoic; this ancestral positive feature is referred to as the Penasco axis or highland. The present expression of the Nacimiento-San Pedro Mountains had its beginning during the Laramide revolution.