 The uplift created two large mountainous islands, known to geologists as Frontrangia and Uncompahgria, located roughly in the current locations of the Front Range and the San Juan Mountains.They consisted largely of Precambrian metamorphic rock, forced upward through layers of the limestone laid down in the shallow sea.The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving extensive deposits of sedimentary rock.