Proterozoic Geology (~1.7 Billion to 544 Million Years Ago)
The oldest bedrock in the greater Phoenix area is principally composed of rocks of the Proterozoic (1.7-1.4 billion years ago). Two distinct ages of Proterozoic rock are present. The earlier Proterozoic rocks are various metamorphosed sedimentary, metamorphic, and igneous rocks (both volcanic and intrusive).These rocks are present in all of the major mountain ranges in and around Phoenix (the Sierra Estrella, McDowell, Phoenix, San Tan, South, and White Tank Mountains). This entire sequence of rocks has undergone at least one episode of regional metamorphism that is recorded as large folds in the McDowell and Phoenix Mountains. An additional pulse of intrusion of igneous rocks with compositions ranging from diorite to granite - the "hump" of Camelback Mountain is composed of rock formed during this pulse - occurred in later Proterozoic time (approximately 1.4 billion years ago). Contact relationships indicate that the granitic intrusions happened after the regional metamorphism that folded preexisting rocks. The plate tectonic setting interpreted for the formation of the Proterozoic rocks is one similar to that currently found in association with the Andes of South America - oceanic crust being subducted beneath continental crust in a southeasterly direction with attendant volcanism, intrusion of magmas, and folding of sediments shed into the subduction zone.

Text by J R.Arrowsmith & W. L.Stefanov photos by W.L.Stefanov