Glacial Erratics

Bedrock Base Map Location 4

Glacial erratics are surface rocks that differ from the type of rock native to the area in which they rest. They are transported long distances by glaciers, and are deposited upon melting. The rocks in this area were extracted from the Ray Road Quarry in Oxford, Michigan. Originally, these rocks were plucked by glaciers from the igneous and metamorphic bedrock of the Superior Province of the Canadian Shield in Ontario, which ranges from 2.5 to 4 billion years old. Ultimately these ancient rocks traveled over 400 miles south by flowing ice and were deposited alongside sand and gravel in southeast Michigan during the last ice age, roughly 16,800 years ago. The underlying sedimentary bedrock of the southern peninsula directly beneath these exotic surface rocks is much younger, ranging from 300 to 400 million years old.

These once deeply buried rocks formed from the slow underground cooling and crystallization of magma resulting in igneous rocks such as granite. Some of the of the more deeply buried rocks were the roots of ancient mountains that recrystallized under conditions of high pressure and temperature, forming metamorphic rocks such as amphibolite, schist, gneiss and migmatite. The latter rocks were buried to depths of over 20 miles beneath Himalayan-sized mountains that have since eroded away.