Discussion

The Geological Map

This map shows the distribution of the main rock formations and rock types in the Great Lakes region.

It is a compilation map constructed from maybe hundreds of more detailed maps by many Canadian and American geologists.

The Rock-Time Chart

A rock-time chart shown here differs from a normal map legend in that it attempts to illustrate those periods for which there no known rocks. These rocks may be missing because they were eroded by rain, wind or ice when the region was above sea-level. These periods of of no known rock deposition are shown in grey on the accompanying rock-time diagrams.