This field guide aims to give anyone interested in geology and Isle Royale an interpretation of things that can be seen outside in this unique National Park. We try to avoid jargon, in spite of some of the words above on this page. Each part of the Earth’s surface offers part of the evidence of past events for us to interpret. On Isle Royale we see rocks which reflect Earth about 1.1 Billion years ago, and we can interpret what this rock record means. These interpretations are speculative and they evolve constantly, reflecting new observations. This field guide is an update of a guide from 1994. One very important source is a geologic map done by N. King Huber of the US Geological Survey. On this geologic map of Isle Royale, this geologic map (see Welcome page) the western part is mostly tan, and the eastern part is mostly green. The colors reflect glacial outwash gravels and moraines that mostly bury the bedrock in the west, while those materials are absent in the east. Because of our interest in the rift lavas, this trip focuses on the eastern part of Isle Royale, which has only minimal glacial cover, although we do pass through Washington Harbor and part of the western portion.