The Geology of Baxter State Park and Mt. Katahdin

Structural geology

The branch of the subject called structural geology is partly concerned with the positions and conditions of rocks, such as the jointing and layering which have been previously discussed, and the interpretation of these structural features as they relate to geologic history. In the discussion of the sedimentary rocks exposed on South Branch Pond Brook, it was shown that the present attitude of these rocks is the result of tilting and erosion of originally horizontally layered rocks. The geologic cross-section, Plate 1C, shows that to the north, just south of Wadleigh Mountain, these same rocks are tilted toward the south, forming a fold or bend in the rocks. Many geologists consider that such folds are the result of compressive forces in the earth's crust which accompany the formation of mountains. Layering, originally horizontal but now inclined, is shown in the exposures at the gorge below Ripogenus Dam and along the road north of Wadleigh Mountain in the Grand Lake Matagamon area.

If the compressive forces which produce folds in layered rocks exceed the strength of the rocks, the rocks are broken along faults. Active or recently active faults, such as the San Andreas fault in California, may be recognized by the obvious displacement of such surface features as streams, roads, and fences and by earthquakes which are produced by the release of energy along the faults. Ancient faults, however, are often difficult to recognize, and their existence may be shown only after careful geologic field work. The work of Rankin in the area north of Trout Brook indicates that the sedimentary and volcanic rocks exposed there are broken by several faults, two of which are shown on the geologic cross-section, Plate 1C. It may be assumed that the forces which produced the folded structures in the layered rocks were also responsible for the faults in those same rocks.

The folding and faulting of the rocks in the Mt. Katahdin area evidently occurred before the intrusion of the Katahdin granite, because microscopic examination of the mineral grains in the granite indicates that no great compressive force has acted on the granite. Had the granite been subjected to the same force which produced the folds and faults in the nearby layered rocks, mineral grains such as quartz in the granite would be crushed and strained. The lack of evidence of strain in the granite indicates the intrusion of the Katahdin granite occurred after the folding and faulting of the layered rocks.

Introduction Bedrock Glacial geology Geologic features Acknowledgments Glossary References Plates

Last updated on January 11, 2008

Contents

The Geology of Baxter State Park and Mt. Katahdin

Structural geology