Geology near the Augusta Civic Center

Augusta has many generally accessible localities to examine many aspects of local geology. One such locality is within and around the large retail complex associated with the Augusta Civic Center. With many large parking lots, the area is generally accessible for viewing some interesting aspects of Maine's geology.

Figure 1

For those who have been following these Site of the Month entries for the last few years, it should be no surprise to learn that most of the state is underlain with metamorphic rocks and igneous rocks that intrude them. Metamorphic rocks, of course, are those that originally were another kind of rock and were changed by pressure and temperature to become the rocks exposed at the surface today. Gneiss and schist are examples of metamorphic rocks common in Maine. Igneous rocks are those that form when molten lava (or magma) is injected into the crust where it cools or spills out on the surface of the earth. Igneous rocks that form from lavas cooled within the crust we collectively call intrusions, and those that form from lava on the surface we call volcanic rocks. Granite and gabbro are examples of intrusive igneous rocks common in Maine.

Near the Augusta Civic Center (Figure 1) are excellent exposures of the metamorphic and igneous rocks that are found throughout this area of the state.

Metamorphic Rocks

Figure 2

Much of central interior Maine is underlain with metamorphic rocks that had their origins as sediments in a deep ocean basin about 430 million years ago. What began as layers of sand and mud in that basin has been metamorphosed through heat and pressure associated with a continental collision that produced the Appalachian Mountains 400 million years ago. The sand layers became gneiss (or in this example, granofels, which is like gneiss except it lacks banding) and the mud layers became schist. The forces of continental collision also folded the rocks so that they are now tilted on edge. At the outcrop shown in Figure 2 you can see the edges of the layers and that the lighter colored granofels layers stand up a little higher because they are more resistant to erosion. The darker schist layers erode more easily.

Igneous Rocks

While there is great variety in the igneous rocks exposed throughout Maine, a primary type in the Augusta area is granite (Figure 3). Granite is an intrusive igneous rock. That is, a molten lava intruded into the crust at depth and cooled there to form solid rock. Eons of erosion have now exposed the granite at the surface. In this particular granite, the primary minerals quartz and feldspar are nearly equal in size, giving the granite a texture that geologists describe as equigranular (Figure 4). The granite here is also interesting because it contains two kinds of mica - the white mica is called muscovite and the black mica is called biotite. By mapping around the area, geologists have established that the granite cuts across the metamorphic rocks, indicating that the granite is younger. It's around 400 million years old.

Figure 3

Figure 4

The exposures of granite here are full of fractures that are nearly parallel to the surface of the earth. There are a few vertical fractures as well. These fractures are the primary routes taken by groundwater as it flows through the rocks. When a driller drills a well into the solid bedrock, the water in the well comes from these fractures. In these outcrops, ice forms in the winter where groundwater flows to the surface along these fractures, clearly showing which ones are the important water-bearing fractures.

Glaciation

Figure 5

Fast forward to about 1 million years ago. At that time, these outcrops were covered by the earth's greatest geological agent of erosion - glacial ice. The best evidence for this can be seen on the outcrop of metamorphic rocks. This outcrop shows striations that formed when rocks held in the bottom of the glacier scraped along the rock surface from northwest to southeast (Figure 5). Striations like this are common throughout Maine and are part of the evidence for the great Ice Age.

Text and photos by R. Marvinney

Originally published on the web as the February 2003 Site of the Month.

Last updated on October 6, 2005