Geology of Augusta's Third Bridge Access Ways

After years of planning followed by years of construction, Augusta's Third Bridge was opened in November 2004 to whisk traffic across the Kennebec River to Belfast and points beyond. Although the highlighted benefit of the third bridge is the improvement of traffic on congested downtown streets, an added benefit for geologists has been the creation of several large road cuts that display the contrasting geology of the Capital area (Figure 1). As testament to Maine Department of Transportation traffic forecasting capabilities, please note that in spite of having been open only a few months at the time of this writing, the new bridge already experiences heavy traffic. If you build it, they will come! Viewing the exposures here should be done only with a careful emphasis on safety.

General geology of the Augusta area

The bedrock geology of the Augusta area is the subject of an on-going mapping project supported in part by the National Cooperative Geologic Mapping Program. Although a detailed bedrock geologic map of Augusta and vicinity is still a few years off, early reconnaissance mapping provides a general framework for the current study. From this earlier work, geologists have established that the greater Augusta area is underlain with a variety of metamorphic rocks and granites that intruded them. The strongly layered metamorphic rocks were originally layers of sand and mud that were deposited in an ocean basin around 430 million years ago. A continental collision that produced the Appalachian Mountains around 400 million years ago destroyed the basin between what was then the eastern margin of North America and a small continental landmass in the ancestral Atlantic Ocean. Heat and pressure associated with this event metamorphosed the original sedimentary material. Sand layers became granofels, a variety of metamorphic rock similar to gneiss with more or less equally sized crystals of quartz, feldspar, and mica, but lacking the prominent banding. Mud layers became schist, a metamorphic rock characterized by strongly aligned mica crystals. The forces of continental collision also folded the rocks so that the layering, presumed originally to have been nearly horizontal, is now tilted on edge. Into the metamorphosed and tilted layered rock rose a magma produced during the continental collision by melting some of the lower crust. This magma forced its way into the metamorphic rocks and eventually cooled to form small granite bodies.

Metamorphic Rocks

Our geological tour begins on the west side of the river where a very large road cut exposes about 1,000 feet of layered metamorphic rocks. The best exposures are those closest to the western end of the bridge.

Granite

The roadcuts on the eastern side of the access road, east of Route 201, are in granite. Unfortunately the contact between the granite and the metamorphic rocks is not exposed in this area. The granite is a small body, underlying only the small hill through which the road cuts.