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Settlement Quarry, Stonington, Maine
Between Penobscot Bay and Mount Desert Island on Maine's mid-coast lies the island of Deer Isle. It can be reached by car on Maine Route 15, almost an hour south of Route 1. Most of the island, including the town of Stonington, and many of the smaller islands to the south and east are underlain by a large body of granite known as the Deer Isle pluton. Over the past two centuries, a large amount of granite has been quarried from several places in this pluton. One quarry, no longer active, is the Settlement Quarry, on Oceanville Road. In 1996, the Island Heritage Trust purchased the quarry property and has developed a network of walking trails and informational signs that make this a wonderful place to learn about the geology and history of Maine granite.
A small dirt parking area (Figure 1) accommodates several cars. A kiosk at the entrance provides a trail map (Figure 2), information about the site (Figure 3), and a guest book for visitors (Figure 4).
During the last Ice Age (about 15,000 to 25,000 years ago), a continental glacier covered the whole of New England, and moved slowly across the landscape far into the Gulf of Maine. Evidence of this glacier activity is preserved at the Settlement Quarry site. The Glacial Erratic Trail (Figure 5) leads over the north slope of the hill, which is still in its natural state, unaffected by the quarrying on the south side of the hill. Most of the exposed bedrock and loose stones are made of pink granite like the rock in the quarry. A few stones, however, are not. One rounded boulder in the trail (Figure 6) consists of a fine-grained igneous rock called hornblende diorite (Figure 7). Another, very large, angular boulder near the far end of the trail is marked by a sign (Figure 8). It is made of a rock with a strong layered or foliated fabric, very different from the uniform texture of granite (Figure 9). This is not an igneous rock, but a metamorphic rock which belongs to the Ellsworth Formation. This large boulder is out of place; it did not come from the bedrock here. Such an out-of-place boulder is called an "erratic." The nearest bedrock outcrop of the Ellsworth Formation is about 4 miles away, at the northern side of Deer Isle. This erratic was frozen into the ice of the glacier and carried for many miles until the ice finally melted, leaving this erratic resting on granite.
Closer to the parking area, off the main quarry road, a sign directs you to an area of flat bedrock whose surface is marked with grooves and scratches (Figure 10). These are natural features caused by stones frozen in the base of the ice being slowly dragged along the underlying rock surface. The direction of the grooves and scratches indicate the direction that the glacier ice was moving, in this case about 30 degrees east of South, which is typical for the ice flow directions measured along this part of the Maine coast.
Another remnant of the last Ice Age is the stony, gravelly deposit which rests upon the bedrock and beneath the topsoil (Figure 11). This deposit, which blankets the region, is called "till." Till is the rocky material that was left behind when the ice melted, and is characterized by a wide variety of rocks of different shapes, sizes, colors, and rock types.
The granite of the Settlement Quarry is a light pinkish color, which is characteristic of the Stonington area quarries. The color and texture have made this stone distinctive and desirable around the world. A typical view at arm's length (Figure 13) shows that the individual mineral grains are about the size of a dime, with a few larger ones scattered about. A close-up (Figure 14) shows the three main minerals of granite - two types of feldspar and quartz - in roughly equal proportions. The pinkish color of the rock comes from the pink feldspar. White feldspar occurs both as separate grains and also as thin overgrowths on pink feldspar (Figure 15). Quartz is gray and transluscent. A small amount of black mica (biotite) is sprinkled through the rock, nestled between the other minerals. A few of the white feldspar grains have tiny bits of biotite and quartz inside them, suggesting they had a more complicated history of formation as the molten granite was solidifying (Figure 16).
Enclaves of Dark-colored Rock
At several places in the quarry, small blobs of dark-colored rock are embedded in the granite (Figure 17, Figure 18). These have been called "enclaves" because they have a different composition from the granite, and are scattered through the granite, but retained their identity and did not become mixed with the granite. These enclaves were called "knots" by early quarry workers, because they can cause the rock to split unevenly, much like a knot in wood. It is only recently, however, that their origin has been understood as remnants of a different magma that intruded into the granite magma when they both were still molten. Compelling evidence that they were both molten at the same time is provided by the presence of feldspar grains of the granite being completely engulfed within enclaves (Figure 19). This could only happen if the enclave was not yet completely solid at the time the granite minerals had formed.
Another type of rock in the Settlement Quarry is a fine-grained, smooth-looking pink rock called "aplite." It occurs in long, thin sheets that cut through the granite (Figure 20). These sheets are called "dikes." The aplite is composed of the same minerals as the granite, but the mineral grains are very much smaller, less than a millimeter across (Figure 21). Aplites are interpreted to represent the final stages of crystallization of the pluton, when small pockets of remaining molten rock were injected through the mass of granite which had mostly crystallized. Some aplites, especially the larger ones, can affect the quarrying, not only because the rock looks different from the granite, but also because the aplites can be more strongly fractured and difficult to work (Figure 22).
Joints and Fractures
For generations, the quarrying of granite blocks was done by splitting the rock along natural fractures or planes of weakness. While modern quarrying methods may include actually cutting through solid rock with wire saws or torches, much of the stone in the Settlement Quarry was removed by drilling and blasting along natural fractures, or joints. The most important joints are the horizontal ones, which break the rock into sheets (Figure 23). The spacing of the sheeting joints determines the thickness of blocks that can be removed (Figure 24). Workers then drill sets of vertical holes a certain distance back from the quarry face, and in the direction of the rock's grain. Small explosive charges in the holes then break the blocks free. At many places in the quarry, sets of three drill holes can be seen, both in the quarry wall, and in blocks that were excavated (Figure 25). Many other blocks left at the site have irregular shapes where the rock did not split evenly (Figure 26). The pier at the waterfront, visible from the Settlement Quarry, is still used for transportation of granite from other active quarries in the area (Figure 27).
Stewart, David B., and Tucker, Robert D., 1998, Geology of northern Penobscot Bay, Maine: U.S. Geological Survey, Miscellaneous Investigations Series Map I-2551, 2 sheets, map scale 1:62,500. [Shows extent of Deer Isle pluton, and reports the Devonian age of the granite as 371 ± 2 million years old.]
Hooks, Ben P., Lux, Daniel R., Gibson, David, and Hogan, John P., 2010, Felsic magma chamber dynamics: A process-oriented interpretation of the mineralogical and textural diversity of outcrops within the Deer Isle Granite Complex, Deer Isle, Maine, in Gerbi and others, editors, Guidebook for field trips in coastal and interior Maine: New England Intercollegiate Geological Conference, 102nd annual meeting, University of Maine, Orono, p. 1-14.
Text and photos by Henry N. Berry IV
Originally published on the web as the June 2011 Site of the Month.
Last updated on July 25, 2011
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