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La Verna Preserve, Bristol
Along the southern Maine coast between Casco Bay and Penobscot Bay, a series of irregular, rocky peninsulas jut seaward south of Route 1. On one of these peninsulas, between Damariscotta and Waldoboro, lies the town of Bristol (with South Bristol to its west). Certainly the most popular destination for visitors to Bristol is the town park at Pemaquid Point, with its picturesque lighthouse and scenic ledges. A lesser known, but more expansive shoreline site of geologic interest is the La Verna Preserve, on Rt. 32 south of the village of Round Pond. (See brochure with directions.) The La Verna Preserve includes several miles of walking trails that lead to the shoreline of open ledges with ocean views toward the east (Photo 2). The 120-acre Preserve is one of the properties of the Pemaquid Watershed Association and is open during daylight hours for public enjoyment (Photo 3).
The most abundant type of rock along the shore of the Preserve is metamorphic rock, which means rock which has been transformed in a significant way by heat and pressure at some time in its history. Metamorphic rocks come in many varieties of color, composition, and texture. Most of the rocks here are thinly layered, with the layers tilted in various directions (Photo 5). A close look (Photo 6) shows that each layer is composed of somewhat different proportions of minerals, so that layers with more light-colored minerals (primarily quartz and feldspar) have a lighter gray color, and layers with more dark-colored minerals (primarily biotite, or black mica) have a darker gray color. Some layers even have a green mineral (diopside) mixed in, giving those layers a light greenish hue. It is the different proportions of minerals that give each layer its own characteristic composition and color. These layers are interpreted to have originated as sedimentary layers, probably sandstones and siltstones with small amounts of clay or lime mixed in. While the original minerals and textures have been largely transformed by the metamorphic process, the layering has survived.
A second variety of metamorphic rock, noticeable along the southern part of the Preserve shoreline, contains a small amount of iron sulfide which weathers to produce a reddish-brown or orange-brown rusty surface (Photo 7). A closer view of this rock (Photo 8) shows that it does not have a well layered structure, and its mineral composition (micas and sillimanite) is different from that of the more common layered gray rocks described above. These rusty weathering rocks probably represent a thick layer of shale or mudstone that has been converted by the metamorphic process into the schist we see today.
The most striking feature of the metamorphic rocks is that the layers are not lying horizontally, but are steeply tilted (Photo 5) and even folded (Photo 9, Photo 10, Photo 11). This pattern of complicated shapes was produced during the metamorphic process. Intense pressure was exerted on the rocks for a long time while they were hot enough to deform internally by bending and buckling. At lower temperatures, rocks under such pressure would fracture and break rather than bend and fold. The resulting structure of the rock, in addition to being amazingly beautiful, requires an amazing geologic history. These rocks at one time were hot and deep in the earth - probably 550º to 600º C and two to three thousand times atmospheric pressure (several miles depth).
The second type of rock that makes up the coastal bedrock of the Preserve is igneous rock. In distinction from metamorphic rocks, which were transformed in a solid state, igneous rocks form by solidification of melted rock. The molten rock may flow through cracks in the solid metamorphic rock, then cool and solidify into igneous rock. The result of this process is illustrated in Photo 12 which shows an igneous rock with fairly straight sides cutting through a folded metamorphic rock. In some cases the edge of the igneous rock has a more irregular shape, where the molten rock did not follow a single clear pathway through the metamorphic rock (Photo 13).
While some of the igneous rock layers cut diagonally across the metamorphic rock layers (as in Photo 12), most of the igneous rock is in sheets that are parallel to the metamorphic rock layers. Where the sea has begun to erode blocks of metamorphic rock, it is clear that the white igneous rock forms a three-dimensional sheet that extends below ground, at the same angle as the metamorphic rock layers (Photo 14). It is this parallel alignment of the main structural features of bedrock that causes the long ridges and valleys of this region - and much of the coastline - to be aligned in a north-south direction.
Most of the igneous rocks in the Preserve are a variety of granite called pegmatite. It is a snow-white rock in which the mineral grains are quite large. Whereas the size of the mineral grains that comprise typical granite may be a tenth to half an inch across, the mineral grains in pegmatite may be several inches across (Photo 15).
Deformed Igneous Rocks
Many of the igneous rocks appear to have been preserved essentially as they formed. Some of them, however have been folded (Photo 16), stretched into lumpy segments (Photo 17), round blobs (Photo 18), or completely separated (Photo 19). Where the igneous rock has been stretched apart, the metamorphic rock curves into the space between the lumps. This is because the two kinds of rock behaved differently under pressure. The pegmatite is more rigid, and tends to pull apart at particular points, whereas the metamorphic rock is more ductile, and behaves more like a fluid. By studying complex rocks like these that have been subjected to stress at depth in the earth and then exhumed, geologists can learn how the earth's crust is behaving now, beyond the reach of the deepest drill hole.
More Recent Geologic Features
The layered bedrock originated as sediment in a marine basin during the Silurian Period of geologic time, approximately 430 million years ago. This particular set of deposits has been mapped as the Bucksport Formation, which extends from the Boothbay-Pemaquid region northeastward to the town of Bucksport. The time of subsequent metamorphism and the formation of the igneous rocks are not known precisely, but research in the surrounding region suggests it was probably in the Devonian Period, between about 420 and 370 million years ago.
In contrast, a continental ice sheet last modified the New England landscape during the last Ice Age, from about 25,000 to 14,000 years ago (more). Notice that we're now talking about thousands of years rather than hundreds of millions. And the current shoreline is even younger, having moved and changed significantly as sea level has risen in Maine approximately 150 feet in the past 11,000 years. So while the rocks are very old, the surface exposure is quite new. Some of the evidence for the recent and ongoing activity includes large boulders left by the glacier (Photo 20), and jumbled blocks of loose rock in the surf zone (Photo 21).
Note to Visitors
Hussey, A. M., II, and Marvinney, R. G., 2002, Bedrock geology of the Bath 1:100,000 quadrangle, Maine (pdf format - 4.9 MB). Maine Geological Survey, Geologic Map No. 02-152. (Bedrock geologic map of the Pemaquid region)
La Verna Preserve brochure, Pemaquid Watershed Association.
Field work and photos by Henry N. Berry IV and Susan S. Tolman.
Originally published on the web as the September 2011 Site of the Month.
Last updated on April 11, 2012
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