Stretched Metamorphic Rocks, Friendship Boat Launch

Introduction

Friendship (Figure 1, Location Map) is a small Maine town of about 1200 people nestled among the peninsulas of the mid-coast region. The village sits on a low ridge looking southeastward over Friendship Harbor. Bradford Point Road leaves Rt. 97 just north of the village and runs around the east side of the harbor, ending at a small hand-carry boat launch (Figure 2). This peaceful spot is our geologic site of the month.

Location
Figure 1	Location Map

Friendship Boat Launch
Figure 2

If you plan to visit, please keep in mind that, while public access is allowed, there is only enough parking beside the road for a few cars, so be considerate of those wanting to use the boat launch. We also implore visitors not to intrude on the lives or private property of people who live next to the boat launch. This is a real town, not a tourist stop.

Rock Structure

From a distance (Figure 3A), the strongly layered structure of the rock makes it clear that this is not granite. The rock has an internal grain, or foliation, that causes it to break easiest along one direction into flat slabs or sheets. In the intact bedrock, the foliation is inclined about 60 degrees from the horizontal, tilted down toward the southeast.

A closer look at the face of one of the foliation surfaces (Figures 3B and 3C) shows an even more striking feature of the rock structure. The light and dark colored areas of rock are drawn out into long, thin streaks. This feature, of elongated rock streaks, is called lineation (lin' ee ay' shun).

Foliation and lineation are identifying properties of metamorphic rock. They form in response to pressure when rocks are heated to high enough temperatures that they can be flattened and stretched, but not to high enough temperatures that they melt. For these particular rocks, the temperature was probably between 1000 and 1200 degrees Fahrenheit for a million years or more, at significant depth in the earth's crust. Such heat and pressure over time cause rocks to change, and they take on new properties such as foliation and lineation. As the metamorphism becomes more extreme, the original features of the rock become correspondingly more obliterated.

The beautiful character of the rock at this site is due to its metamorphic structure.

Rock Structure
Figure 3

Rock Composition

Figure 4

Several interesting features give clues to what the rocks may have been before the metamorphic event. Hunting for such clues requires looking in various places and from different angles (Figure 4). Some of the dark-colored layers have black mineral grains of hornblende (Figure 5A) that are large enough to see with the naked eye. Actually, the mineral hornblende is abundant in these outcrops, but most is in small grains that require magnification to be seen. Hornblende is a mineral typical of metamorphic rocks derived from the volcanic rock basalt. Therefore, we infer that in their early history (before metamorphism) these rocks probably started out as volcanic rocks.

A second type of rock (Figure 5B), in layers interspersed with the dark volcanic rocks, contains streaks of a pale apple-green mineral (diopside) and a cinnamon-brown mineral (grossular garnet). A rock with these minerals is typically produced by metamorphism of sandy limestone, a marine sediment that was presumably interspersed with the volcanic rocks at the time they formed.

On some upper surfaces (perpendicular to the lineation) there are pebble-sized blobs of light-colored rock (Figure 5C) embedded in darker gray rock. These blobs are thought to be remnant volcanic rock fragments of a different composition than the enclosing basalt. In most places at this site, the metamorphism has so distorted them so as to be unrecognizable streaks, but their original shapes are fairly well preserved in a few places.

Rock Composition
Figure 5

Late Stages of Rock History

Thin white veins (Figure 6A) and thicker granite dikes (Figure 6B and 6C) cut across the metamorphic foliation, indicating these features were formed at some time after the major metamorphic event (since they are unaffected by it). The final stages of the history, to cool, uplift and erode the overlying rocks, and to form the present landscape, were less dramatic events from the standpoint of the rock.

Late-Stage Features

Figure 6

A Sense of Time and Space

Figure 7

A bedrock map (Figure 7) of the Friendship region shows that the volcanic unit exposed at the boat launch is believed to be of Ordovician age (Hussey and Marvinney, 2002). This is indicated by the map label "Ouv," where the capital O stands for the Ordovician Period of geologic time, and "uv" stands for "unnamed volcanics." This age was when the original volcanic rocks and sediments formed, in an ancient ocean basin that has since been closed by geologic processes. The major metamorphic event occurred during a time of protracted regional mountain-building that eventually formed the Appalachian system of eastern North America. At that time, the rocks we see here at the surface were carried down to significant depths (a few miles or so) and heated to high temperatures. The age of this metamorphism has not been established for the rocks here in Friendship, but metamorphic events in this part of Maine have been dated to the late Silurian and early Devonian Periods, ranging from 380 to 420 million years ago (West and others, 1995). The thin seams of granite may be offshoots of the large Waldoboro granite (see bedrock geology map - Figure 7), which has been dated at 368 (±2) million years old (Tucker and others, 2001). The gradual erosion of the overlying rock, to bring these rocks to the present land surface, occurred during the remaining few hundred million years.

References

Hussey, Arthur M., II, and Marvinney, Robert G., 2002, Bedrock geology of the Bath 1:100,000 quadrangle, Maine (pdf format): Maine Geological Survey, Open-File Map 02-152.

West, David P., Jr., Guidotti, Charles V., and Lux, Daniel R., 1995, Silurian orogenesis in the western Penobscot Bay region, Maine: Canadian Journal of Earth Sciences, v. 32, no. 11, p. 1845-1858.

Tucker, Robert D., Osberg, Philip H., and Berry, Henry N., IV, 2001, The geology of a part of Acadia and the nature of the Acadian orogeny across central and eastern Maine: American Journal of Science, v. 301, no. 3, p. 205-260.

Last updated on March 3, 2006