The Rangeley Conglomerate

Photo 1. Conglomerate of the Rangeley Formation, south of Rangeley, Maine (larger image)

Introduction to Conglomerates

A conglomerate is a sedimentary rock that is made up of a collection of rocks that have been cemented together to make one composite rock. Conglomerates are fascinating because of the variety of features that can be seen in an outcrop without need of sampling, magnification, or laboratory analysis. The variety of shapes, sizes, colors, and textures make a conglomerate a great rock for the field naturalist to study.

The two essential parts of a conglomerate are the older rock fragments it contains, called clasts, and the sediment that fills in around the clasts, called the matrix. Careful study of the clasts and their relationship to the matrix gives clues to a three-stage geologic history. The kinds of rocks that make up the various clasts give information about things that happened before the conglomerate formed. The size and shape of the clasts, and the nature of the matrix give information about things that happened while the conglomerate was forming. Features that affect both the clasts and the matrix give information about things that happened after the conglomerate had formed. We will look at the Rangeley Conglomerate as an example of how to decipher this three-stage history.

Location

Location Map

The field site is an outcrop on the east side of Maine Route 4 that was blasted for the construction of Route 4. It is immediately south of Overlook Road, and about 0.8 miles north of South Shore Drive, which goes to the State Park. The outcrop is in Sandy River Plantation, south of the town of Rangeley (See Location Map). The conglomerate here is in the oldest part of the Rangeley Formation, named Member A (Moench and Boudette, 1987). Conglomerates also occur in other parts of the Rangeley Formation, notably in Member C. Not all these conglomerates are the same.

This site is described as the last part of Stop 4, field trip A-1, p. 17, by Boone and others (1970), and also as Stop 7 of the field guide by Moench and Boudette (1987).

Source Area

The clasts in the conglomerate consist of many different kinds of rock. They must have been eroded from older bedrock, and then transported to a place where they were deposited together. Looking at the kinds of rock in a conglomerate, therefore, gives a picture of the source area that was being eroded. The Rangeley Conglomerate includes a wide variety of rock types, including sedimentary, volcanic, and plutonic rocks (Photo 1, Photo 2, Photo 3). These rocks can be found intact to the northwest of Rangeley, in the present-day Boundary Mountains region. This means not only that the rocks to the northwest are older than the Rangeley Formation, but furthermore, that they had been uplifted and were being eroded at the time the conglomerate was being deposited.

Photo 2

Photo 3

Sedimentology

Sedimentology is the study of sedimentary rocks, including their description and origin. For conglomerates, the size, shape, and arrangement of clasts is a reflection of the way they were transported and deposited by water. Large pebbles and cobbles can be moved only in high-energy environments, such as in fast-moving streams or by storm waves. The Rangeley conglomerates at this site have a variety of clast sizes from layer to layer, some dominantly pebbles (Photo 4) and some dominantly sand (Photo 5). Still others are poorly sorted by size, and contain large and small clasts mixed together (Photo 1 and Photo 2). All of these factors together imply a dynamic environment with changing conditions.

pebble conglomerate showing strong alignment

Photo 4

Photo 5

Clast shape, and particularly the presence of sharp edges or corners, is a reflection of how long the clasts have been transported. During transport, clasts hit each other, progressively breaking off sharp corners and making the fragments more rounded. The conglomerates at this site include many angular to sub-angular grains, suggesting they have not traveled far (Photo 1 and Photo 2). As with size, though, the degree of roundness is not uniform, and there are many rounded clasts as well.

Photo 6

Finally, the arrangement of clasts in the matrix is worth noting. For sediment in which flowing water is the dominant transport mechanism, larger clasts generally rest directly on each other, packed tightly together, with finer-grained matrix filling the spaces (Photo 1 and Photo 2). In debris flows or slides, all particles flow together regardless of size, and the clasts are commonly supported by the matrix (Photo 5 and Photo 6).

Taken together, all these features indicate that this part of the Rangeley Formation accumulated in a dynamic environment in which a variety of older rocks to the northwest were being eroded, transported a short distance by fast-moving streams, and deposited near the edge of a marine basin by submarine currents and debris flows. The ages of rocks in the region indicate that this occurred in the Silurian Period of geologic time (approximately 435 million years ago).

Deformation and Metamorphism

Tens of millions of years later, in the Devonian Period of geologic time, all the rocks of central New England, including the Rangeley Formation, were involved in a significant mountain-building event. During this process, the rocks of the Rangeley Formation were altered by heat and pressure, a process of change called metamorphism. The most obvious features of metamorphism at this site are (1) the fact that the layers are tilted steeply (Photo 3 and Photo 5), and (2) that in many places the clasts and matrix have been stretched, flattened, or bent due to the heat and pressure (Photo 3, Photo 4, and Photo 6). These factors make it all the more remarkable that so many original features of the conglomerate have been preserved.

References

Boone, Gary M., Boudette, Eugene L., and Moench, Robert H., 1970, Bedrock geology of the Rangeley lakes-Dead River basin region, western Maine. In Boone, Gary M. (editor), Guidebook for field trips in the Rangeley lakes Dead River basin region, western Maine: New England Intercollegiate Geological Conference, Trip A-1, p. 1-24. (Note: the entire guidebook is available for download from the UNH library.)

Moench, Robert H., and Boudette, Eugene L., 1987, Stratigraphy of the Rangeley area, western Maine. In Roy, David C. (editor), Geological Society of America, Centennial Field Guide, v. 5 (Northeastern Section), p. 273-278.

Osberg, P. H., Moench, R. H., and Warner, Jeffrey, 1968, Stratigraphy of the Merrimack synclinorium in west-central Maine. In Zen, E-an, White, Walter S., Hadley, Jarvis B., and Thompson, James B., Jr. (editors), Studies of Appalachian geology, northern and maritime: Interscience Publishers, New York and London, p. 241-253.

Smith, Edward S. C., 1923, The Rangeley conglomerate: American Journal of Science, 5th series, v. 5. p. 147-154.

Text and photos by Henry N. Berry IV.

Originally published on the web as the December 2012 Site of the Month.

Last updated on January 4, 2013