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Beach Pebbles Tell a Story
The smooth, sandy beaches of southern Maine are popular with summer sun-seekers. In contrast, most beaches along the middle and eastern Maine coast are made of stones. This month we look at a typical small, uninhabited island off the Maine coast with a well-preserved gravel beach. By looking closely at the shape of the beach and at the stones themselves, we can learn about the coastal processes that continually shape and re-shape beaches in Maine.
A Look at The Beach
The beach is at one end of the island sheltered by the main part of the island to one side, and protected by small bedrock knobs to the other (see map of the island (Figure 1) and photo of the beach (Figure 2)). This means that waves cannot move the stones very far to either side where they might be washed out to deeper water. Instead, the stones are caught in a "pocket" framed by bedrock. As waves continually wash the stones up the sloping beach and roll them back down again, the stones (Figure 3) become naturally rounded and polished. Believe it or not, geologists use particular names for stones depending on their size. The term "pebble" is precisely defined as a stone between 4 millimeters and 6.4 centimeters across. So technically, this is a pebble beach.
The stones on the beach are neatly arranged in a series of flat-topped deposits, (Figure 4) called storm berms, that are stacked on each other. A view looking back along the beach (Figure 5) from the end of the point shows that each berm has a steeper lower slope, a flatter upper slope, and a line of seaweed and other debris along its top edge. The next higher berm then repeats the shape: steeper lower, flatter higher, with debris on top.
A straight-on view of the beach face (Figure 6) shows another interesting feature. In the lower, steeper part of each deposit the stones are slightly smaller than the stones in the upper, flatter part of the deposit. Then, above the line of seaweed, the lower part of the next deposit again has smaller stones. The differences are subtle, but consistent all along the length of the beach. Furthermore, the stones on the steeper, lower slope are quite round, whereas there are several flat stones on the flatter top area (Figure 7).
So How Was The Beach Shaped?
Each storm berm represents a deposit from a single storm. As the waves break, they release enough energy to scour rocks from the base of the beach and throw them up on the shore. As the tide increases, the waves reach higher and higher levels, pushing stones, seaweed, and driftwood up the beach. As each wave crashes it propels large and small stones up the beach, but as it recedes, it has less energy and can only pull the smaller and rounder stones back down the slope. In this way, the larger, flatter stones are sorted out from the smaller, rounder ones, ending up at the top of the deposit. As the height of the storm passes and the tide begins to fall, the large stones and floating seaweed are stranded at the height of the deposit.
If the next storm is larger than the first one, then the older berm will be wiped out and the stones will be re-formed into the new one. Only the deposit from the larger storm will be preserved. On the other hand, if the next storm is smaller than the first one, only the lower part of the first berm will be affected, and the upper part will be preserved. In this case, the younger, lower berm will rest partway up the older, higher berm. On the beach we are looking at, there are at least five berms preserved, with the highest one being the oldest, and the lowest one being the youngest. Notice that the highest berm is about 15 feet above normal high tide, which would have flooded the whole beach. Most storms of this magnitude in Maine occur in the winter months. Some of the higher berms may be many years old.
Where did the Stones Come From in the First Place?
On this particular beach, any handful of stones is likely to include several different types of rocks. For example, some stones are dark-colored volcanic rocks, (Figure 8) some are light-colored volcanic rocks, (Figure 9) some are volcanic rocks containing broken rock fragments, (Figure 10) and some are granite of different types. (Figure 11) The obvious place for the stones to have come from is the bedrock next to the beach. After all, every stone was once part of some bedrock before it was broken loose.
The bedrock (Figure 12) next to the beach is dark-colored volcanic rock It is broken by many cracks, so it probably has broken apart to supply some stones to the beach. But most of the stones on the beach are different from the bedrock on the island, so they must have come from somewhere else. In fact, there are more prominent out-of-place rocks on the beach, (Figure 13) and on the bedrock next to the beach, (Figure 14) namely, boulders of granite. These boulders, like thousands and thousands of boulders strewn across Maine, were deposited by the glacier that covered New England in the last Ice Age. About 14,000 years ago, as the ice melted, it dropped a mixture of stones of all shapes and sizes across the landscape. The stones that make up this island beach must have been part of a glacial deposit that has since been washed by modern ocean waves.
So What's the Story?
By looking carefully at the shape and elevations of the beach deposits, the sizes of the stones, and the kinds of stones on the beach, we can deduce that:
This small pocket beach is one of hundreds on the Maine coast. Each one has a similar story, although the details may be different.
Further reading on stony beaches:
Shape evolution and fabric in a boulder beach, Monument Cove, Maine, by Charles J. Waag and David E. Ogren. Journal of Sedimentary Petrology, volume, 54, number 1, 1984, p. 98-102.
Orientation of cobble and boulder beach clasts, by David E. Ogren and Charles J. Waag. Sedimentary Geology, volume 47, 1986, p. 69-76.
Morphology and stratigraphy of small barrier-lagoon systems in Maine, by William Duffy, Daniel F. Belknap, and Joseph T. Kelley. Marine Geology, volume 88, 1989, p. 243-262.
Don't touch that rock! - Maine's cobblestone beaches, by Tammis Coffin. Coastal Journal, July 1986, pp. 30-31, 58-60.
Originally published on the web as the September 2000 Site of the Month.
Last updated on October 6, 2005
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