How do Square Blocks become Round Rocks on Mount Desert Island?

Boulder covered beach at Monument Cove NPS/S.Stalker

As you travel around Mount Desert Island you will encounter long steep sided mountains, many with mounds of angular boulders at their base and on their slopes. They offer a dramatic view of several geologic events that have happened here over a span 450 million years.

These events began with plate tectonics, volcanism, followed by stream erosion and more recently by a period of glacial activity.

All these events over that long span of time have left us with a fractured island of mostly granite rock that has slipped, slid, and fallen from the steep valley sides carved by glaciers. Many of these blocks of granite have tumbled down along the shores of the island and then another process begins, weathering.

Mount Desert Island has a tidal range of 10 to 11 feet meaning the waves of the ocean act on these blocks of granite. During storms out in the Atlantic Ocean these waves can be very powerful pounding the rocks against one another breaking the edges gradually rounding them over time.

Waves breaking on boulder covered beach. NPS/S.Stalker

Definitions:

Boulder: a rock fragment with size greater than 256 millimeters (10.1 in) in diameter. (Larger than a basketball)

Cobble: a rock fragment between 64 and 256 millimeters in diameter, especially one that has been naturally rounded. Tennis ball to basketball.

Pebble: a rock fragment between 4 and 64 millimeters (0.16 and 2.51 inches) in diameter, especially one that has been naturally rounded.

Erosion: process in which rock, sediment, and soil are worn away and transported by natural forces such as wind or water.

Weathering: disintegration or alteration of rock in its natural or original position at or near the Earth’s surface through physical, chemical, and biological processes induced or modified by wind, water, and climate.

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