Mapping Geologic Features

(Published in the Quoddy Tides, July 14, 2000)

If we could drain the water out of Cobscook Bay, as if it were a huge bathtub, what would it look like? The shape of the bedrock and texture of the soils that we would see are the result of 350 million years of erosion that has left behind geologic features as unusual as some of the creatures that call this place home. A picture of the different types of rock, gravels, and mud that form the bottom environment in the bay is actually being created by marine geologist Joe Kelley of the University of Maine.

This picture is in the form of a map of the bay's surface geology beneath the water.

Kelley has mapped the geology of offshore areas of coastal Maine for the past several years. Maps of the rest of the coast are published and available, but this recent work in Cobscook Bay will complete the series.

Like maps of upland areas, these present information that can guide people's decision-making. The maps will provide clues to the most appropriate location for various marine activities, and along with other information known about the bay, can perhaps help avoid conflicts over use.

Kelley's maps are created from images that are produced through the use of a torpedo-shaped device called a sidescan sonar that reflects sound waves off of the seabed.

"The sonar device sends sound out up to 100 meters, or a football field, to either side, as it is towed behind a boat," Kelley explains. "If the sound hits rock, it reflects back very strongly and records it as dark images. If the sound hits mud, almost nothing comes back, because the mud absorbs sound. If it hits gravel, the images look salt and pepper like, revealing objects as small as a basketball. Sand often shows ripples on the surface."

Working from the boat of local fisherman Butch Harris, Kelley collected the sidescan sonar images during the spring of 1998. The device was dragged during high tide to cover as much area as possible; the intertidal areas of the bay were previously mapped using aerial photos. The following spring Kelley went out with another fisherman, Jeff Smith, aboard Jeff's fishing boat, Cat Pointer, to gather actual samples of the bottom sediments to test the accuracy of the sonar images.

"In the deepwater areas of Cobscook Bay, we found a lot of gravel and real hard bottom presumably because the current is so strong. Unlike other areas on the coast, here virtually all the gravel is found in subtidal areas, " Kelley reports. "Most of the fine soils accumulate in shallow water. Typically, estuaries show the opposite pattern."

In Cobscook Bay, the sea floor is largely shaped by tidal currents scouring the bottom as the water is pushed between confining walls of bedrock. No recently formed soils are added to the system; instead, mud is released from the erosion of sediments deposited as long ago as the Ice Age. The mud found in Carrying Place Cove and the many other mudflats exposed during low tide may have been first deposited 14,000 years ago!

In deeper areas, the water moves too quickly to allow any significant settlement of soils carried along with the current. An exception occurs where eddies form after water flows through very narrow channels, such as Reversing Falls. Swirls of muddy gravel, pebbles, and sand are deposited in South Bay and, to a lesser extent, in East Bay.

"Cobscook Bay has bedrock left behind when the European continent split off from North America," Kelley says. "The rock is relatively soft here, unlike the granite rocks elsewhere along the coast that were subject to more heat and pressure and are not as easily eroded."

The presence of a sandstone called the Perry Formation, one of the youngest rocks found on the coast of Maine, has helped determine the shape of the bay.

"Over millions of years, the sandstone eroded very easily and left behind much of the harder bedrock you see today," explains Kelley. "It is a quite unusual feature on this coast and very distinctive because of the way the bedrock now narrows and pinches the channels." The peninsulas stretching out in to the bay and the narrow channels in between were formed as a result of this process.

Beyond geology, Kelley's side scan sonar device also reveals some of the human history of the bay. It reflects back the shape of shipwrecks, old moorings, lobster traps, aquaculture pens and even marks from dragging activity.

"I have never seen an area along the coast so thoroughly marked up by drags," Kelley reports. "There may have been more mud on the bottom before this type of fishing became so common. Once the mud has been lifted or stirred up, the currents carry it away, and the bottom is left more hardened than it was before."

Of course, Kelley's side scan sonar maps are not the only source of information about the geology of the bay. Local fishermen hold a lot of information in their heads about Cobscook's bottom sediments. By using that local knowledge to refine Kelley's maps, a more complete picture of what the surface geology looks like could be created. When completed, Kelley's maps will be available at the Cobscook Bay Resource Center in Eastport.

This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information. 

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