Maine's blue clay
As the massive continental ice sheet was melting away more than 16,000 years ago due to rapidly rising global temperatures, the edge of the ice sheet retreated north to the present coast of Maine. Since the ice sheet was so massive, the weight of it caused the earth's crust to bend downward. As the ice melted and retreated, the ocean flooded into the depressed areas that were no longer covered with ice. It took thousands of years for the land (Maine) to rise above the ocean again. In the meantime, seawater flooded far inland. Amazingly, most of Maine's human population now lives on what was the bottom of the sea only 15,000 years ago.
This gravel pit in southwestern Maine exposes the inside of a glacial-marine delta. Photo Courtesy of W.B. Thompson, Maine Geological Survey. Glacial streams from the melting ice deposited great quantities of sediment varying from coarse to fine material. Fine particles, such as silt and clay, were deposited as a blanket of mud away from and over the top of the coarser materials. The most widespread marine deposit is the mud of the Presumpscot Formation, named for exposures along the Presumpscot River in Portland. It contains the ground-up fragments of minerals that make up the bedrock of Maine.
The Presumpscot Formation is generally called "blue clay" by the public; however, it's color is usually gray. The mud of the Presumpscot Formation has been reported to be as thick as 200 feet, and anyone who has had to dig through this mysterious clay can say that it is a sticky mess! However, the "blue clay" and glacial marine mud have been put to many different uses over the years. For many years, glacial marine mud was used for making bricks, at places with names like Brickyard Hill and Brickyard Cove. It was used by Native Americans to make clay pottery, as well as by early Maine settlers for ceramic products. These days, the "blue clay" is used by local potters and artisans for decorative pieces.