Marine “blue clay” deposited in the late-glacial sea covers much of coastal and central Maine. It is formally known as the Presumpscot Formation, named after exposures in banks along the Presumpscot River Valley near Portland. The clay deposits can be as much as 100 to 200 feet thick where they have filled preglacial valleys. They are composed of clay, silt and sand-size particles derived from glacial abrasion of bedrock surfaces and rock fragments incorporated into the base of the ice. The end product is a finely pulverized “rock flour,” the analog of wood dust produced by sandpapering. In Maine, this fine sediment was carried by meltwater rushing through subglacial ice tunnels and discharged into the sea along the glacier margin. Then it slowly settled out on the sea floor. Icebergs broke loose from the glacier margin and floated out to sea. Rocks trapped in these icebergs eventually melted out and fell to the sea floor. This process explains the occurrence of large and small boulders that randomly occur in the marine clay. As the glacier margin retreated inland beyond the limit of the ocean, meltwater rivers continued to transport the mud to the sea.

The marine clay typically has a natural blue-gray color, but near the ground surface, it has weathered to a brownish color. The unweathered marine clay preserves a wide variety of fossil organic remains, ranging from the remains of large whales, walruses and seals to smaller mollusks (clams) and gastropods (snails), and even smaller micro fauna, including foraminifers and diatoms. Seaweed and tree remains have also been found. All of these species of marine organisms fossilized in Maine can be found alive today in the Arctic, demonstrating that our late-glacial sea resembled those found today in Arctic regions.

The “blue clay” supported the now largely defunct brick and tile industry of Maine. Marine clay continues to be excavated on a small scale for use in making pottery. The downside of the clay is its ability to hold moisture, causing expansion and contraction in freeze/thaw cycles and resulting in bumps and potholes in Maine roads. Marine clay is highly vulnerable to coastal erosion, which can be made even worse through landslides triggered by steep shoreline bluffs.

To read more about Maine in the Ice Age, check out these more indepth explanations: Eskers and Deltas, Salt Marshes and Recent Sea Level Rise, Glacial Retreat, Moraines, Chronology, Abrupt Climate Change