An esker (often known locally as a horseback) is a relatively narrow raised sedimentary deposit that winds across a landscape. Eskers are formed from glacial deposits. When a glacier is melting, rivers can form inside or underneath the ice. These rivers, like a normal river, carry sediments of all sizes. These sediments may be trapped within the ice of the glacier, or may be eroded from underneath the glacier. When a glacier recedes, the sediments deposited by the rivers are left behind. The body formed by these sediments is called an esker.

120 million years after the formation of what is now called Maine, huge glaciers covered the northeast region of what is now called North America. These glaciers were thousands of feet thick and covered mountains, even Ktahdin and Sugarloaf. They dragged across the landscape and left striations on exposed bedrock. They carved mountains and valleys with their weight.

Glaciation started about 1.5 million years ago and didn’t stop for 1.49 million years (10,000 years ago). During this period of glaciation water flowed underneath the mile-high glaciers. We call these sub-glacial rivers eskers.

Deposition from the eskers was carried out to the ocean and became marine sediment. When the glaciers started to retreat the eskers started depositing their minerals on what has now become land. The glaciers stopped in down town Unity Maine. Sometimes the deposition was scattered (Kames) and sometimes it was uniform. The uniform deposits are also called eskers. You can follow an esker right out of Unity by driving along Horseback Road. Maine’s system of eskers is over 100 miles long and is one of the longest in the country. Eskers appear as long ridges that stand above the surrounding area. They are composed of mineral deposits and are used for highways, gravel pits and such.

Esker Sediments

An esker (often known locally as a horseback) is a relatively narrow raised sedimentary deposit that winds across a landscape. Eskers are formed from glacial deposits. When a glacier is melting, rivers can form inside or underneath the ice. These rivers, like a normal river, carry sediments of all sizes. These sediments may be trapped within the ice of the glacier, or may be eroded from underneath the glacier. When a glacier recedes, the sediments deposited by the rivers are left behind. The body formed by these sediments is called an esker.

Esker sediments can range in all sizes, and usually do not match the parent material where they are located. An esker river may transport the smallest clays to the largest boulders. The esker left behind by a glacier tells us a lot about the river it was formed by. When an esker is cut in cross-section, as often seen in gravel pits, different sedimentary layers are visible in an esker. Areas of finer sediments indicate much slower river flow, while areas of heavier sediments indicate a much faster flow. Different sediments create different layers of the esker. It is possible to find a silt layer with virtually no particles larger than sand, and 2 feet above find nothing but cobble and boulders. On top of this layer may then be nothing more than silt again.

Drawing by Gregg Shedd

This sedimentation indicates the speed and force of the river during the time of deposit. The differences in the layer compositions may indicate seasonal changes and melting rates in the glacier.