Sebago Lake has a measured water depth of 316 feet, making it the deepest water body in the state of Maine. With a water depth 49 feet below sea level, why is this lake so deep?
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| As a large sheet of ice retreated northwest across Maine during the Ice Age about 15,000 years ago, it left behind the basin which created Sebago Lake, the deepest water body in the state of Maine. COURYESY OF MAINE GEOLOGICAL SURVEY |
At the end of the last Ice Age (about 15,000 years ago), the large ice sheet retreated northwest. The ice sheet weighed so much, it depressed the earth’s surface, Spigel said. The land surface was lower than it is today, so the ocean covered parts of southern Maine.
All of the meltwater from the glaciers contained sediments and ground-up rocks. As the meltwater flowed into the glacial sea, the sediments were deposited to form a delta which dammed the southern side of the Sebago basin, making it impossible for the water to keep traveling south. A large lake then formed behind the dam.
Proximity to the glacial sea and composition of the rock are two factors for high versus low topography, Spigel said.
Different types of rock have varied resistance to erosion. Areas that were once under the glacial sea tend to have subdued topography because the bedrock is covered by thick ocean sediments. The muddy and sandy deposits we see now were once deltas, beaches, or the sea floor.
Many areas of Maine are underlain by granite, which is an igneous rock and can be more easily eroded.
Hacker’s Hill is a popular place in the Sebago Lake area to view the sunrise or sunset. It stands 300 feet above the surrounding landscape, allowing visitors to view many lakes such as Sebago and the White Mountains.
This hill is composed of Sebago granite which underlies large expanse of land in Southwestern Maine. Hacker’s Hill also has metamorphic rocks which would not have been easily eroded by glacial activity.
The Maine Geological Survey has dated the granite back to 296 million years ago using radiometric
techniques. This suggests that Hacker’s Hill was created during the final stages of the Appalachian Mountain formation.
Pleasant Mountain, another tall peak in the Sebago Lake Area “is something different in a sea of granite,” says Spigel. Similar to Hacker’s Hill, this landscape formed as a mountain because its rock composition was more resistant to erosion.
These geological aspects affect more than just topography and landscape. Spigel says that water quantity and quality are also dependent on the local geology.
Arsenic and radon in water can be common due to the type of bedrock that a well is in.
Areas where the retreating glaciers deposited sand and gravel are excellent sources of water. The Portland Water District uses Sebago Lake as its main water source, but it also has pumps in sand and gravel deposits called eskers in case the lake water is compromised.
Sandy glacial meltwater deposits are also known as excellent farmlands today. These areas are much easier to farm than rocky glacial till that was dumped off by glaciers.
Geologists know what the landscape was like millions of years before us and how that impacts what it’s like today. How do they know this?
“Hundreds of years of observation” says Spigel. People started studying the geology of Maine in the early to mid-1800s.
Marine seashells were found way above modern sea level, so scientists started to ask questions and eventually they were able to determine the ages of rocks and other deposits and got a much bigger picture.
George Stone wrote important publications in 1899 about glacial deposits in Maine. There were no geological maps yet, but he understood that there was once a glacial sea by observing different glacial deposits.
The last big mapping effort to create the state geologic maps occurred in the 1970s-1980s. Now, the Maine Geological Survey is working to provide more detailed maps which can be used for environmental monitoring and land use planning.
Spigel has researched the Sebago Lake area during her time with the Maine Geological Survey. About 12,000 years ago, there was a huge landslide. Spigel figured this out by radiocarbon-dating the trees that were killed by the landslide.
After the Ice Age everything was shifting and the earth would have been in a state of imbalance, causing earthquakes and landslides.
This landslide took place on what is today the Sebago Lake Land Reserve in Standish. This property is owned by the Portland Water District and is open to the public for recreation.
Walking on this property, visitors will notice the dramatic contrast is elevation, especially between Bobcat Trail and Horsetail Trail, a direct result of the landslide.
This stark difference in elevation leads to contrast in plant communities. Ponds, streams, and wetlands are found at the bottom of slope, formed by depressions in the landslide deposit.
To explore this fascinating geology and ecology, you can visit the Sebago Lake Land Reserve daily, dawn to dusk. <
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