Androscoggin River – the Ancestral Outlet EarthCache
Androscoggin River – the Ancestral Outlet
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This boat accessible earthcache will bring you to the location where before the last great glacial period there was the outlet of Sebago Lake and the path of the Androscoggin River.
There are many ways to access this earthcache but the easiest is to use the State Public Boat Launch that is located in Raymond at an eastern point on Sebago Lake north of the new outlet. Here you will be able to launch a boat so that you can view the area that was once the outlet of Sebago Lake and the ancestral Androscoggin River. Having a Delorme Atlas will make this earthcache that much easier to understand. You will want to look at Maps 4/5 and Maps 10. Sebago Lake, although second in surface area to Moosehead Lake, is Maine's deepest lake. With a water depth of 316 feet, its deepest part is 49 feet below sea level! It is located along the transition between the Central Highlands and the Coastal Lowlands physiographic regions of Maine.
The story of this once great river path begins, as do so many other landscape stories in Maine, with the last glacial retreat. The last glacial episode in Maine began about 25,000 years ago, when the ice sheet moved south overspreading New England and Eastern Canada. During its peak development, this ice sheet was centered over northern New England slowly flowing to the southeast into the ocean. The ice itself was thousands of feet thick, with its massive weight shaping the land as it slowly traveled. Climatic warming forced the ice sheet to stop and begin to recede as early as 21,000 years ago. This melting of the ice sheet released more water then is possible to imagine taking with it anything that the current could pickup. As the retreating glacier halted for a time the margin would stabilize, thickened as it did several time in this area leaving behind moraines as it deposited the materials.
Charles H. Hitchcock in his report 1861 report on the geology of Maine was the first to theorize that the ancestral Androscoggin River flowed from Bethel through the Sebago Lake basin and to the sea. In 1899 George H. Stone further studied the extent and thickness of the glacial gravels in the Sebago Lake region. Stone was the first to propose that the preglacial drainage of Sebago Lake flowed from the southern end of the lake near the village of Sebago Lake. In a 1922 paper Irving B. Crosby discussed both Hitchcock's and Stone's theories in a paper on the former course of the Androscoggin River. Crosby mapped the ancestral Androscoggin River coming into Maine at its present location but then flowing south from West Bethel, down the valley of the present Crooked River through Sebago Lake and on through the Little River valley and then into either the Presumpscot or Nonesuch River valleys to the sea. Research done by Leavitt and Perkins in 1935 concurred that the outlet for Sebago Lake originally drained southeastward from Lower Bay before switching to its present location. Seismic reflection work done in the early 1990's by Bruce Hansen and William Nichols of the US. Geological Survey shows convincing evidence from the subsurface geology that the southern end of Sebago Lake served as a large river channel in the past. A V-shaped channel, now buried under 200 feet of sand and gravel has been carved into the bedrock. They theorize that a major river may have eroded this in preglacial times.
Now that we have looked at the research and theories on this great river lets take moment to look at why the river changed it course. We first have to look at the bedrock geology to see why Sebago Lake formed in the first place. The Bedrock Geologic Map of Maine shows the Sebago Lake region to be underlain by both granite (in the north) and harder metamorphic rocks (along the southern edge.) The metamorphic rocks along the southern shore were hardened by heat and pressure making them more resistant to erosion than the granitic rocks located under most of the present lake. As the glacier passed over the area the mass of the ice pushing down and moving toward the sea gouged away at the softer granite forming a deep basin. Finally the glacier stopped and began to retreat. As the glacier moved back and forth it released a great deal of melt water carrying both sand and gravel. The glacial retreat was first halted in the area of Sebago Village where an end moraine deposited during the last glacial episode, changed the lake's drainage pattern. Stone suggested that the lake, in pre-glacial time, before the formation of the end moraine would have been about 100 feet lower than it is today. Using as evidence the lack of bedrock outcrop in the Sebago Lake Village area, the extensive coarse-grained gravel deposits rimming the lake, and the similar elevation of the lake bottom to the stream channel on opposite sides of the Sebago Village glacial deposits, Stone concluded that a large preglacial river flowed from north to south through the Sebago Lake basin.
According to the Maine Geological Survey the modern Sebago Lake originally formed at the edge of the continental ice sheet over 14,000 years ago. During the later stages of glaciation, ice melted from the highland areas and a massive ice block filled the lake basin, a type of giant kettle pond formation. As the ice melted, water and ice filled a basin that had been scoured out by a combination of glacial ice and stream erosion. Moving ice and melt-water steams deposited massive amounts of sand and gravel at the southern end of the lake northeast of Sebago Lake Village forming an end moraine. This end moraine acted as a dam causing a large lake to form behind it. Unable to flow in its old path to the south, a new lake outlet formed over bedrock outcrops at the east side of the lake, just west of North Windham.
As the glacier began to retreat once again the same process took place at west Bethel causing the river to follow the path of least resistance and change course moving toward Rumford and then continue onto the ocean. So as you see it was a series glacial events during the last glacial period that charged the course of one of the largest rivers in the northeast.
Access to the lake can be found at the state owned Public Boat Launch in Raymond just off route 302. Please make sure not to enter Lower Bay as it is protected waters for the Portland Water District. This is not an earthcache to be done in the winter as it should be done by boat only. The view form the lake will allow you to see the whole picutre. While the posted coordinates will bring you to the entrance to of Lower Bay, this is as close as you need to go in Sebago Lake, the earthcache can be claimed from north of this point looking south. At the posted coordinateds the water is approximately 100 feet deep, the bottom of what was once the original outlet. If you look at the shore you will see it is lined with rocks, which were dropped by the glacier as it melted. If you look to the south you are looking at the end moraine that not only formed present day Sebago Lake but began the change that cause the Androscoggin River to find a new path to the ocean. Remember this is an earthcache so there is no container just an earth science lesson at an amazing glacial feature. In order to fulfill the requirements of this earthcache you need to: email through my profile the estimated height of the end moraine to the south that blocked the river. Please be sure to include the name of the earthcache in your email. After answering the question to log this cache, you must post a photo of yourself with your GPS with a recognizable feature of the moraine in the background. Logs where the question has not been answered or a photo is not posted will be deleted. Please enjoy your visit to Sebago Lake Region and please remember Lower Bay is protected waters. For further information about the theories and research about the Sebago Lake formation and the past course of the Androscoggin River go to the Maine Geological Survey Website: “Why is Sebago Lake so deep?” at (visit link)
If you enjoy this earthcache you may want to check the Maine Geological Survey located at (visit link)
They have developed a number of information sheets or field localities giving a great deal of information about geologic features. They also have a number of books and maps about Maine’s natural history/ geology that you might find interesting.
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