The north end of Lake Dunmore is surrounded by 250 acres of flat, level land which is less than 25 feet higher than the lake. The soil is gravely sand, and lobes of sandy soil bulge into the lake at the Keewaydin and Songadeewin summer camps. I assumed these sandy lobes were deltas built into the lake as the Laurentide glacier melted away to the north, but now I’m not so sure.
Note: click or tap the images to enlarge them.
The lake margins are not the only features near the lake that are gravelly, sandy, and flat. Elongated terraces cling to the sides of the slopes above the lake. The terraces are 100 to 400 feet above the lake and their arrangement and gentle slope toward the south indicate that they are river terraces. The hillside served as the eastern bank of the river and glacial ice was the western bank. There is no other way to explain these high, dry, hanging terraces. These perched river beds were built in contact with glacial ice 14,000 years ago and are classic examples of kame terraces.
As the southern front of the Laurentide glacier melted north in the nearby Champlain Valley, a lot of water flowed into the Lake Dunmore basin. For maybe a decade it flowed from between the bedrock ridges north of the lake (Figure 3). Later, when the shrinking glacier made way for rivers along Upper Plains Road, water flowed in from the west for maybe another decade (Figure 3). Kame terraces mark the different elevations of these rivers, and the broad, stony riverbeds that remain today indicate that these were raging rivers capable of carrying a huge sediment load.
A large amount of glacio-fluvial sediment (sand, gravel, cobbles, stones) was brought by these rivers. Some extant landforms suggest how much sediment was moving around in those decades. As water poured in from the west, it spread out and deposited wide outwash fans north of the lake. A 12 acre remnant of one outwash surface is 25 feet higher than the surrounding, younger, outwash surface (Figure 4, Surface A). Surface A might have originally covered a large area north of the lake but was eroded away by the water that formed surface B (Figure 4). These rivers delivered tons of sediment to this place and then carried away almost as much as they brought.
The rivers that moved sediment around 14,000 years ago were not like today’s rivers in Vermont. Surface B (Figure 4) is still scarred by the river channels that were active while the glacier was nearby. The rivers carried so much sediment from the glacier and the unvegetated landscape that channels would fill quickly and force the water to find new paths (Figure 5).
The eight figures below begin with a 3D printed model of LiDAR topographic data. The plastic model is hand painted to highlight the existing flat surfaces where glacial rivers deposited sandy/gravelly sediments 14,000 years ago. In subsequent figures, I used Photoshop to add the position and extent of the glacial ice margin that determined where the rivers could flow. I am guessing that this entire sequence of six stages took a decade or two. This sequence is hypothetical and based on interpretations of field observations (some of which were inspired by the LiDAR data).
When the Champlain lobe of the continental glacier melted away from the hillside where today’s Middlebury River meets East Middlebury (a few miles north of Lake Dunmore), the proto-Middlebury river was able to flow directly into Lake Vermont for the first time (Figure 2.5). Meltwater from the glacier was no longer shunted south along the hillsides south of East Middlebury. That was the end of big rivers flowing into the Lake Dunmore basin.
When the stagnant glacial ice in Lake Dunmore finally melted away, the lake was more than 100 feet deep and it is still that deep today. Maybe the sandy shoreline lobes at Camp Keewaydin and Camp Songadeewin are small deltas built after ice-melt had opened the edges of the lake, and rivers (especially C and D) were moving their last loads of sediment. That would have been a final gasp of deposition before the north Dunmore outwash fans were abandoned forever. Otherwise Lake Dunmore might have filled in and become just another flat place beneath Upper Plains Road.