The Champlain Sea existed in Vermont from about 12,000-10,000 years ago. It formed as the glacial dam, which blocked water in the Champlain Basin from draining to the north, gave way. Once the dam broke – and this was no slow leak – water rushed out of the basin in a few hours to a couple of days and the water levels dropped about 300′, from 620′ to 320′ above sea level. Even though we know the new shoreline was at 320′ above sea level (sea level by today’s standards), we know that it was a salt water sea. You can raise sea level by either adding water to the sea (melting glaciers) or pushing the continent down into the ocean. So while yes, the glaciers did melt. They’ve melted a heck of a lot more since the Champlain Sea was around, so we would expect if this were the case that the Champlain Sea would still be around.
With more water trapped in ice than today, ocean levels were still significantly lower than they are today, about 300′ lower at the dawn of the Champlain Sea. So we look to the second possibility: because the continent had been depressed over 600′ by the weight of the glaciers, land that is today 320′ above sea level was below sea level just as the Pleistocene was coming to an end. As a result, the Champlain Basin filled in with salt water and a panoply of salt-water-loving critters. Lets make this a bit more concrete:
- UVM’s campus is at 350′ in elevation above modern sea level
- 10,000 years ago the continent was depressed 600′ by glaciers. UVM was 250′ below today’s sea level (350′ – 600′)
- Tons of water was trapped in glaciers 10,000 years ago, so sea level was ~300′ lower than it is today
- 10,000 years ago UVM was 50′ above water (-250′ + 300′)
The melting glaciers that drained GLV and opened the seaway would ultimately be the Sea’s demise. As the glaciers melted, the weight on the continent was effectively removed and the continent began the slow process of rebounding to preglacial elevation. After about 2,000 years the drainage point at the north end of what’s now Lake Champlain rose to about sea level. Once above sea level, salt water no longer filled in the basin and water in the Champlain Valley began to flow out into the ocean, beginning a long slow process of desalinization. The continent continues to rise and the surface of the lake now sits around 100′. The image below depicts the different rates of rebound as our continent continues to rise.