After the Glaciers | The Champlain Sea

12,000-10,000 years ago

The Champlain Sea follows immediately on the heels of Glacial Lake Vermont. But it can be tough to keep all these dates straight. So here’s a sequence of events in life of the Champlain Sea

  1. Draining of Glacial Lake Vermont
  2. Filling of the basin with salt water
  3. Isostatic rebound of northern part of continent
  4. Champlain Sea fossils
  5. Refugia/relict species
  6. Sedimentary evidence

Draining Glacial Lake Vermont

Imagine a swelteringly hot day in July, 12,000BCE. You just bought a cabin in Jericho at about 650′ in elevation, right at the edge of Glacial Lake Vermont. Your lake front cabin has a lovely little porch made from black spruce that grows down here in the Champlain Valley. You’re watching a band of caribou grazing off in the distance. One gives a start and they all turn and look out over the water. Air rushes up from the south and you follow their gaze. The water is dropping. Fast. Your beachfront is retreating. Somewhat peeved, definitely curious, you start to follow the retreating shoreline. But it’s retreating faster than you can walk! You start to jog just to keep up. About 7 miles later, at the western edge of Essex, VT the water finally stops retreating. Within a few hours Glacial Lake Vermont has dropped over 300′ in elevation! The deltaic gravel and sand sediments deposited in Glacial Lake Vermont by the Winooski River are now visible and you can gaze at the wide domed arc of deposits in disbelief. Beyond the delta sediments mucky mud covers the rest of the exposed area.

You wanted waterfront property and waterfront property you’ll have, doggone it!! So you head back to your cabin and set to work moving your cabin down to the new shoreline. It takes a few months, maybe a year to complete the work (good thing you domesticated that woolly mammoth to help you with the haul). With the work done, you sit on your porch and look out over the lake. And just in time to catch a white flash out in the water. Champ? No, couldn’t be. Another flash. Were those belugas breaching?! Your befuddled brain scans its catalog of mammal knowledge. Aren’t whales marine animals? And now that you think about it, the air has a distinctly salty smell to it. Lake front? Nope. Looks like you’ve just settled ocean front! But how did the lake turn into an ocean while you were at work?

Enter the Champlain Sea

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.

Isostatic Rebound

Depressing the continent : Doing the math

Imagine the North American plate sort of like a canoe floating in a highly viscous pool of something like cold molasses. Now imagine that it starts to snow, but it only snows on half of the canoe. The snow piles up more and more in the bow and eventually the weight of all that snow starts to depress the canoe – but only the front of the canoe where the snow is piling up – down into the molasses. Now melt all that snow, removing the downward force of its weight, and the canoe will slowly start to rebound and then teeter up and down as it returns to even.

This is essentially what happened to the continent under all the added weight of the glacier. So for our analogy, the North American plate is the canoe, the molasses is the asthenosphere on which the plates ‘float’, and the snow is the continental glacier. Seems improbable that a glacier could actually weigh down the continent enough to sink it, but consider the weight of the glacier:

  • The density of glacial ice at its max is 917kg/m^3, or about 57 lbs/ft^3.
  • We’ll assume the glaciers were about a mile thick (5280′) in Vermont
  • The volume of a column of glacial ice 1 mile thick = 1′ long x 1′ wide x 5280′ tall = 5280 cubic ft
  • Mass = Density * Volume
    = 57lb/ft^3 * 5280ft^3
    = 300,960 lbs of weight resting on each square foot!!

And if you need a visualization, here’s a cross-section of the chunk of ice covering Vermont. I set it to 1 mile thick, so if you were standing on the summit of Mt Mansfield (4083′), you’d still be under 1,200′ of ice.

So every single square foot in Vermont would’ve felt the weight of a 300,960 pound column of ice pressing down on it. And the ice was 2 miles thick up in Canada! And the Laurentide Ice Sheet covered more than 5,000,000 square miles!! So according to my super rough, back of the envelope calculation, the weight of the Laurentide Ice Sheet was some staggeringly incomprehensible number, like 4.2e19 (42 quintillion or 4.2*10*18 more tens) pounds of weight. Which is not on a scale that means much to us mortals.

Maybe now it’s more conceivable that all that weight could cause the continent to be depressed. The lifespan of the Champlain Sea – 12,000-10,000 BCE – reflects the amount of time it took for this part of the continent to slowly rise from being depressed 320′ below sea level to just over sea level. The continent didn’t just stop there and sure enough over the past 10,000 years the North American plate has continued to rebound. The rate isn’t evenly distributed (look at the image above that shows the rate of rebound. Here in New England we’ve risen another 100′ in 10,000 years and continues to rise about 5mm per year.

Fossil Evidence

A number of fossils have been found throughout the Champlain Valley that support the theory that a cold sea infiltrated the basin. Perhaps the most famous is the Charlotte Whale, which was unearthed in August of 1849 while workers were constructing a rail line between Rutland and Burlington. Apparently workers thought it was strange horse. Whenever anything interesting was discovered, naturalist on the ground, Zadock Thompson, was called in. He knew something fascinating was hidden in those bones, and eagerly completed the excavation of the rest of the skeleton. He consulted with Louis Agassiz (the same Agassiz described here) and others who helped confirm his suspicion that the bones belonged to a beluga whale. The Champlain Sea was far more extensive than just Vermont and other fossil belugas have been found in Champlain Sea sediments in eastern Ontario and Quebec.

Bones have been unearthed – though not necessarily in Vermont – from other cold water marine mammals as well. You can find research, maps, and information on fossils found throughout the world at Here’s the entry for a finwhale discovered in Quebec: link. Ring-necked, harp, and bearded seals also would’ve lived in the sea. As topline predators, we can assume that their prey were also present in the Champlain Sea (lots of fossils of fish no longer in Lake Champlain have been discovered as well).

The more I started to dig into what species were here, the more excited I got. I’ve wanted to go see a puffin colony on the east coast since I moved here in 2008. It turns out I could drive the 20+ hours up to Canada to go see a rookery. Or I could just head to Mt Philo and then travel back in time 12,000 years. The glaciers had effectively pushed everything farther south. Trees. Insects. Soil types. Birds. Mammals. The distribution of organisms today is not what it was during the Champlain Sea era. Being much much colder and marine, the habitat here in the Champlain Valley would much more akin to the modern ecosystems of coastal Newfoundland and Labrador than to our Northern Hardwood Forest complex that we think of as typical Vermont. Not too many sugar maples to be tapped back then. Walking along the shore you could encounter polar bears (even brown bears) and arctic foxes. Birders would’ve lost their minds!! A day of birding might get you a bird list something like this. Imagine a colony of eiders, murres, arctic terns, auks, and the like nesting on those islands poking up out of the sea. Oh, and heart be still, dare I say, yes, puffins. Imagine raucous colonies of puffins.

Unlike in Glacial Lake Vermont sediments, where fossils are rare, when digging through silts and clays from the Champlain Sea, it’s not uncommon to dig up fossils. While you’re probably not going to find a beluga whale – well, you never know… – it’s not uncommon to find the shells of marine clams. And not just a couple shells here and there. We’re talking oodles and oodles of ’em. I’ve found a few sites with fossil clams, including Rock Point in Burlington and near Macrae Farm Park in Colchester (pictured above). I’m not sure of the species, but the Perkins Geology Museum has a fossil of Nucula abyssicola, which looks similar. But the Report of the State Geologist on the Mineral Industries Geology of Certain Areas of Vermont Volume 7 from 1909/10 (link) lists over 20 different species of bivalves. And I’m not a malacologist, so I’m not sure of the species.

Refugia and relict species

Indirect evidence for the Champlain Sea also comes from the presence of species currently living in/near Lake Champlain that are normally only found in marine environments, or that depend on marine environments for at least part of their life history (e.g. Atlantic salmon spend most of their lives in the ocean but migrate to fresh water to spawn). Relict species are remnant populations of species that historically had a much wider distribution. Refugia are the remnant habitats that support these species. Just as the Laurentide Ice Sheet moved south, so too did ecosystems as they followed climate south. What’s now the high elevation desert of the mountain west, was once a contiguous forest stretching between mountain ranges (it actually would’ve been inverted from what it is today, with lower elevations covered in endless stretches of boreal forest interrupted briefly at the barren summits of the high peaks). The post-glacial warming climate has since transformed the lower elevation forested areas into deserts and open plains, leaving high elevation islands (called sky islands) of forest in isolated peaks. If you were a small wood rat, pocket gopher, or pinyon mouse, you now found yourself in fragmented islands, or refugia. It’s how you get beavers at the 10,000′ in the Ruby Mountains with nothing but dry inhospitable desert around.

Here in Vermont, this looks a little different than out west. Rather than desertification isolating species, it was isostatic rebound and the closing of the Champlain Sea that isolated aquatic species in the refugia of the Champlain Basin. The Richelieu River became a barrier preventing populations in Lake Champlain from moving in and out of the Atlantic. As the basin began the slow process of becoming less and less salty, species trapped in the lake slowly adapted to tolerate the changing conditions. Those that couldn’t adjust (all marine mammals, including the beluga whale) died out. Birds and terrestrial species that depending on the fish would have died out/migrated from the area as the glaciers melted and continued to expose new land to the north.

An incomplete list of species that were extirpated from the Champlain Sea region:

  • FISH: cod, tomcod, eelpout, capelin, smelt,spoonhead sculpin, lake cisco, lake char, wrymouth, long-nosed sucker, lumpfish, 3-spine stickleback,
  • WHALES + PORPOISES: Humpback whale, beluga whale, finback whale, bowhead whale, harbor porpoises
  • SEALS: Harp seal, bearded seal, ringed seal, harbor seal

Not all the species that lived in the salty waters of the Champlain Sea died or left as the water level dropped and salinity changed, and these mighty few still reside in Lake Champlain, a distant echo singing a verse from the Champlain Sea. Here’s an incomplete list of some of these relict species:


  • Atlantic salmon
  • Sturgeon
  • Lamprey (previously thought to be invasive)


  • Beach pea
  • Champlain beachgrass
  • Beach heather

Sedimentary Evidence

Much of what I wrote about Glacial Lake Vermont applies here, but just 300 in elevation lower. Our big rivers would’ve been draining till and other sediments down off of the Green Mountains depositing at their mouths in big deltas of the Champlain Sea at 320′. But while the land surround GLV was mostly barren or occasionally pockets of permafrost and open tundra, the land surrounding the Champlain Sea was beginning to be more forested. Plants stabilized upland soils and fewer sediments were being unloaded into the basin. GLV drained in the blink of an eye, so the rivers would’ve had to carve their way first down through the sandy sediments of those big GLV deltas before forging their own deltas in the Champlain Sea. If you look at the flow of these rivers, they’re more or less pretty straight as they’re bounded by bedrock as they flow through the Green Mountains. Once they hit 620′ and the Champlain Valley they pretty much just lose control and start to wend and weave, meandering their way across soft sediments down towards what would’ve been the edge of the Champlain Sea. Much like the Mississippi River jumping channels and creating new paths, our rivers jump and create new paths through the GLV deltas.

Resources on the Champlain Sea

  • Google earth file of field sites (link)
  • Marine Mammals in the Champlain Sea (pdf)