Illustration by Brucie Rosch

Paleoclimatologist Curt Stager explains how ice sheets shaped the Adirondack landscape


Imagine an environmental
challenge to the Adirondacks harsher than acid rain, invasive species and global warming combined. Imagine something that would not merely damage every lake, forest and meadow but completely crush them, then grind and bury their remains under a mile-thick slab of sealant. Imagine the entire region looking as flat and featureless as a parking lot for tens of thousands of years, then like a gritty demolition site for centuries more. Incredibly, it actually happened here, not just once but dozens of times in the not too distant past. 

I’m referring, of course, to an ice age.

The global warming we are experiencing today is unusual not only because we are its main cause, but also because it runs counter to a general cooling trend over the last 50 million years or so. By two or three million years ago, the Arctic was occasionally cool enough to allow leftovers from each winter’s snowpack to persist through summer. Slow cycles in the tilt, wobble and orbit of the Earth amplified that summer cooling over millennia until the lingering drifts grew into two gigantic mounds that covered Baffin Island and Labrador. 

Gradually thickening to hundreds, then thousands of feet, the heavy mounds squeezed their deeper snow layers into a putty-like version of ice that oozed sideways like a blob of moist dough that flattens as it flops onto a kneading board. Woe betide the baker who drops the dough next to a wayward insect so the spreading edge engulfs it. Every 100,000 years or so, the Adirondacks went under like that hapless bug.

Ice sheets are not glaciers. They are large enough to flow under their own tremendous weight and cover vast landmasses, like the huge remnant ice sheet that still covers Greenland today. Glaciers are smaller rivers of ice that are pulled down mountainsides by gravity. Several dozen times during the last two to three million years, the ice sheet in Labrador grew high and wide enough to fuse with the one on Baffin Island while pressing its southern rim against and then over the Adirondacks. During the most recent ice-age cycle that commenced more than 100,000 years ago, the combined “Laurentide Ice Sheet” pressed all the way to the New England coast. After it stopped growing about 18,000 years ago it began a long, slow meltdown that exposed bow waves of debris that we now call Long Island and Cape Cod. The southern edge of the dying ice sheet shrank northward by nearly a mile per decade until, roughly 14,000 years ago, the Adirondack Mountains emerged from their icy prison, too.

Signs of that destruction are everywhere in these mountains. One of the first scientists to recognize them as such was Swiss paleontologist Louis Agassiz, who popularized the concept of ice ages during the mid-19th century. While visiting the Adirondacks in 1858 with Ralph Waldo Emerson and colleagues for a “Philosophers’ Camp” excursion to Follensby Pond, he found large boulders at their forest campsite and likely explained to the group that they were left there by a “great engine” of ice, not Noah’s Flood as geologists had previously surmised. 

Today we call such boulders “erratics,” imported chunks of rock that often differ from the bedrock beneath them as the Latin errare (“to wander”) suggests. Plucked from local outcrops or distant corners of Canada, they are ubiquitous in the Adirondacks, even atop the highest peaks. Some are as big as houses, like a pair of massive chunks beside the hiking trail to St. Regis Mountain. A smaller one marks the grassy gravesite of abolitionist John Brown near Lake Placid.

Less common are scratches, or “striae,” that were gouged by stones embedded in the sliding base of the ice. A spectacular example stands beside Route 458 where the road curves around the side of a hill near Santa Clara. On a rainy day the wet rock face can seem as smooth, dark and shiny as a whale’s flank. Walk up close enough to press your hand against it, and you can both see and feel dozens of slender pebble-streaks like a meteor shower frozen in time, their parallel paths still aligned north-south along the Laurentide ice route.

The work of past glaciations on bedrock was more like planing and polishing than carving because ice, after all, is softer than anorthosite. Each advancing stage brought another forward stroke that turned cobbles and grit into chisels and sandpaper. Whenever Arctic summers became warmer again, the ice stopped moving and began to melt in place, and the work ceased until the next cold advance commenced. The deep, U-shaped notch in the town of Keene that now holds Chapel Pond and the broad cliffs that draw rock climbers was smoothed by this repeated, incremental grinding over the ages. Long, pale scars of landslides on the steep slopes of Colden, Giant and other peaks show how easily a montane forest can lose its grip on the slick stone beneath it. 

Glacial geologists revel in the diversity of such ice-derived landforms, and they wax eloquent when describing the aftermath of deglaciation here. Riverbeds that once lay in, on, or under the ice became sinuous “eskers” like the wobbly ridge that now divides Clear Pond from Rainbow Lake. Hill-sized piles of sand and gravel are “kames” and similar-sized pits are “kettles,” many of which have since become lakes. Wide, flat expanses of sand like the one occupied by the airport at Lake Clear are “outwash plains” sorted and spread by flowing meltwater. Fine “varve” layers of pulverized silt at a cemetery in Keene were deposited on the floor of a deep glacial lake that once filled the Ausable Valley. Sticky gray clays under Plattsburgh show that the newly ice-free Champlain Valley was still so down-warped by its former burden that seawater poured in to form a temporary arm of the Atlantic. Excavations in those “Champlain Sea” deposits have yielded the bones of seals and beluga whales.

What would it be like to live here during an ice age? Well, how would you feel about living on top of the Greenland Ice Sheet? You would face a lifeless plain of ice and snow that was bitterly cold not only because of the climate cycles, but also because you would be standing a mile above sea level. Far below, where the creeping belly of the ice scraped the ground, nothing could survive in the slowly churning wreckage of rocks and dirt, either. 

 To share the Adirondacks with living beings, you would have to wait for an “interglacial” warm spell to melt the ice down and allow plants and animals to return. We don’t know much about the creatures that moved back when the Adirondacks first appeared from beneath the last ice sheet. Cellar holes, canals and road-cuts are rarer in the park than in the rest of the state, so most of the ancient remains of ice-age animals and plants are still buried out of sight here. 

Several decades ago, however, a vertebra found in a gravel pit near Elizabethtown caught the attention of David Franzi, a noted glacial geologist at SUNY–Plattsburgh. “I wasn’t sure at first that it would turn out to be very old or interesting,” he told me recently, until carbon-14 dating and the keen eye of a specialist showed that it belonged to a musk ox 13,000 years ago. “It probably drowned in a meltwater lake or river and was buried by gravel in a delta,” he explained. For now, that lone bone is our oldest mammalian relic from the Adirondacks. 

Pollen grains preserved in local lake deposits provide more clues by showing that tundra and thickets of low-lying spruce, fir and birch colonized the moon-like landscape shortly after the ice left. We can therefore infer from fossil discoveries in similar habitats elsewhere that classic ice-age mammals may have lived here, too. That means mastodons, at least two kinds of mammoth, ground sloths and giant Castoroides beavers the size of black bears. All are now extinct, but many other species from the earliest days of heavy forest cover still remain, including white-tailed deer and gray foxes. As a basic rule of thumb, ancestors of any native Adirondack mammals you know today were living here soon after deglaciation.

What happened to those great beasts of the distant past? The ice sheets never reached far beyond the southern border of New York State, and for much of each glacial cycle they didn’t even extend beyond northeastern Canada. That left plenty of room for wildlife to go about their business as usual in most of North America during ice ages and interglacials alike. When ice covered the northern forests that spruce-munching mastodons favored or the grasslands that mammoths preferred, those same species persisted in forests and steppes farther south. Mammoth bones, for instance, have been found as far south as Texas. During warm interglacials, habitats and species easily drifted north again. For them, migration was the solution to climate change—at least until the last ice age ended.

But by roughly 12,000 years ago there were no more mastodons, mammoths, sloths or Castoroides left in New York State. Gone, too, from the continent were widespread species who had likewise survived previous glaciations and interglacials, from horses and camels to cheetahs and sabertooth cats. For some scientists, climate change seems to be the most likely cause, but the last deglaciation was no more severe than dozens of others that preceded it without causing mass extinctions. The most unique feature of the end of the last ice age was the arrival of people from Asia. 

With so much ice locked up on land, sea level dropped by about 300 vertical feet and dried the Bering Strait between Siberia and Alaska. Paleolithic hunter-gatherers entered North America about 14,000 years ago and quickly spread throughout the ice-free regions. Some reached the Adirondacks shortly after the ice left, as a distinctive grooved projectile point found on the shore of Tupper Lake attests. We know too little about the various cultures and worldviews of the earliest North Americans to know for sure what might have caused some of them to become species-exterminators, but most experts surmise that over-hunting was the main reason why we can now hike in the Adirondacks without being trampled by hairy elephants.

What lessons might this legacy offer us today? Truly extreme climate change is no party, and the global warming that we are dealing with now is arguably less devastating than those ancient ice disasters, at least for the glaciated regions. But our current situation is also more troubling in some ways. Rather than slow sunlight cycles, today’s changes are being driven by a quick buildup of greenhouse gases, causing ocean acidification as well as rapid warming. With so much humanity on the planet there are fewer refuges for species to migrate to as their ancestors did long ago. And because we now know that we are causing these changes, climate deniers aside, they have moral implications unlike the work of oblivious ice sheets.

Such a deep-time perspective reveals few silver linings in our own climate crisis, but it may offer some meager comfort if you’re feeling crushed under a glacier’s weight of environmental despair. Yes, we’ve gotten ourselves into a mess, but at least we’re not facing an ice age. In fact, experts calculate that our heat-trapping carbon emissions will likely warm the atmosphere long enough to stop the next ice age, which would otherwise begin about 50,000 years from now. Lucky us. 


Scientist, educator and author Curt Stager holds an endowed research chair at Paul Smith’s College. His research deals with climate change and the ecological histories of lakes around the world. His work is published in technical journals such as Science as well as publications such as National Geographic and The New York Times. He co-hosts “Natural Selections,” a weekly science program on North Country Public Radio. Stager is the author of four books, most recently Still Waters: The Secret World of Lakes (W. W. Norton & Company, 2018). He’s a frequent contributor to this magazine.


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