A few summers ago, I hiked a hundred mile section of the Pacific Crest Trail, not far from Lake Tahoe. The previous winter had seen record storms in the Sierras, so sometimes, under the bright July sun, dirty snowfields hid the trail. I thought: look at this. It’s an almost baby glacier.
That snow melted before the next winter. But the interruption in the snow cover must have been brief. If the next round of storms had fallen on white ground, and the same thing had happened the next year, and the next, and the next, and the next, and the next … and the next, for several thousand years, a thickness of ice would have formed, taller than a skyscraper and wide as a continent. That’s all a glacier is: an accumulation of storms, across more seasons than we could ever imagine.
Why, during some ages of the Earth, do winters allow this? Reasons are nested within reasons, but the overriding cause is plate tectonics, that endless, apparently random reshuffling of the continents. Look at a globe: land divides the oceans at the north and south poles from warmer water towards the equator. Balmy currents can’t easily reach the top and bottom of the world. These frigid polar seas cool their parts of the world to the point that vast glaciation becomes possible. This span of millenia, when the configuration of the land makes the great ice sheets possible, is a glacial age. We live in a glacial age. It will end, in several million more years, when the continents have again rearranged themselves.
But (as my italics keep noting) the crushing of continents under ice is only a possibility, even during a glacial age. The South Taconics are now over a thousand miles from the nearest glaciers. But eighteen thousand years ago, they were buried under five thousand feet of ice. Why, within the span of the glacial age, do glaciers come and go?
Everything, even the orbit and tilt of the Earth, is subject to change. Our planet is subject to a 105,000 year cycle in which the shape of its orbit shifts from an elongated ellipse to a less elongated ellipse. There’s also a 41,000 year cycle in which the tilt of the Earth’s axis changes, and a 21,000 year cycle in which that moment when the Earth is closest to the Sun shifts forward from January to March and back again. (Yes, the entire Earth is a bit closer in January.)
The effect of all these cycles is that sometimes, for thousands of years, the summers are a bit cooler than that Sierra season I walked through. They are just cool enough to preserve a layer of white. This happenstance, given enough millennia, produces ice mountains that move in all directions from their own immense weight, punish landscapes into new shapes, erase life, depress the surface of the planet, and, when they retreat, form vast and shifting lakes that empty into miles-wide torrents, and allow the Earth’s surface to rebound. Sometimes this springing-back of the land is dramatic enough to change the course of rivers.
In other epochs of this glacial age, other glaciers gripped the mountains, retreated, gave way to other warm eras where other flora and fauna flourished. But evidence of these distant days is scanty, since glaciers are exceptionally good at destroying evidence of what came before them. Let’s begin this story with the most recent glaciation (termed the Wisconsin) which peaked about 18,000 years ago.
It’s simpler to imagine glaciers slowly advancing across the landscape (all the way to Long Island) and then slowly retreating (back to Labrador) but nature rarely tells a simple story. Evidence indicates the Taconics were covered in ice (beginning about 30,000 years ago) then freed, then covered again, freed, then assaulted once more before the glaciers returned to the far north. Even this narrative is a simplification: like a bulldozer digging away at a hillside, glaciers push forward, pull back, push forward again as microclimates come and go.
How can anyone look back through millenia and detail the doings of glaciers? Simple: the last glaciers left abundant evidence of their own careers, and no new glaciers have come along to destroy it. Simple, from my twenty-first century vantage point, but hardly simple to the pioneer geologists who climbed and peered and measured and recorded and pondered.
Students of Ice
One compelling example is Edward Hitchcock, who died during the Civil War, and in 1841 published a Final Report on the Geology of Massachusetts. It’s an exhaustive study of not only soils, rocks, river gorges and minerals, but also scenery. Hitchcock seems to have visited every corner of the Bay State, and devotes a whole section of the book to vivid and loving descriptions of natural sites. It’s disarming, in our day of sober and analytic science, to see the South Taconics turn a renowned geologist into a romantic poet. But listen to Hitchcock’s description of the view from atop Mount Everett:
“Oh what a glorious display of mountains all around you! And how does one in such a spot turn round and round, and drink in new glories, and feel his heart swelling more and more with emotions of sublimity, until the tired optic nerve shrinks from its office.”
After (with much effort) finding his way to Bash Bish, he heartily recommends a two day visit to Everett and the falls:
“To one who has a taste for the wild, the romantic, and the grand in nature, those two days will be a season of delightful emotions.”
But Hitchcock was not so swept away that he neglected his work. Painstakingly and meticulously, he recorded the depth and compass angles of striation marks in exposed rock, the locations of erratic boulders, the distribution of sand and gravel. One table lists eighty examples of “Diluvial Grooves” in Massachusetts (and neighboring) rock. One line from his list:
“Locality: Copake, NY, west side of Taconic Mountain
Rock: Argillaceous Slate
Direction: N 15 W, S 15 E
Remarks: Very distinct and extending down the slope some hundreds of feet.”
Hitchcock didn’t collect data just to publish it; he had a theory. At some time in the distant past, an immense ocean had flooded across the northeast. Plain evidence: the northwest by southeast grooves in mountain rock, the misplaced boulders, the worn and smoothed surfaces found everywhere. However — in 1840, Louis Aggasiz had used similar evidence to advance the idea of glacial action. Hitchcock became aware of Aggasiz’s work after he’d completed his manuscript; in a postscript he discusses it, chews on it thoroughly, and dismisses it, essentially because no-one in 1841 had any idea how the Earth’s climate could have changed so dramatically. Glacial action was a “dreamy hypothesis” which would pass on into the “caves of oblivion.”
It’s tempting to snicker at Hitchcock’s innocence. But the story of glaciers pushes the limits of imagination even today: the Empire State Building buried under ice? Six hundred centuries of glaciers advancing and melting back? A vast leap was demanded of the man’s mind.
Geological evidence also pushed his reasoning into tension with his devout Christian beliefs. He was “… aware that (his) conclusions might seem at variance with the sacred record.” But he reconciles the Bible and science with a kind of Deism: God set the wheels in motion, and nature took its course — but the Almighty designed those wheels to set a course towards human happiness. The Earth has become a fit habitation for humans because God willed it to become so. It’s an awfully comforting concept: plant remains become coal, animal remains become limestone, volcanoes diminish, because these events are useful to humans. “In all this we see indications of that same benevolent foresight and care … to which our daily experience of God’s goodness testifies.”
Ice Dominates and Disappears
In the twenty-first century, knowledge has grown, while faith in God’s goodness has diminished. For now, I’ll focus on the knowledge.
Somewhere around 100,000 years ago, ice began to grow out of the north. It’s difficult to write this without picturing a gleaming white wall lumbering forward, but of course, if you’d been a witness to that era, you wouldn’t have noticed any dramatic change. Maybe the winters might have seemed a little colder? The shift was subtle, but tectonic. Many, many human lifetimes later — 70,000 years (930 lifetimes?) — the ice would have reached the Taconics.
Probably an initial glacier was overtopped by an even greater one. Year by year, the combined mass swelled. Around 30,000 years ago, after inching down the Hudson Valley for centuries, ice began to claim the Bash Bish gorge, climb the slopes of Alander and Brace, and drown the shallow valley to the east. After retreats and returns, it finally left Everett’s crest, for a time, an island in a gleaming frozen sea. Until it too was swallowed. Then, for millennia, the South Taconics would have looked like nothing more than a swell in the patient deluge.
Ice came first from the northwest? Why guess that? One reason: the ice sheet probably originated in the area of Hudson Bay — north and west of the Taconics. Another has to do with “glacial plucking.” Picture a glacier climbing the west side of the Taconics. Friction increased as it hit obstacles. Ice at the base melted; water flowed towards the eastern side where pressure was less. But this water refroze, gluing rock to glacier. The eastward advance ripped great sheets of stone free. Walk the Appalachian Trail along the Taconics’ eastern edge (Race Mountain, especially) and you see the result: cliffs. By contrast, a climb up the west flank is steep, but ropes are unnecessary.
A line across Long Island, through northwest New Jersey, marks the culmination of glacial advance, around 18,000 years ago. Like a very slow-motion ocean, waves of ice crept back and forth, for centuries, but the glacier’s final conquests kept receding in the distance. By 15,000 years ago, the ice edge had shrunk back to the Taconics region. For a time, it may have been stuck there. In the far north, the weight of still-accumulating ice continued to shove the entire mass towards the ocean. But the front edge of this juggernaut melted quickly enough to cancel out the forward thrust: a “stable front.”
When a push broom retreats across a floor, it leaves a line of dirt and dust. Glaciers also do this, but their detritus is made of gravel, sand, rocks, boulders. If their edge stays in one place a while, meltwater streaming from underneath and over the edge adds more and more stuff to the pile. “Moraine” is the term for this left-behind, and a moraine crosses Dutchess and Columbia Counties, just north of 199 east of Red Hook and south of 7A through Ancram and West Copake, a low hill running in a steady line pointing towards the southern edge of the Taconics. I imagine the ice-imprisoned mountains gazing down on this holding action, impatient.
Once the rising warmth defeated that stable front, and the glacier slipped north, tumult ensued. For millennia, glaciers had robbed water from the ocean, the ultimate source of all that snow. During the height of the ice age, the edge of the Atlantic lay far east of our current coastline. But now the glaciers gave it back, drenching the landscape. Melting ice in the valley above flushed torrents down every ravine. Down Sages Ravine, Bear Rock and Race Brook Falls, and especially Bash Bish gorge, a thunderous gush pummeled and chewed rock, flung spray, and spread a delta in the valley below. Braided streams criss-crossed what came to be called Copake Flats, and the Sheffield plain. The soil in my Copake vegetable garden is stony, but at least I know why. The outwash from Bash Bish gorge, and smaller streams nearby, left behind coarse sand, pebbles, fat stones. For many years, the Taconics looked down on a sodden world of ponds, bogs, swamps and sluggish creeks. Some aquatic plants took hold in the valley, but vanishing ice left bare mountainsides gleaming in the sun.