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Paddling Articles In the Same Boat

Hidden in Plain Sight

Legacies From the Last Ice Age

By Tamia Nelson
tamia@paddling.net

April 24, 2007

You can't spend time in Canoe Country without getting up close and personal with the landscape, a landscape that was shaped by ice. Lots of ice. And the last time it came, it came to stay, advancing and retreating for a couple of million years in response to forces that are still imperfectly understood. Then, some 10,000 years ago — a long span by our reckoning, but only an eye-blink in geologic history — the ice retreated for the last time. Meltwater carved new channels through the land. New growth blossomed, painting freshly-exposed earth in hues of green. Animals and birds moved in and made themselves at home. Humans followed. Soon the harsh stillness of the Ice Age, a silence broken only by rasping wind and blowing snow and the creaking of great glaciers, was no more. Once again, the landscape echoed to a myriad of living voices.

The Big Chill At its greatest extent, the Empire of Ice covered much of northern North America with a mile-thick mantle whose approximate boundaries are shown in blue on the little map to your left. So much water was locked up in this ice that the sea sank lower, exposing the portion of the continental shelf contained within the dashed lines. Later, as the massive glaciers melted away, their substance poured out in countless streams, and these filled thousands of pools — ranging in size from tiny potholes to great lakes — before the water continued its relentless journey to the sea. The continent itself, freed of its great burden of ice, heaved upward. This "isostatic adjustment" continues even today, its stately progress signaled by occasional earthquakes.

Of course, the rivers of meltwater weren't the only tools shaping the land. Glaciers aren't static crystal palaces. They flow. And while these great rivers of ice move far more slowly than their liquid counterparts, they, too, leave their mark on the landscape. If you paddle in Canoe Country you've seen the results. But did you know what you were seeing? Much of the evidence of glaciation isn't obvious. You could say it's hidden in plain sight. Still, it's not hard to educate your eye. Let's begin with one of the Ice Age's most striking calling cards:

Glacial Erratics

Resident Alien Erratics are migrants from afar. You could call them resident aliens, I suppose. They can be as small as pebbles or as large as a Greyhound bus, or any size in between. Obviously, the bus-sized boulders are the most impressive, and when you come across one like this 15-footer, you can't help but notice it. An erratic is as hard to miss as a newt in in your water bottle. Erratics differ from the local bedrock, and the differences can be striking. Suppose you're paddling down a central New York river that flows between walls of dull gray limestone. Suddenly, you see a truck-sized boulder of streaky pink gneiss perched on a ledge. The New York limestone is blocky, but the gneiss is rounded. It's obviously a stranger to these parts — an erratic, in other words. And just where did it come from? That's a harder question to answer, and you'll need a little local knowledge. The boulder looks like it was quarried from the bedrock in eastern Ontario, hundreds of miles away from the New York river where you found it. How did it get so far from home? You can rule out the river. It doesn't even flow through Canada, and, anyway, it isn't muscular enough to move a boulder of that size such a long distance. And, no, Sisyphus wasn't responsible. A glacier was.

Here's how it happened. As the glacier slowly ground its way south, it plucked up a good-sized chunk of Canadian country rock and imprisoned it in ice. There it remained, locked inside the moving glacier. Hundreds of mile further on, the glacier halted, all forward progress checked by warmer temperatures. Then the ice began to melt. Sooner or later, the boulder was freed from its frozen tomb and found a new home in New York — without benefit of passport or customs clearance. What's that? You think you've seen an erratic that grounded far from any river? Well, that's not surprising. You probably have. Erratics can be found anywhere within the former boundaries of the Empire of Ice, even on mountain summits.

 

Having probed the mysteries of erratics, you continue your interrupted journey. But the next time you go ashore to stretch your legs, you look down at your feet. And what do you see?

Carvings in Stone

Chatterrocks Not all the rock entombed in glacial ice is as big as a bus. Some pieces are no bigger than your fist, or even the end of your little finger. In fact, the underside of a glacier can be studded with such fragments, many as sharp and angular as a drill bit. And what happens when the glacier grinds forward? The stony drill bits' stuttering progress leaves its mark in the underlying bedrock. Literally. The result? Crescent-shaped gouges known as chattermarks. The picture on the left shows chattermarks in sandstone; a camera's lens cap provides the scale. Notice how the marks curve. Since the bellies of the curves point "upstream," the ice advanced from left to right.

Glaciers also sign their work in other hands. Wherever you see chattermarks, look for glacial striations, grooves cut in bedrock parallel to the direction of glacial advance. And don't ignore the striations' larger cousins, glacial grooves, wide troughs milled in rock by larger cutting tools. Lastly, look for evidence of the characteristic polish left on bedrock by the glacial "milling paste" of fine-grained sediment and water.

On This Rock...
Bedrock in the backcountry

You'll have plenty of opportunities. Bedrock is a familiar sight in Canoe Country. In many places, the ancient ice rivers scoured away all organic soil, and it takes time to recover what was lost. Even 10,000 years is just a start. Happily, rocky points of land make fine campsites in hot weather, a welcome alternative to the stultifying confinement of lowland spruce. And there's a bonus: The breezes that play over the rock keep many biting flies at bay. In swamps, a "whaleback" of bedrock offers a dry refuge, not to mention a commanding view of the route ahead — useful indeed if you find yourself temporarily confused as to which way to go.

 

So far, so good. But the ice did more than strip soil and scour bedrock. It also gave us…

Eskers

When the ice melted, it deposited its burden of scoured soil and rock dust on the land beneath. Countless tons of sediment were left behind in the low hills called drumlins and in the sinuous ridges that snake through much of Canoe Country, following meltwater channels that once coursed under the glaciers. These ridges are eskers, and we've met them before. From the seat of a small boat an esker looks just like any other steep-sided ridge. But a topographic map gives the game away:

Why Don't You Esker?
Portrait of an esker

Cultivate an eye for eskers. Like bedrock whalebacks, they make good campsites in swampy lowlands. They're high, dry, and (relatively) bug-free. Climbing up the steep sides can be difficult, however. To minimize the likelihood that your trail will become a gully in the next heavy rain, zig-zag your way up the slope.

 

Not all glacial sediment ended up in hills and ridges. Much of it settled down in…

Swamps, Bogs, and Stranded Beaches

Hack off a hunk of glacial ice, put it in a bucket, and bring it into a warm tent. Now let it melt and see just how much sediment collects at the bottom. When the ice retreated from Canoe Country, meltwater rivers distributed such sediment far and wide, and a lot of it accumulated in poorly-drained areas. Swamps and bogs were the result.

Bog River
A river runs through it

This sweeping panorama of a wildlife-rich lowland was photographed from atop an esker, a high, dry refuge in an otherwise waterlogged landscape. Other refuges can be found in unlikely places: beaches and deltas left behind by long-vanished glacial lakes and inland seas.

High and Dry
An ancient beach

A swampy swale occupies the foreground. Can you see the small pond and weathered wooden dock in the middle distance? The sands of an ancient beach-and-delta complex rise high above it. Canoe Country soils are fragile. In this instance, the thin blanket of organic soil overlying the deltaic deposits has been stripped bare by ATV traffic. (Man is also an agent of geologic change. Surprised?) Of course, such features are often easy to overlook under way, and even harder to identify on a map, no matter how large the scale. But some glacial legacies are unmistakable. Consider…

U-Shaped Valleys

When glaciers overran the landscape, they sanded away many of the rough edges. Fast-flowing rivers tend to carve steep, V-shaped valleys. When a glacier moves down such a river valley, however, it grinds away the flats, leaving a characteristically U-shaped cross section. Take a look at the mountainside on the right of the picture below:

Glaciated Valley
Glacial valley

Note the gentle, concave curve of the valley wall. It's most evident on the right, but the opposite slope mirrors its contours. Before the glacier swept over these hills, a river followed a concealed fault line. The advancing glacier then rounded off the steep sides of the fault-line valley. Lastly, glacial meltwater dammed the valley outlet with a gravelly mix of sediments called till. Now a lake fills the valley, a windswept monument to the passage of an ancient river of ice. Canoe sailing, anyone?

Canoe Country was once buried under hundreds of feet of ice. The signs of this arctic invasion are everywhere, awaiting your next voyage of discovery. They're hidden in plain sight, so all you need to do is look for them. Then, the next time you take refuge from the wet on an esker or defy the bugs on a bedrock whaleback, you'll be pleasantly reminded that all you see around you was once a provincial outpost of the Empire of Ice. How things have changed!

Copyright 2007 by Verloren Hoop Productions. All rights reserved.









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