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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.
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
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
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.
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:
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.
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.
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:
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.