Eskers
Remembrance of Rivers Past
By Tamia Nelson
The day was drawing to a close. All of us were tired, and some of
us were getting decidedly cranky. We'd been paddling for hours along a
narrow river, through country that had the character of a waterlogged
sponge. In fact, that's really what it was—a well-watered lowland
forest, rich in black spruce, shrubby leatherleaf, and bog flowers. A
great place for turtles, herons, ducks, muskrats and beaver. Not so
great a place for camping, however. Still, we knew that a good
campsite wasn't far away. We were looking for an esker.
The sky darkened. Our shadows lengthened and then disappeared
altogether. The sun sank below the horizon. Finally, just as the last
of the light was going, we rounded a bend and saw a long, low ridge
rising up alongside the river. We'd found our home away from home. We
weren't the first folks to do so. The scattered remnants of a
fish-drying rack could be seen on the river bank, lying where some
earlier gale had brought a wet-footed gray birch crashing down upon
it.
We dragged ourselves and our gear up a short, steep trail to the
summit of the ridge. The narrow crest was level and dry—scarce
amenities in the watery lowlands of central Ontario. Birch and aspen
leaves trembled in a gentle breeze that helped keep the evening
mosquitoes at bay. Chickadees wished us good night. Then they were
silent. The first notes of the frog chorus began. Somewhere, not too
far off, a loon called. In no time at all we'd pitched our tents on
the stoney, sandy soil and were busy making a late supper. It wasn't
the first time I'd camped on an esker, of course—eskers are
common throughout the once-glaciated regions of the northeastern
United States, eastern and arctic Canada, and Scandinavia—but it
was the first time I'd actually gone looking for one.
Often known locally as "hogbacks" or "horsebacks," eskers aren't
among the most spectacular landscape features. From the seat of a
canoe or kayak, an esker looks like nothing so much as a long, low,
undulating ridge. Seen from near its base, however, an esker rises
steeply. If there isn't a landing carved into the flank or a small
beach at its foot, you'll have a hard time scrambling out of your boat
and up the slope. As you climb, you'll notice that the esker is formed
from gravel, cobbles and sand. Look closely at the stones at your
feet, and you'll see that they have very few sharp edges. Each stone
looks as if it's been tumbled about for a long time, like a
rough-polished gem. And so it has. The gravels and cobbles of eskers
have been polished by moving water, just like the stones in a
riverbed. And, indeed, eskers mark the paths of long-vanished rivers.
This becomes more obvious when eskers are seen from the air. Look
down on one, particularly in the treeless barrens of the high arctic,
and you'll see a narrow, sinuous ridge winding over the landscape. An
esker can extend for miles, lying on the land like a discarded feather
boa on a Victorian ballroom floor.
So how, then, did eskers form? Some 10,000 years ago, the
continent-spanning glaciers of the most recent Ice Age were in
retreat. As the mile-thick ice warmed, rivers of meltwater ran through
caverns deep beneath the surface. These hidden torrents picked up and
carried the sand and rock formerly trapped in the ice. Where the
ice-walled channels were steep, the sand and rock were tumbled smooth.
As the gradient of the channels lessened, or as the flow of water
slowed from a torrent to a trickle, these secret rivers dropped their
burdens. First the heavier stones—cobbles and large
gravel—settled out. Next came the smaller gravel. Last to be laid
down was the sand. Finally, over many years, as the glacier retreated,
the once-hidden channels were freed from their icy prison. As the ice
walls melted away, the river-bed cobbles, gravel and sand were first
exposed and then deposited on the underlying land. The margins of the
piled sediments then slumped, leaving the sinuous casts of fossil
rivers behind. The retreating glaciers had given birth to eskers.
But how can eskers be distinguished from the now-empty channels of
ancient surface rivers? Easily. The rivers we see today are shaped by
the land through which they flow. Their courses are determined by
topography. Even when long dry, their beds reflect this earlier
constraint. The hidden rivers which gave rise to eskers, however,
flowed in suspended channels, buried deep in the ice and yet still
high about the surface of the ground. When the great continental
glaciers melted back, they deposited the sedimentary casts from these
secret channels willy-nilly over the newly-exposed land. Eskers
therefore show a glorious independence—one forever denied the
beds of more conventional rivers. Eskers go where they will, often
cutting across the "grain" of the landscape.
All well and good. But we're not all geologists. Why would any
practical canoeist or kayaker care about eskers?
That's a fair question. Let's go back to the start of my story. We
were paddling through a spongy, waterlogged lowland forest. Dry
campsites were few and far between. But we weren't condemned to spend
a soggy night. Find an esker, and we'd find a well-drained and
attractive camp. Eskers are made up of cobbles, gravel and sand. Even
when all the surrounding country has the consistency of cold oatmeal,
an esker camp will be high, dry and comfortable. That's something that
even the most pragmatic paddler will appreciate.
So how do you find an esker? If you keep your eyes open, you'll
learn to know one when you see it, but it's even better to be able to
plan ahead. Begin by examining the large-scale topographic maps for
your trip. Remember that an esker is a sinuous ridge, snaking across
the landscape. That's what you're looking for. Here's an example,
taken from a USGS 1:25,000 metric map depicting a portion of the
10-mile-long Jenkins esker in the northern Adirondack
Mountains:
See the narrow, winding ridge in the highlighted section, just to
the left of center? That's the topographic signature of an esker. Note
how the contour lines bunch together on the steep east and west
flanks. (North is at the top of the map, and the contour interval is
five meters.) While the esker is as much as 25 meters high from base
to top, it rises and falls along its length. Do you see the
eastward-trending, finger-like ridge splitting off to the north?
That's most likely a tributary of the main esker, breached by the
river to the east but re-emerging near the marked shelter. Compare the
profiles of the nearby hills with that of the esker. Though some of
the hills are as steep as the flanks of the esker, their peaks are
broader and higher.
Of course the landscape depicted in the map above exhibits good
relief, with many possible campsites. Not all parts of canoe country
are so blessed, however. When the surrounding land is low and marshy,
an esker stands out unmistakably on a topographic map. It's also a
very welcome sight at the end of a long and weary day. So the next
time you plan a trip up North, take a good look at your maps
beforehand—and then keep your eyes peeled as you paddle along. A
fossil river is well worth looking for!
Copyright © 2000 by Verloren Hoop Productions. All rights reserved.