Glaciers Rock

Nothing reveals topography like a bicycle.  You can walk a stretch of road a hundred times and not notice a grade that is immediately clear after one ride.  And in my neck of the woods, nothing has created topography like the Laurentide ice sheet.  The weight of a full mile of ice smoothed the north-west side of bedrock outcrops leaving deep grooves; the south-east side left jagged.  Ten to twenty thousand times the several car loads of leaves I take to the dump each year could not cover the scarred rock.

The bedrock outcrops make for a good class trip, but all the loose rock worn smooth on the trip down from Vermont and New Hampshire make for a good ride.  It's tempting to imagine the ice sheet over Boston as a blue-white expanse of Arctic snow.  At least part of it was more like what you find piled at the edges of a mall parking lot in spring:  an abrasive mix of ice, rock and dirt.

It's also tempting to imagine the retreating glacier disappearing like a heavy snow in spring.  A mile thick of ice and debris melts in some spots and stays stubbornly frozen in others.  It creates rivers and vast lakes both on its surface and deep below.  The channels sweep enormous boulders clear in some places and fill the river bottom with layers of rounded stones in others.

The channel beds and lake bottoms that remained after the glacier's retreat became the eskers and drumlins that make such good riding and the sand fields that separate them.  An esker is the inverted bed of debris left by a stream or crevasse.  Drumlins are similar, forming mounds instead of elongated hillocks.

When I first moved to the Boston area, I thought the long, tall piles of rock and dirt were somehow a result of the highway on one side or the reservoir on the other.  It seemed impossible that the long ridges of debris with sides too steep to climb at places could be natural formations.  And it seemed equally implausible that they were connected to the great reed flats flanking the Charles River.  After hiking and then riding the trails that climb the ridges and skirt the ponds, it was thrilling to learn that this landscape was all created by a glacier.  It's sort of like finding a fossil in your backyard covered with miles of technical, swoopy trails.