Erratics: The continental glacier’s calling cards
Acadia National Park on Maine’s “Down East Coast” is a place of mystical wonders: sheer granite cliffs rising from the sea, washed in the salt spray of enormous waves crashing against them; barren mountain peaks that seem much loftier than they really are, climbing upward from sea level and splashed with the colors of flowers normally found hundreds of miles farther north; spectacular stone bridges in the most unlikely places in the Acadian forests, gifted by John D. Rockefeller and looking like leftover sets from The Lord of the Rings films; and miles of fir and hemlock coastal forests, the dark green boughs hanging with the lichen called Old Man’s Beard, wreathed in fog and dripping in a silence broken only by the haunting cries of gulls and the clanging of buoys.
But Acadia is also an enormous outdoor natural history classroom, and educational institutions such as the College of the Atlantic in Bar Harbor frequently use its features for courses in geology, marine biology, botany, and environmental chemistry, among others.
The mountains of Acadia and other even higher summits in interior Maine must have been among the last places in the northeastern United States to have had glacial ice when the vast Wisconsinan glacier — the most recent continental glacier — retreated as Earth warmed over the last 15,000 years. U-shaped valleys, the odd asymmetrical mountains called “roches moutonnees” (which translates as something like “sleeping sheep”), bedrock showing the striations (scratches) caused by the advance of the glacier, and immense deposits of glacier-borne rock material all testify to the recent — in geologic time — Ice Age.
Glaciers are immensely powerful agents of erosion, and much of the upper part of the continental United States is covered in the debris they left behind.
The poet Robert Frost wrote frequently of the hardship of being a New England farmer — not the least of whose problems was the immense quantity of rock left behind in the landscape presenting an appalling challenge to the horse-drawn plough. His poem “Mending Wall” deals with the boulders — “some like loaves and some…nearly balls” that are ubiquitous in fields and which farmers for generations have used for stone walls and house foundations and churches.
The beaches of the Acadian Coast are famous for the almost infinite variety of types and colors of rocks found on them, many of which may have been transported for hundreds of miles from their point of origin: For it is a rule of glacial geology that an advancing glacier acts as a bulldozer, scraping away loose sediment and vegetation right down to the bedrock and then proceeding to abrade it, too, away; but a melting or evaporating glacier acts as a dump truck, leaving behind all sizes of rock particles from tiny clay and silt to huge cobbles and boulders in one heterogeneous mess called glacial drift.
The larger particles — the cobbles and boulders — are known by the quaint name glacial erratics. The high summits and slopes of Acadia are dotted with thousands of erratics, and on many of the peaks such as Cadillac Mountain, the highest mountain on the East Coast of the country, they sit in rugged isolation, often looking like the remnants of the mysterious rocky circles found in Ireland and England.
One of them is a famous landmark for visitors to Acadia: It is called Bubble Rock and it perches picturesquely on the steep slope of the summit of a small mountain known as South Bubble. It is an enormous chunk of the Lucerne Granite, a beautiful black-and-white speckled stone with large crystals that forms the bedrock some 40 miles north of Mount Desert Island
The sight of it challenges credibility: The slope appears to exceed the physicist’s “angle of repose” and it looks as if the slightest touch would send it careening noisily into the valley below it, smashing rocks and trees as it went.
Many visitors to Acadia National Park have appeared in the apparently obligatory photograph in which they attempt to dislodge it. Legend has it that, on a Halloween night some years ago, the entire football team of a local high school hiked up and attempted to push it off South Bubble. But there it stands.
A charming children’s book (now, alas, out of print) attributed the precarious position of the rock to Acadian trolls. But it must be remembered that on what is now the coast of Maine, the continental ice sheet was greater than a mile in thickness and thus had immense power to transport rocky materials: Retreating, it left this erratic and many others behind, sometimes in the most unlikely of locations.
Our own Helderberg hills and the Adirondacks also feature vast numbers of erratic cobbles and boulders, but, given the thickness of the forests that grow on them, erratics are often hard to spot, except when they are exceptionally large.
A relatively low mountain to the west of the Great Sacandaga Lake has a ridge running from its summit that features dozens of fairly large boulders of gneiss and anorthosite, transported from the High Peaks of the Adirondacks. Just below the mountain’s summit is a gigantic erratic, split down one side by frost wedging and the action of tree roots.
Far larger than Bubble Rock, it sits on a much gentler slope, and, located as it is in some fairly remote wilderness, it has likely had few visitors. Doubtless, hundreds more like it are scattered throughout the Adirondack fastness.
But we in the Albany area do not have to look far to find glacial erratics. Anyone who lives outside of the city of Albany — especially in the Altamont and Voorheesville area on the west side of the Hudson and in the Wynantskill and East Greenbush area on the east side — has found the same problem as the New England farmer in attempting to dig on one’s property: the enormous quantities of rocks in the soil.
They may have many different points of origin, of course, but one that is very common is a purplish rock often found as a smooth cobble or boulder, deposited directly by the ice or perhaps rounded and polished in a vigorous stream pouring off the melting glacier. These are samples of what is called the Potsdam sandstone and they originate far to our north near the border with Canada.
The fact that erratics’ points of origin can be determined with precision is useful to glacial geologists, who sometimes refer to deposits of such rocks as boulder trains. Appearing in more or less straight lines that strike roughly north-to-south, they allow the directional motion of the glacial ice to be determined very accurately.
When I am conducting geologic field trips, I usually toss out this question to participants: Just how long ago was the Ice Age?
Unless respondents are exceptionally knowledgeable, answers are usually accompanied by a shrug of the shoulders: “A million years ago?”
That is not a bad answer as a million years ago the ice was, indeed, in control of much of this part of the world.
But people are often unaware that even 10,000 years ago there was glacial ice on some of the highest peaks in the Northeast, and that we are living in what geologists call a glacial interlude.
But aren’t we in a time of global warming? Yes, there is evidence of that assertion. But all evidence is that the warmest Earth has been in the last 20,000 years occurred 7,000 years ago in a time geologists call the “hypsithermal,” and the glacial ice is poised for its return in a few thousand years.
In a few weeks, we will be entering the first stages of fall, and the average day-to-day temperatures will begin to drop. From time to time, we may see exceptionally warm days — we can all remember days in October and even November when the thermometer incongruously hit 80. But no one is foolish enough on such days to think that summer is coming back.
We are living in a geologic period of continental glaciers and the little purple stones in your backyard not only tell us that the last glacial advance was not so long ago; they may be harbingers of cold, cold days to come!