Those of us who have hiked through forest or traveled across plains in the northern U.S. or Canada have occasionally noticed a large rock or boulder that seemed totally out of place. Surrounded by trees or half-buried in the soil, these monoliths appear to have either risen from the ground or dropped from the sky. On closer inspection, we find that their composition (usually igneous or metamorphic) is different from other rocks in the area.
These alien boulders are "glacial erratics," having been transported to their current location by the Pleistocene glaciers. Wrenched from a mountainside or scooped from the bedrock, they were carried atop or within the Ice Sheet as it advanced southward and then released as the glacier melted back. Often transported hundreds or even thousands of miles, these massive rocks are a testament to the power of flowing ice. Found across the glaciated areas of North America, they are but the largest and most obvious component of glacial cargo; smaller rocks, well known to farmers as "field stones," pebbles and fine till were produced and deposited in the same way.
A few of the more famous North American erratics include the 5000 ton Madison Boulder in Madison, New Hampshire, Doane Rock in Eastham, Massachusetts (on Cape Cod) and the Okotoks Erratic in Alberta, Canada. The latter, weighing in at 16,500 tons (the largest known glacial erratic on Earth), was transported from the Canadian Rockies by a mountain glacier. Not limited to terrestrial areas, these well-traveled boulders also dot the floor of the northern oceans, dropped there by icebergs that calved from coastal glaciers.
