A glacial erratic is a rock carried and transported by glaciers millions of years ago. When the glaciers melted and receded, it left behind the rocks sticking out like sore thumbs, and at the most oddest of places. Glacial erratics are often carried over from distances of hundreds of kilometers, because of which they are almost always different from the type of rocks native to the region. This makes them easy to spot, but not unless you are a geologist. For most observers, a glacial erratic looks like any other regular boulder.
Photographer Fritz Hoffmann isn’t a geologist but he is a keen observer. Once when scanning the New England landscape while driving along in the winter, Hoffman noticed a repetitive quality in the large rocks on the land. “With the leaves off, I could see them tucked into the woods. They were definitely boulders and not rock outcrops,” said Hoffman. “That piqued my curiosity and it was then that I began to learn what a glacial erratic is.”
The “aha” moment came as we were driving to Cape Cod one weekend and passed the exit to Plymouth, MA. I had already written my proposal but needed a strong hook to lead with. I made a u-turn and found the nearest tourist center. I raced in there with one question. Is Plymouth Rock a glacial erratic? The woman behind the counter replied, “A glacier moved it here if that is what you mean” … Once I made the connection of glacial erratics to rocks like Plymouth Rock, Glen Rock and the Rollstone Boulder, I knew I had my story and I became hooked on erratics.
Fritz Hoffmann’s photo-story about glacial erratic - Pet Rocks – came out in the March, 2012 National Geographic magazine. Here are some spectacular pictures of inconspicuous rocks with dramatic travel stories.
Looking as if it fell from the sky, a 40-ton erratic stands on the Colville Indian Reservation in Washington State. Such boulders are sometimes called rubbing stones because bison scratched up against them.
Sundown over Olmstead Point in Yosemite National Park, California. Glacial erratic boulders litter the glacially polished bedrock and are characteristic of the high altitude landscape of Tioga Pass in Yosemite National Park. As the ice sheet pushed across the rock mountains it plucked off the leading edge of the mountains and entrained these rocks in the ice. When the ice melted, the rocks were left in these erratic locations. Olmstead Point was named after Frederick Law Olmstead. Half Dome can be seen in the distance.
Having just scaled his favorite rock, a glacial erratic located in Central Park, NY with a view over the sheep meadow and the east side, 5 year-old Luca uses his hands to express his joy to his mother, calling out, "Thank you mommy!". The glacial erratic sits on bedrock at the southern edge of the Sheep Meadow. The boulder is clearly identified as an erratic from its contrasting material with the stone that it sits on. While the boulder was surely left in the park by the ice sheet 18,000 years ago, it may have been moved to this location during park construction.
This erratic stands where it was deposited by a glacier; the town of Glen Rock, New Jersey, grew up around it, and Doremus Avenue makes its way on either side. Take a right at the stop sign and you're on Rock Road, which leads to the center of town. Glen Rock's rock has several names: the Rock, the Great Rock, and the Big Rock among them.
Boulders plucked from a nearby mountain rest on the polished expanse of Yosemite's Olmsted Point, elevation 8,400 feet. The glacier that carried them flowed into adjacent Tenaya Canyon and grew to thousands of feet thick.
Boulders perch randomly at Olmsted Point in Yosemite National Park. A glacier sculpted the 92-million-year-old bedrock here and left the boulders, plucked from a nearby mountain, when it retreated. The rocks, along with grooves in the bedrock, show the path of the glacier.
Brenda Diaz and Jessica Ruiz take a break near the rock during a road trip. Indians used to mark boulders with carvings. Today's artists, some of whom have immortalized the years of their high school graduations here, prefer paint.
Yeager Rock (at right), in north central Washington State, helped geologists map how far south the ice sheet pushed—and provided a surface where local graduates could paint important dates.
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