Boulders Left Behind By Glaciers Are Known As?
For centuries, people puzzled over the presence of large boulders in places they clearly didn’t belong based on the type of stone they were made of. How did a granite boulder the size of a house get into the middle of a field when the nearest quarry with similar stone was hundreds of miles away? How could boulders from a mountain get to the plains when it was clearly too far for them to have simply rolled down the distant mountain?
Originally, the explanation for such abnormalities was a vast flood approximately 10,000 years ago, similar to the legendary floods described in the texts of ancient cultures including Mesoamerican, Sumerian (Epic of Gilgamesh), Hebrew (Old Testament) and Indian culture. We now know, however, that these massive stones, known as “erratics” in geological terms, were moved by geological forces, primarily glaciers. Tens of thousands of years ago as glaciers slowly crept across most of the world, they acted as enormous earth movers, crushing, churning, and sometimes lifting everything in their path. Occasionally, a glacier would lift up a boulder or its pushing against rocks would break pieces off and trigger avalanches. The pieces would then be carried forward, sometimes for hundreds of miles, before time and climate change eventually triggered a melt that deposited the boulder wherever it happened to be.
Erratics aren’t just a geological curiosity befitting nothing more than a trivia question though. Studying erratics is useful to geology because it reveals a wide range of information including the patterns of prehistoric glacial movement, patterns of glacial flooding (as the glaciers melted and flooded prehistoric lakes, many erratics were carried forward on huge rafts of ice), and they can even use them to track historical ocean temperatures and current patterns as erratics carried off by icebergs would eventually be dumped into the ocean as the icebergs melted.
Image courtesy of Coaxial.