What's a drumlin, and how do these geological features form?
Yesterday I hiked Dexter Drumlin in Lancaster, MA, which is an excellent and very visible example of a drumlin.
Here it is! Yes, it is indeed a hill, in a field—though some drumlins can be in forests or in other environments. So what’s so special about it, and how can you recognize if you have one near you?
Look at the shape of the hill. See how it’s got a more gradual slope on the right side (the “lee” side), which rises to the top, and then has a steeper slop on the left (the “stross” side)? That’s one thing that identifies it as a drumlin. Geologists call this an “inverted spoon” shape. This is caused by the movement of glaciers over the surface of the Earth. In order to be called a drumlin, they need to be at least 100 meters wide, and underneath their surface, they can be made from sediment consisting of basically anything from silt and gravel to clay. Most of their construction is of local material, meaning that the glaciers which formed them did not carry and deposit materials from a long way off to form them.
In many if not most places there are entire fields of drumlin, but here at Dexter there is the one obvious one, which is surrounded by farmland. There may have been more at some point, but with the old former Atlantic Union College across the street, and roads around the area, others may have been excavated to the point of obscurity.
How do these land features form? There are many theories, and scientists haven’t agreed on anything yet. Knowing which is right is made all the more difficult by the differences in drumlin composition between various fields. The one thing most scientists can agree upon is that they were formed by the flow of ice sheets over land, and that the steeper side is on the “up glacier” side, and the hill tapers off in the direction of the ice flow (so in the above photo, the glacier would have flowed left to right). Most drumlins that have been studied were formed during the last ice age, several thousand years ago. However, recent climate change has made it possible for scientists to see an active drumlin field in Iceland, formed between when the glacier surged in 1992 and when it retreated in 2009. It’s expected that soon, we may be able to see more drumlins in Antarctica as well, as the ice sheets recede there. While this recession is not really a good thing, one small perk is that we can learn more about how the features of our planet were formed. From studying the Iceland drumlins, we can see that the formations are more streamlined the further back from the glacier they are, suggesting they’ve experienced more glacial surges that wore them down then those closer to the glacier, which had the more pronounced snub-nosed shape.
Here are some resources if you want to learn more about these land features!