2/1/2005:
North Dakota geologist John Bluemle writes, “Drumlins are small hills, elongated in the direction that the glacier was flowing. They are abundant in Ireland, so it is appropriate that the name ‘drumlin’ is derived from an Irish Gaelic word, ‘druim,’ meaning ‘back’ or ‘ridge.’ I’ve always thought of classical drumlins, like the ones I’ve seen in Ireland, as kind of like inverted spoons, but there they are commonly referred to as ‘basket of eggs’ topography.”
Bluemle says, “A typical ‘classical’ drumlin is about 100 feet high and maybe 500 feet long by 200 feet wide. They are commonly about two or three times as long as they are wide. A drumlin usually has a blunt nose pointing in the direction from which the ice approached and a gentler slope tapering in the other direction.
“Several years ago when I was in Ireland,” he writes, “I spent some time working with an Irish geologist, studying drumlins north of Belfast. The drumlins he showed me typically have a base of gravel and sand material that was obviously deposited by running water along with a cap of glacial sediment that must have been plastered on by the moving ice, which also shaped the features.”
A number of drumlins exist in central North Dakota, as well. In many ways, they are similar to classical drumlins, but are also very different. Our best-developed drumlins are in McHenry County between Minot and Harvey. Bluemle says they range from drumlin-like hills similar to those in Ireland, to features that are much more elongated. Much more.
In 1987, he did a study of about 200 extremely elongated drumlins in the area between Verendrye and Balfour. To the uneducated eye, these appear to be ridges rather than elongated hills. For example, in Ireland, most drumlins are 3 times longer than they are wide. The McHenry County drumlins are more like 30:1 and 50:1. And then there’s Hogback Ridge, which is 17 miles long, about 375 feet wide, and 25 to 40 feet high along most of its length. That’s a ratio of 240 to 1.
Hogback Ridge and the other McHenry County drumlins run from northwest to southeast, which is the way the glacier moved through that region. The most striking thing about these drumlins is that they are so straight they look like they’re man-made. In fact, from the air, Hogback Ridge looks like a large railroad or highway grade. At one point in time, the locals took advantage of this, and did use a segment as a raised roadbed.
Bluemle and his team made several excavations on Hogback Ridge to try to determine how it was formed. “We decided that Hogback Ridge formed quickly,” he writes, “probably over the period of a year or two, and that it was shaped by a very rapidly flowing glacier. The glacier was very thin, and at the point where it crossed what is now the Souris River Valley near Verendrye, it picked up (or “thrust”) a large block of material for a short distance.”
Visualize this large immovable mass scraping out, or carving, a gouge or “tunnel” into the underside surface of the ice. As the glacier continued to move, these cavities got longer and longer as the ice forced its way over the obstacle. If the glacier had been thicker, the underground tunnel would have closed in with more ice, because glacial ice is “plastic.” But that didn’t happen in McHenry County; the glacier was too thin. Instead, the ground underneath was a sort of mush of glacier deposits mixed with sediment left from a previous lake bottom. The weight of the glacier squeezed this mush upward to fill in the cavities, creating the amazingly long straight ridges we now call drumlins.
Dakota Datebook written by Merry Helm