On this date about 12,000 years ago, and for that matter, just about every day for about 60,000 years, some interesting geological features were being created in North Dakota. They’re called drumlins – small hills elongated in the direction of glacial movement. The typical drumlin is about 100 feet high and maybe 500 feet long and 200 feet wide, with a blunt nose in the direction from which the ice approached, and a gentler slope in back.
In 1993, State Geologist John Bluemle, wrote about them. He had studied drumlins in Ireland where they’re quite common. They typically had a base of gravel and sand capped by glacial sediment.
The drumlins in central North Dakota are similar, but they also have differences. The best-developed are in McHenry County between Minot and Harvey. Bluemle observed that they range from hills like the typical drumlins of Ireland to hills that are amazingly long.
In 1987, he did a study of about 200 of the elongated drumlins between Verendrye and Balfour. To the uneducated eye, they appear to be ridges. In Ireland, most drumlins are 3 times longer than they are wide. The McHenry County drumlins are more like 30 to 1, even 50 to 1, and then there’s Hogback Ridge, which is 17 miles long and about 375 feet wide! That’s a ratio of 240 to 1.
The McHenry County drumlins run from northwest to southeast. They’re so straight, they look man-made. From the air, Hogback Ridge looks like the grade of a large railroad or highway. At one point, the locals took advantage of this, and used a segment as a raised roadbed.
Bluemle and his team made several excavations on Hogback Ridge. They decided it “formed quickly, probably over the period of a year or two, and that it was shaped by a very rapidly flowing glacier.”
The movement scraped out a gouge or “tunnel” into the underside of the glacier. As it continued to move, those cavities got longer and longer. If the glacier had been thicker, the underground tunnel would have closed in with more ice. But that didn’t happen. The space beneath the relatively thin glacier filled with a mush of glacial deposits mixed with sediment from a previous lake bottom. The weight of the glacier squeezed the mush upward to fill in the cavity, creating the amazingly long straight ridges we know today.
Dakota Datebook by Merry Helm
