#10 - Recessional Moraines Near Canadian Border in North-Central Montana

The Big Flat . . .
Turner is small farming community on the Boundary Plateau, 12 miles south of the Canadian border in north-central Montana. Known locally as "The Big Flat," the plateau sits a few hundred feet higher than the surounding prairie. Interestingly the Big Flat was NOT covered by the most recent glaciation (Wisconsin Ice Age), but WAS over-run during the previous advance (Illinoian Ice Age). While trying to learn more about the types of glacial deposits on and around the Big Flat, Rob Thomas PhD (geology professor/author) shared the Quaternary Geologic Map of the Havre Quadrangle (USGS) with me. A portion of the map (above) showed several "minor moraines" (red lines) in the area south of Turner, so I decided to check Google Earth to see if they were discernible. See for yourself - Compare the map above with the exact same area shown on the Google Earth image below. Click on the image to enlarge. On the Google Earth image the ridges (minor moraines) look like faint ripple marks on the farmland. The yellow X marks the place I put my drone up to take the photos at the bottom of this page.

Conveyor belt . . .
Moraines are accumulations of rock material carried by glaciers - in this case the Laurentide Ice Sheet. The rocks were transported here from Canada and then dropped at the end of the glacier as it melted (like a conveyor belt). If a glacier is neither advancing or retreating, a significant ridge of rock material can build up - called an end moraine. These minor moraines south of Turner are recessional moraines formed as the ice sheet paused periodically during its retreat. NOTE: Glacial retreat is NOT caused by the glacier moving backwards, but rather melting away from the front (terminus). "Receding" would be a better word.

Boots on the ground . . .
I saw the map. I saw the Google Earth image. Next, I needed to see the land with my own eyes. At the time (april 2024) I was teaching in Hays-Lodgepole High School 70 miles south of Turner, so one Saturday I drove north to check it out. Although they were only a few feet high and covered with soil, the moraines were very obvious! I put up my drone and took the photos shown below. It was pretty cool to look upon the landscape and see ridges of rock material deposited during the Illinoian Glacial Maximum, 190,000–130,000 years ago!

Term: recessional moraine