Drone photo - That's me in the lower left. Click on it to enlarge.
Come and go . . . Earth has experienced several ice ages in the past, with the most recent one starting about 2.6 million years ago - the start of the Pleistocene Epoch. Over and over, continental glaciers from northern Canada grew toward Montana, advancing and then melting away once every 41,000 years. Mysteriously, about a million years ago the timing of the cycle changed from once every 41,000 years to approximately once every 100,000 years. The graph at the below shows the last four advances. They're labelled as "Glacials" on the graph, but are commonly referred to as "ice ages." Geologists believe that it was only during the last two glaciations that the continental glacier grew far enough to reach into present-day Montana, about as far south as the Missouri River. The most recent advance, referred to as the Wisconsin Ice Age, ended about 12,000 years ago (the end of the Pleistocene Epoch), and the one before it is called the Illinoian Ice Age.
Out of place . . . The glaciers left all sorts of evidence as they advanced and then melted on the Hi-Line area. Many of these are hard to discern or unnoticeable to the untrained eye, but everyone who has travelled the area has seen the big random rocks on the prairie or big piles of rocks that dot the farmlands. Rocks like this were dubbed "erratics" by early geologists because they didn't match the type of rock that was natural to the area. "Erratic" comes from the Latin word
errare, meaning to make a mistake or to wander. The rocks appeared out of place in their new surroundings. In fact those big erratics (and many smaller ones too) that travelers see on the Hi-Line hail from northern Canada, many from as far away as the Hudson Bay - and it is clear that they were transported here by coninental glaciers that grew into northern Montana during the Illinoian and Wisconsin Ice Ages.
Like a conveyor belt . . .
The continental glacier that eventually covered much of Canada and the northern part of Montana has been named the Laurentide Ice Sheet. Its primary growth center was Hudson Bay region. As the thickness of the ice grew and the glacier started to spread from from there it "plucked" pieces of rock from the bedrock - mostly metamorphic rocks. The rocks became embedded in the glacier and rode along until the matrix of ice melted centuries or even thousands of years later somewhere in Montana. That was the end of the line for the rocks.
Picking rock . . .
The glacial erratics are a nuisance to farmers who want to grow crops on the land. Farmers must first remove the erratics, either by manual labor or by using specialized machinery. The rocks are typically piled in an area that will have to be farmed around in the future. The photo atop this page is an extreme example of one of those piles - by far the biggest pile of erratics I've ever seen! It is located near the tiny town of Hogeland about 50 miles northeast of Havre (straight-line distance). Interesting these rocks were all brought here during the Illinoian Ice Age, not during the more recent Wisconsin Ice Age. Both Hogeland and neighboring Turner sit on the Boundary Plateau, known locally as the Big Flat, and apparently the Laurentide Ice Sheet was not thick enough to cover the plateau the last time it grew into the area. It did flow around it though.
Term: Pleistocene Epoch
