The Unusual Absence of Ice Age Glaciers in the Black Hills

In many areas of Alaska, Canada, and the northern United States, as well as many other regions of the world, geologists have found evidence of glacial activity in the not-too-distant (geologic) past. In North America, specifically, an ice sheet covered some parts of Alaska, almost all of Canada, and part or all of the American states on the Canadian border. If we focus on the Dakotas, the ice sheet left a rather interesting mark in the form of the Missouri River. The river got diverted to the southeast by the ice sheet, and now the bedrock of the Dakotas mostly follows the pattern of glacial deposits east and north of the river and older rocks west and south of the river.

This geologic map of South Dakota shows how "East River" in the orange to brown differs from "West River" in all other colors. The Black Hills are the oval on the left. Courtesy of the USGS.

It makes sense to me how the ice sheet covered the eastern and northern parts of the Dakotas, given that the north is colder and the east is generally wetter, allowing more opportunities for snow to fall and add to the ice. I don't doubt that the ice sheet never made it to the Black Hills, but what I'm really wondering about is the absence of alpine glaciers in the Black Hills.

Average annual precipitation in the United States. You can see the Black Hills as a small spot on the SD-WY border with as much precipitation as Minnesota. Courtesy of the NOAA.

This is part of a map showing average annual temperature in the United States. The Black Hills once again stand out as a blue spot on the SD-WY border. Bluer means colder. Courtesy of the NOAA.

The Black Hills always have been colder, on average, than the rest of the Dakotas. Even in recent times, it is not unheard of for freezing temperatures to happen in July in the northern, higher elevations of the Black Hills around Lead and Deadwood. The Black Hills is also the only area in South Dakota that gets an average of over 72 inches of snow per year. When so much snow falls in an area that is as cold as the Black Hills, there tends to be evidence of Pleistocene glaciers. If we look at Humphreys Peak in Arizona, that has about the same climate as the northern Black Hills. There is evidence of glaciers there, and the Navajo people of the area have historically called it "the mountain that never melts". If these mountains had glaciers late enough for the Navajo to give Humphreys Peak its native name, perhaps during the "Little Ice Age" from roughly 1300 to 1850, why not the Black Hills? Or did the Black Hills in fact have glaciers in that time? I seem to recall reading that the Lakota people informed Lewis and Clark that the tops of the Black Hills had snow year-round, though I don't remember where I read that.

Apart from comparisons, we also have evidence of animals in the Black Hills that are found only in subarctic or tundra climates. At Wind Cave National Park, there is a site called Persistence Cave which has yielded bones from those kinds of animals, particularly pikas and pine martens. In fact, the webpage I read that information from concludes that "The environment represented at Persistence Cave is more similar to that of Rocky Mountain or Glacier National Park than it is to Wind Cave National Park today." Those parks have glaciers now, and if Wind Cave in the Black Hills had those animals, it should make sense for the Black Hills to have gone through some years with permanent snow, if not true glaciers.

There is also the reasonable assumption that the Black Hills were higher than they are now. On page 214 of Roadside Geology of South Dakota, geologist John Paul Gries says that the Black Hills started rising about 62 million years ago and finished by about 48 million years ago. Much of the rock in the Black Hills eroded away, which is what revealed the older rock formations. Also, much of this sediment has been traced from the plains east of the Black Hills to the parts of the Black Hills it came from. The question is, how much higher was the Black Hills during the Pleistocene? That would be interesting to answer as well.

I still don't have an answer for why there is no evidence of Pleistocene alpine glaciers in the Black Hills, but I still think it's likely that the Black Hills had glaciers at some point. Maybe we just haven't found the evidence yet, or maybe it has all been lost to erosion somehow. Perhaps, at some point in the future, a geologist will support my argument or provide good reasons and explanations against it.