Distinctly Montana Magazine

65 w w w. d i s t i n c t l y m o n t a n a . c o m M ontana is a study in contrasts. The western part of the state is covered with mountains, and this is where Montana gets its name. The east, however, is most- ly known for its flat, wide-open prairies and plains. And this is where Montana gets its nickname, the Big Sky State, because there are no bigger, more brilliant skies than on Montana's eastern plains. Many cataclysmic events over millions of years, however, helped form the nooks and crannies in between where the mountains and plains meet. One of those places is called the Shonkin Sag. And though very few people know about it, every Montanan probably should. The Shonkin Sag is a geological feature located in Chouteau County, Montana. It is well known locally for its unique land- scape and extreme terrain. It is also one of the most important glacial and volcanic features in North America. Perhaps the world. The Shonkin Sag is a large, deep, canyon-like depression flanked by ridges and cliffs. The steep, rugged walls create a striking contrast with the smooth valley floor below. The land on the plateau above this remarkable geological feature is flat and brimming with fields of wheat and cattle. "The Sag," as some call it, was formed during the Pleistocene period, which occurred from about 25,000 to 15,000 years ago. But the Sag would not have been created if several other events had not taken place millions of years before. First, a body of magma formed the Highwood Mountains. About 50 million years ago, an intrusive volcano ejected layers of magma into the overlying sedimentary rocks. The sediments were later eroded, exposing the more resistant magma layers that now form the backbone of the Highwoods. Second, about two and a half million years ago, we entered the Ice Age, when ice sheets advanced from the north and covered huge swaths of North America, including eastern Montana. As the ice sheets covering Montana formed and retreated every 100,000 years or so, the shape and course of the Missouri River also changed. North-flowing rivers, like the Missouri, were dammed by the ice sheets and formed several large glacial lakes in the region. Glacial Lake Missoula in northwestern Montana is probably the best known glacially dammed lake, but there were others, including Glacial Lake Great Falls, which stretched from as far west as present-day Choteau, Montana and as far east as Geral- dine. During the last ice age, the Laurentide ice sheet reached the north side of the Highwood Mountains and formed a barrier to the Missouri River. Glacial Lake Great Falls was created up- stream of this ice dam. At some point, or perhaps even repeatedly, the ice dam broke, and large amounts of water were released from the glacial lake. Glacial Lake Missoula abruptly released so much water that enormous floods carved the scablands and rolling hills of the Palouse region in eastern Washington. Glacial Lake Great Falls also catastrophically released, and huge amounts of water carved out the Shonkin Sag, including the dry falls which now form the west side of Lost Lake. story and photos by TODD KLASSY A look over the edge at the 200 to 300 ft. tall cliffs on the northern face of Lost Lake between Shonkin and Geraldine, Montana. Thousands of years ago this was the precipice of a waterfall that some have compared to the likes of Niagara Falls.

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