The Shonkin Sag in central Montana is one of the most spectacular remnants of the Ice Ages in North America.  It is a deep valley that winds around the northern edge of the Highwood Mountains; but occupied today mainly by several minor streams and several brackish lakes.  It is generally supposed to have been formed by a massive overflow from a lake dammed by Ice Age glaciers.  There are several problems, however, with the simple picture in which this immense gorge was formed as the result of a single event.  An alternate model is presented here, based on long periods in which the Shonkin Sag was the preferred channel of the Missouri River.

The stages in the development of the Shonkin Sag are:

• Introduction
  • The Standard Explanation
  • An Alternative Explanation