We now take a side trip on Montana State Highway 80 to see one of the most spectacular remnants of the Ice Ages to be found anywhere in North America.
During the maximum extent of the continental glaciers the climate was
very cold and very arid, so some rivers may at times have stopped
flowing completely.  The Missouri River, whose source lies far to
the south of the glaciers, probably flowed throughout the year.
When the ice blocked the path of the river, a large lake began to form
behind the ice dam.  This lake, Lake Great Falls, and Lake Agassiz
in present North Dakota, Minnesota, and Manitoba were among the largest
lakes of the Ice Ages.  Lake Agassiz at its greatest extent covered
a greater extent than all the Great Lakes together.  However the
water in Lake Agassiz was mostly melt water, while Lake Great Falls
was fed by both the Missouri River and melting ice.  As Lake Great Falls
continued to rise it cut an overflow channel along the north side of the
Highwood Mountains and flowed between the ice and the mountains, creating the
immense valley of the Shonkin Sag.  The Missouri was probably
diverted into the Shonkin Sag many times during the past million
years.  Today the Shonkin Sag contains only several tiny streams
and several shallow brackish lakes.
The map shows the probably course of the Missouri at the maximum extent of the glaciers.  The channel begins at the location of Lake Great Falls, and winds around the north side of the Highwood Mountains.  Though at least one major glaciation reached as far south as the Highwood Mountains, the ice need not have reached those mountains to block the flow of the Missouri.  Because of the configuration of the rivers and mountains, any advance of the ice sheets from the north could have dammed up the Missouri River.  Then the River was diverted along any of numerous channels toward the east.  The entire channel of the Missouri River for several hundred miles east of Fort Benton appears quite new, and very rugged.  This channel originated as one of those overflow channels.
The Shonkin Sag in places is gigantic, suggesting that it was not excavated by a single catastrophic overflow event; but was formed over many thousands of yearsperhaps over a time interval greater than what has elapsed since the end of the last ice age.  One of the highlights of the field trip is a gigantic dry water fall, four times the size of Niagara Falls.
See a detaled map of the Shonkin Sag
Channels, with a discussion of their possible significance.
It is possible to spend days exploring this region.  Besides the
Sag itself, there are many subtle traces of pre glacial river channels.
See detail map of route
The Shonkin Sag is over 60 miles in length, and one could spend days exploring it fully.  Therefore, we have two choices of routes:
Much of the rock exposed in the Shonkin Sag is an unusual mineral called Shonkinite (named after this location).  Shonkinite is found in only a few places in the world, most of which are associated with volcanic intrusions in continental interiors.  Several of the mountain ranges of central Montana, the Highwood Mountains, The Bears Paw Mountains, and the Adel Mountains, consist principally of intrusions of Shonkinite.
The composition of Shonkinite is similar to Basalt, except that it is rich in Potassium.  Basalt is found associated with volcanic eruptions from fissures and ocean spreading centers, while Shonkinite is usually found far from any evident fissures, spreading centers, or hot spots.
A property of Shonkinite that may have influenced the formation of the Sag is that it is often very brittle or friable.  You will not need a geologist's pick in the Highwood Mountains to break off pieces for examination.  That property probably made it fairly easy for a powerful river to tear huge chunks from the walls of the Shonkin Sag.
Photographs by G.  Davidson