The story begins sometime before the Ice Age glaciers spread over the northern Great Plains.  The actually timing of events is difficult to determine, because there have been at least four major Ice Age cycles, and each glacial advance tended to erase the evidence for the previous cycle.  The time of this map could be anywhere from 200,000 to 1,000,000 years ago.  All we know for certain is that there was one episode of glaciation in which the ice advanced as far as the Highwood Mountains and set the stage for the creation of the Shonkin Sag.
The ancestral rivers are shown in blue-violet.  Since the modern reader's geography is formed more by highways than by rivers, the principal modern roads are shown in red for orientation of the view, which is bordered on the northwest by the foothills below Glacier National Park, on the southwest by the Rocky Mountains near Helena, Montana, on the southeast by the Snowy Mountains, south of Lewistown, Montana, and on the northeast by the towns of Harlem and Hogeland, Montana.
The 3500 foot (1067 m) and 3850 foot (1173 m) contours are indicated because they are especially pertinent to the later development of the ice-dammed lakes.  Prominent shore lines are found around the city of Great Falls indicating that those were relatively stable levels attained by peri-glacial lakes.  Those levels also roughly coincide with the feet of the smaller mountain ranges and the foothills of the Rocky Mountains.
The ancestral Missouri flowed to the north of the Bears Paw Mountains, in the valley now occupied by the Milk River.  In eastern Montana, off the map to the upper right, the river turned toward the northeast and joined other rivers which flowed into the ancestral Hudson's Bay.  The principal evidence for the former course of the Missouri is found in the traces of the old valley, which are easy to discern over much of the region shown on the map.
The heavy green lines (A) mark the edge of the ancestral Missouri Valley, where it can be seen by anyone who knows what to look for.  That valley today is visible as a relatively flat plain, generally 2 5 miles across (3.2 8.0 km); with the present day valley forming a deep trench near the center.  The old valley has been somewhat broadened and smoothed over by glaciers; but the river working over millions of years must have excavated a relatively large valley.  We know that the Missouri took a long detour toward the north because today there is a broad valley running from the Missouri toward the city of of Havre (where the pre-glacial valley turns toward the east).  In that valley are only several inconsequential streamsmuch too small to have cut a valley of such breadth.  It could only have been excavated by a large river.  Of course the ice age glaciers smoothed off the edges of the existing valleysin some places completely obliterating them.
The courses of other rivers have been greatly altered over time.  The Milk River is not shown on this map because it was a part of the Saskatchewan River system, far to the north.  The light green lines (B) indicate where modern, post-glacial valleys are found.  The ancestral Marias River also flowed somewhat to the north of its present course.  Some traces of the old valley can be seen along US Highway 2.  The old valley of the Marias River can be seen most clearly north of the town of Big Sandy, where it joins the former valley of the Missouri.
Most of the rivers north of the Sun River have had their courses altered and confused.  The Sun River appears to be flowing in its original valley, which is evidence that the glaciers may not have extended that far south.
To the east, there was a low ridge (C) running from the Judith Moutains to the Bears Paw Mountains.  It is shown as a dashed red line because it was a local drainage divide.  The streams on the west side of that divide flowed into the Missouri River.  In some place their flow directions have been reversed since the Ice Ages.  The modern drainage divides are shown as dotted lines.  Except for the Continental Divide, they too have changed greatly over the ice ages.
Especially noteworthy is the ancestral Judith River (D), which flowed in a valley that can still be seen to the north of the present Missouri.  Like the Missouri Valley, that valley has been largely erased by glaciers.  While parts of the present-day Missouri canyon were originally occupied by small streamsflowing in the opposite direction from the present day flowwe cannot know for certain whether there was a stream running away from the low point of the divide (C) which might have provided an easy course for the Missouri to breach the divide.
At point E are several relatively broad, east-west trending
valleys that must have existed before the ice ages.  They can be
traced back to the south side of the Highwood Mountains.  However,
in a classic case of stream capture, the upper streams have been
diverted toward the north.  Those streams cut deeply into the
terrain, so the upper ends of the valleys are now bisected by rugged
young valleys running perpendicularly across them.
Next, the approach of the ice sheets