There is no more impressive, and more accessible, illustration of the power of the great Ice Age glaciers than the changes in the course of the upper Missouri River.  The evidence is easy to see and understand, once we realize what has happened.
The river systems of the Saskatchewan and Missouri Rivers are
very ancient.  At the time of the dinosaurs there was a shallow
sea filling most of the region, and the rivers flowed directly into
that sea.  About 60 70 million years ago the Laramide
orogeny begain raising the Rocky Mountains and filling in the sea.
When the sea became filled with sediments the rivers had to flow across
the plains; some ended up in the Arctic Ocean others flowed toward the
Gulf of Mexico.  The location of the divide between those drainage
systems is not known precisely; but it seems to have been far to the
south of the present day divide.  It seems to have run eastward
from the rapidly rising mountains of Wyoming, through the Black Hills
toward the hills of central Minnesota. The South Saskatchewan and
Missouri River systems were at one time connected.  The vast
system comprising the North Saskatchewan, the South Saskatchewan, and
Missouri Rivers flowed toward the Arctic (before the formation
of Hudson's Bay).  The map shows a plausible reconstruction.
The present-day drainage divides are shown on the enlarged map as black dotted lines.  The pre-glacial rivers flowing in their present courses are shown in light blue, and the pre-glacial rivers that have been shifted out of their course are shown in dark blue.
The relationships between the Missouri and Saskatchewan River systems have been confused by the great continental glaciers that once extended almost to the point where the Missouri now flows into the Mississippi.  There are many peculiarities in the present river courses: there are great valleys running across the prairies that seem to be unrelated to the major rivers.  Some of those valleys have only insignificant streams running in them today.  The North and South Saskatchewan Rivers both flow in a southeastward direction out of the mountains; then turn abruptly to the north.  Topographical maps show that there is a broad valley continuing toward the southeast from the bend of the South Saskatchewan.  That valley connects to the Q'Appelle Valley; but the Q'Appelle River flows in the wrong direction, away from the South Saskatchewan.  Likewise, the small, sluggish Milk River flows for several hundred miles through a valley that is much too large to have been excavated by that river.  The alignment of valleys on topographical maps suggests that the pre-glacial South Saskatchewan turned southeastward to join the Missouri, rather than northward to join the North Saskatchewanas it does today.
The North-South Divide between the river systems has shifted toward the north (shown as a dotted line on the enlarged map).  The Divide now runs from a small mountain in Glacier Park, Montana, just north of the source of the Milk River, then through the Cypress Hills of Alberta and Saskatchewan.  There it follows a low ridge separating the Saskatchewan River system from the Frenchman River; the Frenchman River is the northernmost tributary of the Missouri-Mississippi system.  From the Cypress Hills the divide runs eastward to become the divide for the Souris-Assinniboine-Red River system.
But when the great ice ages began more than a million years ago, the rivers lay in the path of the advancing glaciers.  Wherever the glaciers encountered north-south oriented valleys, they tended to flow along those valleys and broaden them.  The glaciers also blocked many of the rivers, which were diverted around the southern margins of the glaciers.  In several places large segments of the Missouri River were displaced to cut a deep valley through rugged elevated terrain far to the south of its original channel.  The branches of the Saskatchewan River system were cut off from the Missouri, and eventually excavated new courses toward the north, over the depressed terrain that had been scooped out by the glaciers.  The old valleys can be seen clearly on a topographic map; the DeLorme Topographic Atlases of Montana and North Dakota are highly recommended.
also a landforms map
At various points in the field trip, we will be crossing the path of Lewis and Clark, and other explorers. Where the geography illuminates their discoveries, a note will be called out, in a box like this.
Three especially useful guide books are
Our field trip starts at the point where The
Missouri River exits the Mountains southwest of Great Falls,
Montana.  Approximate coordinates of important overlooks
are noted in square brackets: [...].
Stops along the way:
This web site has been created and maintained by Gerald Davidson, PhD.
Most recent revision March 2010
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