At the end of the ice age, the ice sheets decayed rapidly.  The map suggests that the larger glacial tongues remained intact as they shrunk.  This is probably only partly true.  In many places huge blocks of ice and debris were left stranded.  Whatever the details, eventually the ice blocking the Missouri River decayed sufficiently that it allowed the river to again flow in the present-day channel (V), which had probably been initiated thousands of years earlier, at the beginning of the ice age.
Before the Ice Ages, the Milk River flowed eastward, to the north of the region shown on the map.  But there are several large channels within the region that must at some time have carried a large river.  At some time the big rivers began to flow down from the Rocky Mountains, but ice remained to block the eastward flow, so the Milk River was diverted to flow southward to join the Missouri.  It is generally believed that the land was first cleared of ice along an "Ice Free Corridor" to the east of the Rocky Mountains; and the melting proceded northeastward, toward the huge dome of ice over Hudson's Bay.  The final remnants of ice in northern Canada did not disappear until several thousand years after the rest of the world had left the Ice Age.
As the Milk and Marias Rivers began to swell with water from melting glaciers, they must at first have been diverted far to the south.  They may have joined the Teton River, which today is today too feeble to cut the rather pronounced valley in which it flows (W).  At the confluence of the Marias and Teton rivers todayjust a short distance from where they flow into the Missourithe larger Marias River flows in a narrower valley than the miniscule Teton River.  This strongly suggests that parts of the Marias Valley are quite recent, and have probably existed for the past 10,000 years.
The present-day drainage divides are shown to indicate that every
stream south of the glaciers, and east of the Continental Divide, could
have flowed into the Missouri.  One puzzling feature is the
distortion of the drainage divide running northwestward from the Bears
Paw Mountains; there is a "kink" that appears to deflect the Marias
River toward the south.  A tongue of ice, or debris from
collapsing ice sheets, probably persisted to the west of the Bears Paw
Mountains long after the flow was re-established in the rivers.
Next, after the ice age