After the ice ages, the modern landscape emerged, after several minor re-arrangments of the rivers.  The Missouri River has continued cutting deeply into the earth.  The length of the channel is not greatly different from the pre-glacial course; but the channel is significantly deeper.  This is largely due to the increased flow at the end of the ice ages.  The spectacular canyon south of the Bears Paw Mountains is nearly 1000 ft (305 m) deep.
After the ice melted, the Marias found a new course across the plains.  Its current course doesn't cut across any hard rocks, so this minor river has excavated a quite respectable valley.
Notice that the Marias River at one point comes very close to the present drainage divide (X).  There the river takes a sharp bend toward the south.  This suggests that the Marias was forced to make a detour around a large remnant of the tongue of ice that flowed along the west side of the Bears Paw Mountains.
The Teton River played a small trick in breaking through a low ridge (Y) about 5 mi (8 km) west of the town of Fort Benton.  This ridge runs along the northern rim of the ancient valley of the Missouri; and the Teton and Marias Rivers apparently ran along the north side of the ridge at the end of the Ice Age.  What caused the breakthrough is difficult to guess; but it may have been a result of a flood caused by the increased flow from melting snow and ice in the mountains.  The strange result is that the Teton River now flows on the edge of the ancient Missouri Valleyso one wall of its valley is about twice as high as the other.
A peculiar situation occurs where the Marias River flows into the Missouri, which fooled the Lewis and Clark Expedition when they came through here in 1805.  The Missouri has been furiously cutting a deep valley since the last ice age.  Consequently the much smaller Marias was forced to cut a deep channel over its last few milesor enter the Missouri as a water fall.  A fall may have existed at one time.  The present-day lower Marias River descends quite rapidly, in some places at a rate exceeding 53 ft/mi (1 %).  The result is that the Missouri appears rather sluggish, where the Marias enters as a succession of rapids.  This, and the amount of sediment picked up by the Marias River, suggested to many members of the Lewis and Clark party that this was the true course of the Missouri.  A short journey up the Marias river revealed the truth; that that Marias is generally a small river meandering sluggishly across the prairies.
The well battered Milk River ended up with a fine, broad new
valley which it appropriated from the ancestral Missouri.  After
the ice that was blocking its path disappeared, it was again free to
flow toward the east.  But now, rather than meeting the South
Saskatchewan River it found a way across a low ridge that runs along
the U.S.-Canada border east of the Sweetgrass Hills.  Therupon it
now runs southeastward and joins the ancient valley of the Missouri near
Havre, Montana.  The ridge is composed largely of poorly
consolidated sandstones, so the Milk River picks up a disproportionate
load of sediments.  Its name was derived from the milky appearance
of the water where the Milk River flows into the Missouri.
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