When the continental ice sheets spread southward into northern Montana
and the Dakotas, a few isolated areas in Montana stood above the
surrounding plain. These are mostly areas that were uplifted by the
intrusion of igneous bodies long before the streams began downcutting
and carving the land. The northernmost of these isolated mountains, the
Sweetgrass Hills, were surrounded by ice and became nunataks, or islands
of land, in the sea of advancing ice, which pushed southward up against
the Highwood Mountains, near Great Falls, the Bearpaws south of Havre,
and the Little Rockies to the east.
Much of the northern part of Montana is a plain of little relief that is
the surface of a nearly continuous cover of glacial deposits, generally
less than 50 feet thick. This plain has been incised by the east-flowing
postglacial Teton, Marias, and Milk Rivers.
In North Dakota, a high area on the east side of the Williston basin
acted as a barrier to the advance of the ice, most of which was diverted
southeastward. The margin of the ice sheet, however, lapped onto the
bedrock high, where it stagnated. Earlier advances moved farthest south;
the later advances stopped north of the present course of the Missouri
River—their maximum position marked by ridges of unsorted, glacially
transported rock debris (till) called terminal moraines. North of the
terminal moraines is a distinctive landscape characterized by a rolling,
hummocky, or hilly surface with thousands of closed depressions between
the hills and hummocks, most of them occupied by lakes. This is the
deposit left by the stagnant or dead ice, and it is called dead-ice
moraine. The rolling upland in North Dakota that is covered by dead-ice
moraine and ridges of terminal moraines from the last glacial advances
is called the Coteau du Missouri (fig. 20). A gently sloping scarp,
several hundred feet high and mostly covered by glacial deposits
(referred to collectively as drift), separates the Coteau du Missouri
from the lower, nearly flat, drift-covered plains of the Central Lowland
to the east. This escarpment, which is called the Missouri escarpment,
is virtually continuous across the State of North Dakota southward into
South Dakota. The base of the Missouri escarpment is the eastern
boundary of the Great Plains in these northern states.
[Illustration: _Figure 20.—Ground moraine on the Coteau du Missouri,
northwestern North Dakota. Photograph by R. M. Lindvall, U. S.
Geological Survey._]
The advancing ice front blocked one after another of the
northward-flowing streams of the region, diverting them eastward along
the ice front. Shonkin Sag, north of the Highwood Mountains near Great
Falls, Mont., is an abandoned diversion channel of the Missouri River,
occupied when the ice front stood close to the north slopes of the
Highwoods. Much of the present course of the Missouri River from Great
Falls, Mont., to Kansas City, Mo., was established as an ice-marginal
channel, and the east-flowing part of the Little Missouri River in North
Dakota was formed in the same way. These valleys were cut during the
last 2 million years.
The north-flowing part of the Little Missouri River and the east-flowing
courses of the Knife, Heart, and Cannonball Rivers in North Dakota are
for the most part older, preglacial courses. The Little Missouri was
dammed by the ice, and its waters impounded to form a huge lake during
the maximum stand of the ice, but the deposits of this glacial lake are
few and make no imprint on the landscape.
The valley of the east-flowing, glacially diverted part of the Little
Missouri River, however, is markedly different from that of the
north-flowing preglacial river. It is much narrower and has steeper
walls than the old valley. Because it is younger, it is little modified,
except by huge landslides that have affected both walls of the valley.
Tremendous rotated landslide blocks in the North Unit of Theodore
Roosevelt National Memorial Park are some of the best examples of the
slump type of landslide to be seen anywhere (fig. 21).
Melting ice at the front of the glaciers provided large volumes of
meltwater that flowed across the till-mantled surface in front of the
glacier as it melted back toward Canada. This meltwater took many
courses to join the glacially diverted Missouri River, and these sinuous
meltwater channels wind across the dead-ice moraine and the older, less
hummocky ground moraine between the Coteau du Missouri and the Missouri
River. Locally the sediment carried by the meltwater streams was banked
against a wall of ice to form a small hill of stratified drift that is
called a kame. Streams flowing in tunnels beneath the ice formed
sinuous, ridgelike deposits called eskers, and in places the meltwater
deposits form broad flat areas called outwash plains.
[Illustration: _Figure 21.—Rotated slump blocks in huge landslide, North
Unit of Theodore Roosevelt National Memorial Park, N. Dak. Note that
layering of Fort Union Formation in cliffs on skyline, where landslide
originated, is horizontal._]
This rather limited variety of landforms, then, characterizes the
landscape of the Glaciated Missouri Plateau. The landforms themselves
are testimony to their glacial origin and to the great advances of the
continental ice sheets. This is a stream-carved terrain that has been
modified by continental glaciers and almost completely covered by a
thick blanket of glacially transported and deposited rock debris,
locally hundreds of feet thick. Subsequent stream action has not altered
the landscape greatly.