In Miocene and Pliocene time the mountains were deeply eroded by
streams. It was during this time that Chief Mountain, Divide Peak, and
two smaller outliers, and the fenster along Debris Creek were formed.
All of the existing mountain valleys were cut out of the overthrust
block, although not to as great a depth as they have today. The time
required for their formation amounted to several millions of years. The
result of all this erosion was a landscape very similar to the present
day Blue Ridge in Virginia and North Carolina, the type which geologists
call mature.
Near the close of Pliocene time the climate cooled, timberline began to
lower, and increasing amounts of permanent snow accumulated in the
higher parts of the mountains. Finally glaciers formed from the snow and
began to move down the stream-carved valleys. This marked the advent of
Pleistocene time (The Glacial Age) nearly a million years ago. Glaciers
eventually filled all valleys and covered all the park area except the
summits of the highest peaks. Glaciers extended from valleys on the east
side of the Lewis Range far out onto the plains, and from the
Livingstone Range and the west side of the Lewis Range they moved into
the wide Flathead Valley. The forests disappeared and it is probable
that not a single tree remained in the area which is now the park.
Available evidence indicates that climatic fluctuations during
Pleistocene time caused the glaciers to disappear for a considerable
period of time, or at least to shrink to insignificant size and then to
return. At the end of Pleistocene time they began to shrink and about
9,000 years ago, during what is generally regarded as post-Pleistocene
time, disappeared again.
The large Pleistocene glaciers greatly altered the pre-existing
landscape of the park by gouging out valleys to much greater depth, and
making their sides and heads much steeper than the streams had been able
to cut them. Most of the lakes, vertical cliffs, sharp peaks, and
waterfalls which constitute much of the park's magnificent scenery were
created as a result of intensive glacier action.[4]