GLACIER The Geologic Story of Glacier National Park Special Bulletin No. 3

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 a 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.^*

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^*For a complete discussion of glaciers and their effects see Special Bulletin No. 2 (Glaciers and Glaciation in Glacier National Park) of the Glacier Natural History Association.

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