Where to play with huge snowpatches in summer?
-In mountains of high altitude and/or high latitude. Maybe high enough that even glaciers exist. The Glacier National Park (GNP) is one such place to go.
Introduction
The glaciers are indeed an essential ingredient of the spectacular scenes in GNP. Yet, the scenes could be given an extra layer of “awesomeness” if the long geological history were taken into account. In a sense, the GNP is the result of the interactions between the terrain, the glacier, the climate, the fauna and flora, etc. over billions of years. And the most fascinating part is probably how the stage was “prepared” for these interactions by tectonic processes.
This stage is the Lewis Overthrust. And its formation is vividly explained in a booklet “Rocks, Ice, and Water - The Geology of Waterton-Glacier Park”, by David D. Alt and Donald W. Hyndman, 1973. I think it would be better to directly present the excerpt of this book (pp. 21-24), instead of myself, a layman, trying to explain the geological mechanisms.
The Lewis Overthrust Fault - The Beginning of Mountains
Geologists normally expect to find sedimentary rocks stacked in the order they were deposited with the oldest layers at the bottom of the pile and the youngest on top. In Waterton-Glacier Park this expectable order of rock layers is dramatically reversed; Precambrian sedimentary rocks deposited more than one billion years ago are now on top of Cretaceous sedimentary rocks laid down only 70 million years ago! Apparently a slab of Precambrian sedimentary rock several thousand feet thick slid eastward some tens of miles over the much younger Cretaceous sedimentary rocks. The surface on which the sliding movement occurred is called the Lewis Overthrust Fault. Geologists have marvelled at this situation, and attempted to understand it, ever since they first recognized it in the years near the beginning of the twentieth century.
For many years geologists argued that a strong force from the west must have pushed the slab of Precambrian sedimentary rock eastward onto the plains. But this vision poses problems: pushing such a long, thin slab of rock from behind without crumpling it is an undertaking similar to pushing a large carpet across a floor without wrinkling it. The rocks, like the carpet, are not strong enough to withstand the necessary force without folding, and the friction in the sliding surface makes it difficult for movement to occur. The Precambrian slab in Waterton-Glacier Park is not crumpled, neither is there evidence in the area west of the Park of any geologic event that could have exerted a strong eastward push. So it is difficult to imagine how this enormous slab of rock was pushed into its present position.
Now most geologists are convinced that overthrust faults do not move in response to a push from behind but instead slide downhill under the pull of gravity. This eliminates the problem of understanding how the overthrust slab could withstand a force without crumpling because every particle of rock feels the tug of gravity individually.
The problem of overcoming friction in the sliding surface of the fault still remains. If water in the pores of the rock is under sufficient pressure, it will support the rocks above enabling them to move with very little friction. This might have happened in the Cretaceous rocks but would have been most unlikely in the much older Precambrian formations because they were probably much too dry by the time movement occurred on the Lewis Overthrust fault. Some geologists contend that the Precambrian rocks moved because certain layers were weak enough to flow like a viscous fluid carrying the rocks above. They imagine the rocks behaving somewhat like a layer cake filled with soft frosting.
Thousands of feet of sedimentary rocks had accumulated in the region of Waterton-Glacier Park by the end of Cretaceous time. Layer after layer of sandstone, mudstone and limestone deposited over a period of many hundreds of millions of years while the region was a level plain flooded at times by shallow seas. The quiet years ended about 70-90 million years ago, during Cretaceous time while giant dinosaurs still roamed the earth.
Processes operating deep within the earth slowly raised the continental crust of western North America, with its burden of sedimentary rocks, several thousand feet vertically upward forming a broad crustal arch that had its crest about 100 miles west of Waterton-Glacier Park. Thousands of feet of interlayered limestone, sandstone, and mudstone slowly slanted eastward as uplift proceeded. Tilting such a thick pile of sedimentary rocks, some layers strong and others weak, is somewhat like tilting a thick stack of heavily-buttered pancakes. In both cases the entire stack will eventually begin to slide downward on one or more of the weak layers. That is probably what happened about 50-60 million years ago all along the eastern front of the northern Rockies.
Somewhere deep within the pile of sedimentary rocks a weak layer yielded under the strain and the rocks above began to glide slowly eastward down the flank of the broad regional arch still rising in the earth’s crust. The huge slab of Precambrian sedimentary rock that is now Waterton-Glacier Park slid an unknown distance eastward, probably more than 30 miles. It moved at least ten miles across the soft sands and muds deposited in the shallow Cretaceous sea that had receded only a few million years previously.
As the mass of Precambrian sedimentary rock slowly slid eastward, large gaps opened behind it. The same sort of thing happens when slabs of snow slide down a roof leaving gaps where the moving snow detached itself from the stationary mass that remained behind – or the upper end of a glacier pulls away from the mountain at its head. The North Fork Valley, along the western margin of Waterton-Glacier Park, appears to be such a gap pulled open behind the Precambrian slab as it moved eastward on the Lewis Overthrust. It is approximately ten miles wide, probably not enough to account for all the displacement on the Lewis Overthrust fault. In order to find another gap to account for more displacement we must go farther west to the broad Flathead Valley and suggest that the Whitefish Range, west of the North Fork Valley, may also have slid eastward.
Early geologists interpreted the North Fork Valley quite differently, arguing that it is a downdropped block of the earth’s crust. Rocks in the southern end of the valley, where the bedrock valley floor is exposed, do suggest that the valley floor has dropped vertically downward. But there are reasons to believe that the floor of a pull-apart valley might drop so the two explanations are not incompatible.
Mountain ranges north and south of Waterton-Glacier Park also consist of large slabs of rock that slid eastward as the earth’s crust arched up to the west. But these ranges do not consist of a single large slab. Instead, they are made of many smaller slabs stacked on each other like a row of fallen dominoes. This kind of structure is also found in part of the eastern portion of Waterton Park.
More references:
- Wikipedia, where a few illustrative figures are provided.
- Glacier National Park and the Lewis Overthrust