Introduction
The "Great Falls" of the Missouri was first described in writing by Merriwether Lewis on June 13, 1805.
"Šimmediately at the cascade the river is about 300 yds. wide; about ninty or a hundred yards of this next the Lard. bluff is a smoth even sheet of water falling over a precipice of at least eighty feet, the remaining part of about 200 yards on my right formes the grandest sight I ever beheld, the hight of the fall is the same of the other but the irregular and somewhat projecting rocks below receives the water in it's passage down and brakes it into a perfect white foam which assumes a thousand forms in a moment sometimes flying up in jets of sparkling foam to the hight of fifteen or twenty feet and are scarcely formed before large roling bodies of the same beaten and foaming water is thrown over and conceals them. In short the rocks seem to be most happily fixed to present a sheet of the whitest beaten froath for 200 yards in length and about 80 feet perpendicular."
The upper part of this falls has since been inundated by Ryan Dam to produce electricity, but you can still see the geology of the falls in the rocks below the dam and the cliffs on either side of it.
How do water falls like this form?
Besides plenty of water, a natural waterfall needs both of these things:
1. A river or stream that is cutting down through the landscape
In this area, the Missouri River was apparently forced out of its old channel when a continental ice sheet moved into northern Montana during the last ice age. Eventually, the displaced river began cutting a new channel, and it has not yet finished the job. In time, this channel will look more like the Missouri River valley upstream and downstream from the gorge. It will be broader, with room to meander, and the cliffs will be less steep.
The explorers were very impressed with the narrowness of the Missouri gorge and the falls:
"...I beheld those Catracts with astonishment the whole of the water of this great river Confined in a Channel of 280 yards and pitching over a rock of 97 feet..."
[Clark, June 17 at the Great Falls for his first view]
2. A layer of rock that resists cutting more than the rocks above or below it
Hard sandstones are interlayered here with softer muddier rocks. As the river cuts its gorge, it gets "hung up" on the resistant sandstone layers, forming falls.
What about the rocks of the falls?
The falls and gorge walls were carved into a group of layered sedimentary rocks called the Kootenai Formation. They were deposited in Early Cretaceous time (about 100 to 144 million years ago). Almost all of Montana was quite flat and very near sea level then. Rivers flowing out of mountains rising far to the west carried sediment to the plains and sometimes to an inland sea. Most of the Kootenai Formation in the gorge walls is nonmarine. Those river-deposited rocks are layers of alternating mud, sand, and gravel turned to rock. Some of them are quite red from iron oxide. Lewis and Clark noted the bluffs are of red earth" [Clark, June 15, 1805]
The rocks that make up Ryan Island, most of the falls, and the lower part of the gorge wall on the right are different. There is less mud and more pure quartz sand. These rocks record a narrow shallow sea that winnowed out the mud along its wave-dominated shoreline and deposited sand in near-shore settings including tidal channels. The falls near the water line are made of tidal channel sands. The shifting channels cut into each other and left stacked, lens-shaped layers. The Early Cretaceous sea advanced into Montana from the north, along a low area east of the early Rocky Mountains called the Albert Trough. The trough formed where the weight of the new mountains depressed the Earth's crust.
How did this geology influence the Expedition?
An essential landmark. Lewis and Clark learned of the Great Falls of the Missouri from the Minataree at their winter camp near the Mandan villages. Between June 2, when they reached the Marias River, and June 13, when Lewis found the falls, the Expedition was in some controversy over which fork was their route to the Pacific. When Lewis found the Great Falls on June 13 he knew they had taken the right fork.
"...my ears were saluted with the agreeable sound of a fall of waterŠ I saw the spray arrise above the plain like a collumn of smokeŠsoon began to make a roaring too tremendious to be mistaken for any cause short of the great falls of the Missouri..."
A place of great beauty. There are only a few moments in the journals where the explorers are lost for words to describe the natural beauty they are encountering. These events are nearly always triggered by landscapes dominated by unusual geology. Lewis, especially, was overwhelmed by the beauty of the Great Falls.
"...I therefore with the assistance of my pen onlyŠstill hope to give the world some faint idea of an object which at this moment fills me with such pleasure and astonishment..."
A difficult obstacle. The whole Expedition was challenged by the problem of portaging the boats and gear around the falls. Lewis (June 13) recognized the problem immediately:
"...the river was one continued sene of rappids and cascades which I readily perceived could not be encountered with our canoes..."
Ironically, part of their portage route followed the abandoned wider channel of the Missouri south of the present gorge. The Expedition spent an entire month in the falls area, which made them late getting to the Continental Divide and resulted in the mission nearly ending in snowstorms across the Bitterroots.
References
Fashori, M.Z., and Hopkins, J.C., 1989, Sedimentology and petroleum geology of fluvial and shoreline deposits of the Lower Cretaceous Sunburst Sandstone Member, Mannville Group, southern Alberta: Bulleting of Canadian Petroleum Geology, v. 37, no. 4, p. 371-388.
Haney, M. and Schwartz, R.K., 2003, Estuarine Member of the Lower Cretaceous Kootenai Formation, Missouri River Gorge, Great Falls, MT: Geological Society of America Northeastern Section 38th Annual Meeting Abstracts with Programs.
Moulton, G.E., Ed., 1987, The journals of the Lewis & Clark Expedition, v. 4, April 7-July 27, 1805: University of Nebraska Press, Lincoln, NB, 464 p.
Vuke-Foster, S.M., 1987, Marine tongue in the middle Kootenai Formation north of Helena, Montana, in 1997 Guidebook of the Helena area: Montana Bureau of Mines and Geology Special Publication 95, p. 63-64.
Vuke, S.M., Colton, R.B., and Fullerton, D.S., 2002, Geologic map of the Great Falls North Quadrangle, Central Montana: Montana Bureau of Mines and Geology Open File 459, scale 1:100,000.
Walker, T.F., 1974, Stratigraphy and depositional environments of the Morrison and Kootenai Formations in the Great Falls area, central Montana: Ph. D. dissertation, University of Montana, Missoula, 195 p.
