Great Falls in Cascade County, Montana — The American West (Mountains)
Rocks of the Gorge and Falls
Photographed By Barry Swackhamer, August 16, 2022
1. Rocks of the Gorge and Falls Marker
Captions: (top banner) River-deposited nonmarine sandstone and mudstone; (bottom left) Shelving Rocks; (center globe) Paleogeograpjhic reconstruction of the Early Cretaceous, by Ronald C. Blakey.; (middle right) Coastal marine sandstone; (diagram, bottom right) Alberta Trough.
The Early Cretaceous world
The layered sedimentary rocks in the falls and gorge walls belong to the Kootenai Formation. Their age is Early Cretaceous (about 100 to 140 million years old). Rivers flowing from new mountains in the west and south and off the eastern plains carried sediment to a central lowland that sometimes held an inland lake.
Nonmarine rocks
The gorge walls are mostly river deposited layers of alternating mud, sand, and gravel turned to rocks. Because the layers have varying resistance to erosion, Meriwether Lewis described them as "shelving rocks." Some are rusty red from iron oxide: "bluffs are of red earth" - William Clark, June 15, 1805.
Marine Rocks
The rocks of Ryan Island, the falls and the lower gorge walls have less mud and more quartz sand. They record a shallow sea. Waves and tidal currents along the edge of the sea winnowed out the fine mud, leaving mostly sand grains behind.
The upper falls rock is mostly tidal channel sandstone. Shifting tidal channels cut into each other and left stacked lens-shaped layers. Most of the lower falls is composed of
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The Early Cretaceous sea advanced into Montana from the north, along a low area east of the early Rocky Mountains called the Alberta Trough. The trough formed where the weight of the new western mountains depressed the Earth's crust around its like a heavy person on a mattress.
Erected by North Western Power.
Topics and series. This historical marker is listed in these topic lists: Exploration • Natural Features. In addition, it is included in the Lewis & Clark Expedition series list. A significant historical date for this entry is June 13, 1805.
Location. 47° 34.113′ N, 111° 7.232′ W. Marker is in Great Falls, Montana, in Cascade County. Marker can be reached from Ryan Dam Road. The marker is located on Ryan Island, accessed via suspension bridge. Touch for map. Marker is at or near this postal address: 160 Ryan Dam Road, Great Falls MT 59404, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within 6 miles of this marker, measured as the crow flies. It's All Geometry (a few steps from this marker); From Water Comes Power (a few steps from this marker); Formation of the Gorge and Falls (within shouting distance of this marker); The Falls Undergo a Change (within shouting distance of this marker);
Photographed By Barry Swackhamer, August 16, 2022
2. Rocks of the Gorge and Falls Marker
Photographed By Barry Swackhamer, August 16, 2022
3. Ryan Dam
Note the base rocks.
Credits. This page was last revised on May 3, 2024. It was originally submitted on May 2, 2024, by Barry Swackhamer of Brentwood, California. This page has been viewed 47 times since then. Photos: 1, 2. submitted on May 2, 2024, by Barry Swackhamer of Brentwood, California. 3. submitted on May 3, 2024, by Barry Swackhamer of Brentwood, California.
