Great Falls Area Geology Field Trip
Rainbow Falls on the Missouri River
before the construction of Rainbow
Dam. Smokestack at the smelter on
the horizon. Note man standing on
cliff at right for scale. The river flows
over a ledge of resistant sandstone in
the Kootenai Formation. The
sandstone was deposited in a river
channel about 115 million years ago.
The dominant feature in the Great Falls
area is the Missouri River and its set of
former waterfalls and 5 current dams. As
shown in the block diagram to the right,
the river comes out of of the Rocky
Mountains at Cascade and meanders to
Ulm along a remarkable straight path. At
Ulm the river is deflected around the 'Big
Bend' to Great Falls and then flows down a
gorge to Morony Dam (M). The city of
Great Falls (shown in red) is built on the
crest of a gentle fold known as the
Sweetgrass Arch. Resistant sandstone
layers in the Kootenai Formation, now
exposed at the surface along the crest of
the fold, acted like a natural dam, causing the Missouri River to be deflected from its roughly
linear path. Upriver from Great Falls and the 'natural dam', the river is slow and winds back and
forth in lazy meanders. Down river from Great Falls the fast moving water has cut a deep,
narrow gorge, which is about 200 feet deep below Ryan Dam and nearly as deep at Morony
Dam.
Before the Ice Ages the river cut a 450 foot deep gorge through the 'natural dam', shown by the
dotted line in the block diagram. In cutting the gorge, the river followed the meanders to
produce incised meanders. The Sand Coulee Cut-Off on U.S. 87/89, the lower end of Sand
Coulee Creek (SC), and Gibson Flats are in the old river channel on the south side of Great Falls.
The former channel rejoins the current channel below Morony Dam. A continental glacier from
Canada blocked the Missouri River and formed a large lake (called Glacial Lake Great Falls).
The lower end of the old channel from Morony Dam to the Sand Coulee Cutoff was completely
filled with glacial till. The rest of the old channel was filled to within 100 feet of the top with
lake sediments. The current gorge, where the waterfalls are located, was cut since the retreat of
the last glacier. Thick sandstone beds create the large waterfalls that were such an obstacle for
the Lewis and Clark Expedition.
Primary topic:
Secondary topics: formation of erosion surfaces and benches, age of benches and rate of erosion;
channel sands, regional flow of groundwater, Great Falls Tectonic Zone
Route: Ulm Pishkun, Ulm, Smith River, River Road, Gibson Flats, Giant Springs, Rainbow
Dam, Cochran Dam, Ryan Dam, Morony Dam
Definitions of mineral, rock, bedding, formation, joint, cross-bedding, meander, incised
meander, channel deposit, sandstone, limestone, chert, fossil, fossil traces such as tracks, glacial
deposits, till, outflow channel, aquifer, and structural arch illustrated and discussed on the
outcrop.
Reference: Chamberlain, V. R. and Baker, D.W., 1991, Field Guide - Great Falls to Ryan Dam
via Rainbow Dam Road. Little Belt Press, Monarch, 15 p.
Guidebook
Guidebook to the Geology of the Great Falls Area. The 85 page geology guidebook describes
the geological features seen on the field trip and the geologic setting in which they occur.
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Last Updated October 31, 2000 by David Baker
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