This excursion is mostly
confined to Montana Highway 80, a well maintained paved road.
Begin at Fort Benton, at the west end of the highway bridge over the
Missouri.  Remain on Highway 80 for 32.5 miles. Just before
arriving at the town of Square Butte there is a spectacular view to the
west [Site 4] of a valley with high rock walls.  Only an immense
river could have cut this valley, but there is only an insignificant
stream there now.  Leading off to the the west there is an
unimproved country road which takes us to the base of the cliff on the
north side of the Sag.  This cliff is solid rock, and exposes the
side of a laccolith that was eroded by the Sag.  The dark rock is
shonkinite, which has been intruded between layers of sandstone and
other rocks to form a broad, relatively flat laccolith.
See detail map
After viewing the cliff retrace the route back to the bridge that crosses the minor stream of Flat Creek.  Notice that the stream follows a valley that leads away from the Sag toward the North.  While the principal route of the stream flow during the ice age was probably along Arrow Creek to the south, the rivers could have followed this broad valley which also presents a potential outlet for the Shonkin Sag.
Return to the town
of Geraldine and take a local road leading directly west for about 8.4 miles,
where there is an overlook with impressive views of the Sag and Square
Butte in the distance [Site 5].  Both here and near Square Butte it
is apparent that the valley that constitutes the Shonkin Sag is
immense, and must have been excavated by torrents of water working
for a very long time.  It is only slightly smaller than the Ice Age
overflow channels of the Columbia River, such as the Grand Coullee.
The return route again follows Highway 80 back to Fort Benton.  About 5 miles before arriving at the town there is an imposing view looking over the broad pre-glacial Missouri Valley.  The valley walls have been much eroded and smoothed by glaciers, so it takes a bit of imagination to see them.  Just imagine that the deep present-day Missouri channel is not there, and you can visualize a broad trough running in a southwest to northeast direction.
|Start, Fort Benton, Highway 80||0.00||0.00||N47° 48.97'||W110° 40.14'|
|Town of Square Butte, View of Shonkin Sag||32.5||52.3||N47° 39.52'||W110° 12.13'|
|  View of Cliff||37.3||66.7||N47° 32.56'||W110° 17.09'|
|Retrace route to Flat Creek||44.4||71.4||N47° 33.52'||W110° 12.96'|
|Geraldine (left turn)||45.5||73.2||N47° 36.34'||W110° 15.89'|
|  View of Sag||56.9||91.5||N47° 34.72'||W110° 24.65'|
|Retrace route to view point||86.3||138.9||N47° 47.91'||W110° 35.10'|
|Return to Fort Benton||91.3||146.9||N47° 48.97'||W110° 40.14'|
Photographs by G.  Davidson