This excursion follows several rough country roads, so it is
best to follow a more detailed map.  The directions below
refer to this map rather than the rough general map.
This route begins at west end of the highway bridge
at Fort Benton.  However, rather than continuing on Highway 80, we
turn right onto a local road immediately after crossing the bridge over
the Missouri River.  This road takes us to the little town of
Highwood at 35.0 miles.  A short detour on the road that runs
south from Highwood offers some good views of shorelines of Lake Great
Falls on the east side of the valley.  The photograph was made
about 5 miles south of the town.
At Highwood the valley appears to split in two branches; the valley leading off to the east, in whigh the town lies, is the west end of the Shonkin Sag.  The Sag for the first few miles east of Highwood  is quite straight and uniform, and exhibits little subsequent erosion after the ice age.  However the undulations in the valley sides are subtly elongated toward the west, like giant ripples.  That is exactly what they are, indicating that this part of the Sag was once flowing brim full of water, as Lake Great Falls overflowed.  (Note that by starting our trip at this end we are travelling in the same direction as the river.)  This portion of the Sag is an overflow channel corresponding to the highest levels of Lake Great Falls.
There is no road that runs the
length of the Shonkin Sag, so it is necessary to take a slight detour
to the north when approaching the location of the eponymous town of Shonkin
.  Before we arrive at what remains of the town we encounter another
huge valley running off toward the Missouri River to the northwest.
This valley is not nearly as spectacular as the Sag, so it might easily
be overlooked.  It is so broad that it must have played some important role
in the formation of the Shonkin Sag.  The Shonkin Creek Valley
between this point and the town of Fort Benton was probably the main
overflow channel when the lake was at a lower level and the ice
dam lay further north.  That the gradients are much less here than at
the town of Highwood suggests that this was really the main channel
for the Missouri during the thousands of years it took to excavate the
Shonkin Sag.  Shonkin Creek now runs northward in this valley; during
the the Ice Ages the water flowed southward.
A bit further on we come again to the main portion of the Sag, with the remains of the town of Shonkin in the center.  The impression we have is of a much more mature valley than the Sag at Highwood.  The Sag between Highwood and Shonkin is a genuine overflow channel, that may have taken only a few hundred years to excavate.  The Sag east of Shonkin is a huge valley that must have been created over a very long time.  It displays a considerable amount of erosion along the sides, in contrast with other parts of the Sag.  One inference we might make is that this valley was the precursor of the Sag, and existed long before the section near Highwood.
Further evidence that the picture is not as straightforward as it
seems at first, is the fact that there is a relatively high ridge
running from the Highwood Mountains, through the location
of Shonkin, toward the Missouri River.  Water spilling over
the Sag at Highwood would have been blocked by this
ridgeunless a pre-existing river had already broken through the
ridge.  The topography indeed suggests an ancient stream coming down
from the Highwood Mountains and crossing the ridge in a northeasterly
directions.  There is no place to view it from the local
roads, but the remnants of that old valley cross the Shonkin Sag
at an angle of about 20 degrees.  Perhaps the existing valley
was blocked by ice, so the water spilling through had to cut a
At this point we turn southward and make a detour through the Highwood Mountains.  About 4 miles south of Shonkin is a view toward the northeast of a higher spillway channel, probably older than the more ruggedly sculpted main channel of the Shonkin Sag.  This detour is a good route to see evidence of the types of rocks which were made in the huge volcanic outpouring that made the Highwoods and Bears Paw Mountains.  The rocks are mostly shonkinite, a rare mineral that is found in only a few places in the world.  Shonkinite is not extremely hard, and it is rather fragile and readily eroded.  Stop anywhere at an outcropping by the road; and you will find that the rocks can be easily split by simply knocking several pieces together.  The brittleness of the rock might help to explain how such a gigantic valley could have been eroded here.
13 miles from Shonkin we re-enter the Sag .  Here are excellent
views of the topography of the broad lower reaches of the Shonkin Sag;
notice especially the enormous size of the valley, both in width and
depth.  As we begin to climb out of the valley, we have a superb
view of a dike of Shonkinite on our right.  The region is full of
such vertical intrusions of rock; this is one of the most
spectacularrunning almost half a mile without a break.
As we reach the rim
of the Sag we have a grand view, with brackish lakes and Square Butte
in the distance. Anyone who has seen the Columbia River in Washington
will be struck with the resemblance of this valley to portions of the
Columbia Valley.  The valley here is similar to, but even grander
than the Columbia Valley where it makes a grand bend near the cities of
Richland and Pasco.  There is also an obvious similarity to the
Grand Coulee in Washington, another enormous valley with no flowing
river.  The Grand Coulee was an Ice Age overflow channel for the
Columbia River; and also features a gigantic dry waterfall.  The
comparison is important because it is known that the Columbia has been
cutting its valley for many thousands of years.
On coming to the town of Geraldine, we head southward again on Highway 80 until we come to the town of Square Butte.  Just before the town there is a spectacular view to the west  of a valley with high rock walls.  There is an unimproved country road which takes one to the base of the cliff on the north side of the Sag.  If the weather is dry, this is an essential part of the excursion, that should not be missed.  This cliff is solid rock, and exposes the side of a laccolith that was eroded by the Sag.
This Shonkin Sag Laccolith, is about as perfect a cross section of
a laccolith as can be found anywhere.  The dark shonkinite was
intruded as a horizontal layer hundreds of feet thick, below a layer of
sandstone.  The east end of the exposure shows several sills of
shokinite spreading horizontally; separated by layers of ligher colored
sandstone.  This is as near as one can come to the standard textbook
picture of how a laccolith is formed.  It only takes a bit of
imagination to visualize the tremendous forces that could sheared off the side
off this huge pile of rock.
After viewing the cliff we 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 at some time have followed this broad valley which also forms a potential outlet for the Shonkin Sag.
About 4.3 miles
north of Geraldine we follow a secondary road leading off to the
west.  On re-entering the Sag, we follow a dirt road to the
picturesque remains of the town of Montague .  (Don't attempt this
detour during wet weather!  During wet weather a better approach is
from the west.)  Montague was built along a railroad which once served
the city of Great Falls.  The railroad is gone now, except for the
section from the city of Lewistown to the town of Geraldine.
We have set aside the most
spectacular part of the excursion until the last.  About 4 miles
southwest of Montague is a gigantic waterfall without a drop of water
falling over its face.   We take the road up the hill south of
Montague, and stop at a point where there are some rocks exposed on the
south side of the road.  A short hike takes us to the brink of the
falls (be extremely careful!).  During the ice age this must have
been one of greatest falls in North America.  The total height from
the brink of the falls to the bottom of the lake at the foot is over
250 feet.  The falls here were more than twice the height and twice
the width of Niagara Falls.
To the east of the falls is
another cross section of a laccolith.  This, the Lost Lake
Laccolith, is named for the lake at the foot.  Again there are
sills of shonkinite intruding between layers of lighter rocks.
Above the dry falls the channel of
the river is easily made out; though it may be difficult to trace on
topographic maps.  The relief is quite striking to the south,
suggesting that here, too, is a remnant of a valley as great as the
Shonkin Sag at Highwood.  However, this valley lies several
hundred feet above the lower Sag.  Its elevation is consistent with
the highest shorelines of Lake Great Falls, at 3800 to 3900 feet.
Apparently the first overflow followed this channel, until a lower
channel breeched the ridge at the town of Highwood.  The higher
overflow channel has been obliterated by later streams flowing across
it; so it was probably only briefly active.  After the lower
overflow channel was created, it became the main spillway for Lake
Great Falls, every time the water in the lake rose.
We could continue on the rough road past the dry falls, which would take us back to Shonkin and Fort Benton.  An easier way is to retrace the route through Montague, to exit the Sag on the north side and return to Highway 80.  This route has an excellent view of the ancient Missouri Valley, best seen from a point 5 miles  before coming to Fort Benton.
|Start, Fort Benton, Highway 80||0.00||0.00||N47° 48.97'||W110° 40.14'|
|Right on local road, Highway 228||0.3||0.5||N47° 48.77'||W110° 39.83'|
|Local road (veer right)||4.9||7.9||N47° 45.18'||W110° 41.66'|
|Highwood Road (right turn)||8.7||14.0||N47° 42.36'||W110° 43.63'|
|Continue on Highwood Road (left turn)||12.1||19.5||N47° 41.26'||W110° 47.46'|
|Town of Highwood||19.5||31.4||N47° 35.04'||W110° 47.95'|
|Big Sag, turn left up north side of valley||24.0||38.6||N47° 35.91'||W110° 42.78'|
|Local road (right turn)||25.9||41.7||N47° 37.56'||W110° 42.52'|
|Shonkin, continue on local road||32.4||52.1||N47° 37.58'||W110° 34.58'|
|View of upper valley||35.4||57.0||N47° 35.68'||W110° 33.43'|
|View of floor of Sag||45.7||73.5||N47° 34.12'||W110° 26.89'|
|View of Sag and dry lake||47.3||76.1||N47° 34.72'||W110° 24.65'|
|Highway 80 (right turn)||55.9||89.9||N47° 36.34'||W110° 15.89'|
|View of Flat Creek, flowing north||60.1||96.7||N47° 33.38'||W110° 12.92'|
|Local road (right turn)||62.3||100.2||N47° 31.52'||W110° 12.13'|
|View of cliif||67.1||108.0||N47° 32.56'||W110° 17.09'|
|Return to Geraldine||78.3||126.0||N47° 36.34'||W110° 15.89'|
|Local road (left turn)||82.6||132.9||N47° 39.29'||W110° 19.25'|
|Town of Montague||89.7||144.3||N47° 40.79'||W110° 26.93'|
|View of Dry Falls||94.0||151.2||N47° 38.33'||W110° 29.77'|
|Return to Montague, continue north||98.3||151.2||N47° 40.79'||W110° 26.93'|
|View of Sag||100.8||162.2||N47° 41.85'||W110° 29.42'|
|Local road to Highway 80 (right turn)||104.0||167.3||N47° 43.87'||W110° 26.60'|
|View over ancient Missouri Valley||112.4||180.9||N47° 47.91'||W110° 35.10'|
|Return to Fort Benton||117.1||188.4||N47° 48.96'||W110° 40.12'|
Photographs by G.  Davidson