The Shonkin Sag in central Montana is one of the most spectacular
remnants of the Ice Ages in North America.  This is a gigantic
channel excavated by the Missouri River at a time when its path toward
the north was blocked by ice.  There are only a few insignificant streams
running in the Sag today, but at one time it must have carried a torrent of water,
full almost to its brim.  The principal evidence for a flood of water
is in the "U" shape cross section, a shape that is typcal of both overflow
channels and glacial troughs.  The meandering form of the Sag confirms
that it is not a glacial trough.
The Shonkin Sag is a lonely place today.  It winds along the northern
edge of the Highwood Mountains as a huge, barren valley, with several
large saline lakes along its reachesone of which is seen in the
photograph.  It is generally 1.3 to 1.6 miles across (2.1 to 2.6 km)
and about 300 ft deep (91 m).  At the location of the photograph,
the valley is 1.5 mi across (2.4 km) and 309 ft deep (94 m).  The
total length, from the town of Highwood to where it meets the valley of
Arrow Creek is nearly 55 miles (88 km).  Below the point where it
meets Shonkin Creek (Point C in the map below) it runs nearly level for
35 miles (55 km), which is somewhat surprising considering the trememous
velocity of the water that must have excavated this deep trough.
The Arrow Creek Valley is an extension of the channel which eventually
meets the present-day course of the Missouri River.  That valley
shows extensive erosion and modification since the Ice Ages.
Not long ago a railway ran along a portion of the Sag.  The
tiny town of Montague was located on the railway.  That
railway was never very prosperous - a branch line of
the Milwaukee, St. Paul, and Pacific, which dead-ended in the city of
Great Falls, about 30 miles away.  The tracks are gone from the
Sag.  What is left of the railway is a minor, independent line
which ends in the town of Geraldine, near the Sag.  The town of
Montague has become a picturesque ghost town.
Apart from the lakes and other curiosities, what most strike the
visitor are the huge cliffs up to 570 ft high (173 m) along the
Sag.  Some parts of the Sag have been worn away by erosion in the
15,000 years since the Ice Ae glaciers departed.  But in some
places the walls still stand 300 or more feet (91 m) high.  The
map indicates with a thin green line where the valley walls still
stand, winding along the north edge of the Highwood Mountains.
Present-day streams and rivers are indicated by blue lines; the black
lines are roads which are passable in dry weather.  Some of the
roads are not recommended in wet weather.
Actually there are several distinct parts of the Sag, indicated by the
letters A G on the map.  The topography in each differs in
subtle ways that may be important to an understanding of the formation
of the Sag.  Advancing downstream we find:
The standard explanation is that the Shonkin Sag was created by the
catastrophic drainage of a huge periglacial lake, dammed up during
the ice ages by the great ice sheets.  At some time the glaciers
advanced to foot of the Highwood Mountains, thereby blocking the
Missouri and other rivers flowing into the lake from the
southwest.  When the ice began to melt, Lake Great Falls rapidly
rose to a point where it could spill over the ridge at at the foot
of the glacier.  The flow quickly eroded part of the glacier,
and in one great swoosh cut a drainage channel toward the east.
Undoubtedly parts of that story are plausible, and probably account
for some chapters in the history of the Shonkin Sag.
There is much evidence for the existence of Lake Great Falls, including
at least two shorelines at elevations of 3500 3600 ft (1067
1097 m) and 3800 3900 ft (1158 1189 m).  These
shorelines are consistent with two spillway channels, A-B, and
F-G.  Apparently F-G corresponds to an early episode, and the
westward extension of that channel can be faintly traced on the slope
above A-B.  The "gigantic spillway" picture is consistent with the
formation of sections A-B, B-C, and F-G; but there are several problems
in accounting for the remainder of the Sag:
The change in gradient at point C
Imagine the water, loaded with boulders and huge chunks of ice roaring
down the steep slope B-C.  Why should it have suddenly slowed, and
flowed along the nearly level channel beyond C?  Rivers just don't
do that.  We should expect a big hole at C, or at least some
evidence that the water was moving very fast and came upon a place
where it was suddenly forced to change its velocity.
The drainage divide beyond C
Moreover, the actual principal local drainage divide is not pierced by
A-B, but lies just to the east of C.  There is a long ridge - named
Frenchman's Ridge - running from the mountains northward to the present
canyon of the Missouri.  The ridge is at least partly made up of
hard rock, for it forms the upper rim of the dry falls.  In
pre-glacial times that ridge may have been slighly higher than the
ridge at A-B.  It is punctured only by the Sag.  It is difficult
to imagine how that ridge could have been breeched in a catastrophic
flood unless there were already a pre-existing channel there.
The low gradient over much of the length of the Sag
The floor of the Sag is nearly flat for more than 34 miles, from the
Junction with Shonkin Creek to the point where it joins the pre-existing
valley of Flat Creek.  The elevation differences over that distance
are less than 200 feet, which can easily be accounted for by erosion
and filling by sediments washed down from the mountains over the past
10,000 years.  Such a negligeable gradient is common for mature
valleys with meandering rivers, but unlikely for an overflow channel
formed by a catastrophic flood.  If the Shonkin Sag were formed
one catastrophic event we might expect the gradient to be rather
irregular, with many ledges where the rapids formed as the water
flowed over relatively resistant rock.  It seems more more
likely that the Sag below point C was originally formed by a more
leisurely flow acting over a long time.
One complication that must be taken into account is the possibility
of differential settling and uplift.  However the Highwood Mountains
are the worn down remains of a massive intrusion several million years
ago; and there are no active faults in the vicinity.  The igneous
activity was finished millions of years ago.  If there is any
vertical motion it is slow isostatic rebound, due to the erosion
of much of the material in the center of the mountains.  This might
be sufficient to account for variations of tens of feet; but it could
not explain the low gradient of the Sag.
The size and shape of the channels
Further evidence suggesting that the picture is more complex than a
single overflow event comes from the shape of the channel.
The first diagram below shows the profile of the present valley of the
Missouri River, sampled a short distance to the west at several points
between the Falls of the Missouri and the town of Fort Benton.  The
scale at the side indicates elevations in feet above sea level.
The vertical scale is magnified by a factor of 15, so each trace
indicates a cross section 2 miles across.  This channel has presumably
been excavated since the ice sheets decayed, over a period of 10,000
12000 years.  The profiles are very impressive, and are indicative
of a relatively young valley excavated by a vigorous river.  The
most important aspect to notice here is that the channel shape is
relatively constant along this segment of the river valley.
Keep in mind also that below the resistant ledge of hard rock which
forms the Great Falls of the Missouri, the river is flowing through
mainly shales and sandstones and a mixure of poorly consolidated crud.
Now look at the profiles of the Shonkin Sag.  The locations
of the profiles are indicated by thin orange lines on the map
above.  Below the overflow channel, A-B, this is an rather
abnormal valley.  If the valley were excavated by a single
stream, with no significant tributaries entering from the sides,
we might expect the power of the stream to diminish downstream.
But the channel actually becomes broader and deeper.  The
lower part of the Shonkin Sag suggests that rather than a short
lived catastrophic event, this was excated over a very long period.
Moreover, comparison with the Missouri channel, which was cut into
much softer rocks, suggests that the total amount of water that
flowed through the Sag may have been much greater than the amount
that has flowed along the Missouri in the past 12,000 years!
The sinuous channel along the lower part of the Sag
The overflow is presumed to have been constrained by ice, which
enveloped the north side of the Highwood Mountains, and caused
the overflow to find a winding way between the ice and the
mountains.  But a stream powerful enough to break through a
rocky ridge would have been powerful enough to break up some
of the ice blocking its way.  One might expect a catastrophic
flood, like that which resulted fromt the overflow of Lake Missoula,
spreading over a broad expanse at the food of the Highwood Mountains,
carrying away ice, boulders, and everything in its path.
FOR THE FORMATION OF THE SHONKIN SAG
A more complete explanation for the Shonkin Sag is that it was formed
over immensely long periods of glaciation, probably during at least several
Ice Ages.  It was formed not by a single violent event, but in a much
more complex sequence of events, including diversion of the flow of
the Missouri river for most of the duration of the Ice Ages.  For
the past million years the Earth has spent most of the time locked in
ice; so there was sufficient time for a river to cut a channel around the
icea channel that could have been scoured and enlarged by several
violent overflow events.
Rather than a temporary, short-lived channel, the Shonkin Sag may have
been the preferred channel for the Missouri River for immensely long
times.  That it looks so fresh today, unlike the other valleys of
the Missouri, is because it was never eroded by glaciers.
The composition of the rocks of the Highwood Mountains is very important
to the creation of the Shonkin Sag.  Three minor mountain ranges
in north-central Montana were formed as massive intrusions of relatively
rare Shonkinite.  First were the Bears Paw Mountains, then the
Highwood Mountains, and finally the Adel Mountains.  The presence
of Shonkinite is thought to be related to the occurence of these volcanic
intrusions far from a plate boundary or known hot spot.  Shonkinite
is similar to Basalt, except that it is anomalously rich in Potassium.
Basalt is usually found near plate boundaries, hot spots, or fissure
The Shonkinite mountains were formed several million years ago, largely
through the intrusion of laccoliths.  They probably looked rather
like the more recent laccolithic Henry Mountains of eastern Utah.
They have been worn down rather quickly, so that the rock exposures
are mostly just isolated ridges protruding from hills of rubble.
Shonkinite is generally black, but weathers quickly to a dark red.
A feature that may have been critical to the formation of the Shonkin
Sag is that Shonkinite is often extremely friable; it can sometimes be
broken in the hand.  Almost any impact, whether by ice, running
water, or suspended boulders, can readily break off fragments from
the exposed vertical faces.
The alternate model for the formation of the Shonkin Sag involves
several distinct steps:
A pre-glacial stage in which the
Missouri River flowed across the northern part of Montana, in a valley
now occupied by the Milk River.  The ancestral Missouri was
prevented from flowing in its present course by a ridge running
between the Judith Mountains and the Bears Paw Mountains.
A period of gradual glacial advance
which blocked the Missouri River, and caused a lake to rise behind the
ice dam.  Eventually the lake began to overflow the ridge
to the east.  The water would have quickly breached the ridge
and begun to drain the lake; probably establishing the present course
of the Missouri through the Missouri Breaks.
Further glacial advance which cut off
off the previous overflow channel. Despite the greatly reduced flow
of the Missouri River, the lake may have risen further until it
breached the divide through which the Shonkin Sag passes.  The
river must have been diverted through the Sag for thousands of years
in order to create the huge present channel.
Filling of Lake Great Falls as the
ice front reached the foot of the Highwood Mountains.  This cut off the
drainage, allowing the lake to rise another 500 to 800 feet.  The
inflow from rivers was probably much reduced, so the lake may not have filled
completely until the ice began to melt.  The vast lake that formed
is now known as Lake Great Falls.  Eventually the lake began to spill
over a ridge at the front of the ice.
The damming of the river to form Lake Great Falls was not a necessary
consequence of the ice advance.  The ice could have retreated
before this happened, leaving the Shonkin Sag as a relatively young,
Catastrophic drainage of Lake Great Falls as the
spillway channel near the town of Highwood was rapidly eroded and
the water again reached the lower part of the Shonkin Sag.  The
gigantic flow of water may have filled the Shonkin Sag almost to the
brim, scouring the sides and creating the present-day U-shaped
valley.  Any ice blocking the Sag would have been swept away.
Establishment of new Missouri channel as the
ice rapidly retreated and the lake finally disappeard.  The rocks and
soils along the present course of the Missouri are relatively soft, so
eventually the river would have been diverted away from the Shonkin
Sag.  Increased flow from melting ice deepened the present Missouri
channel sufficiently that it became the preferred channel.
After the Ice Age in which there were minor
adjustments of streams, and they cut deeply into the present channels.