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 typical of both overflow
channels and glacial troughs.  The meandering form of the Sag confirms
that it is not a glacial trough; those are highly unlikely to have the tight
meanders we see in the Shonkin Sag: meanders with radii comparable to the width
of the valley.
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, shallow saline lakes along its reachesone of which is seen in
the photograph.  It is generally 1.0 to 1.6 miles across (1.6 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 tremendous
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.  The 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 runs from Lewistown and
ends in the town of Geraldine, near the Sag.  The town of
Montague has become a picturesque ghost town (though with some
of the usual recent shacks and trailer houses that mar so many
unprotected ghost towns in the West).
Apart from the lakes and other curiosities, what usually 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 Age glaciers departed.  But in many
places the walls still stand 300 or more feet (91 m) high.  The
map indicates with a thin green line where prominent 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
roadsparticularly those within the Sagare 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 [which you can read about in The Roadside
Geology of Montana, by David Alt and Donald W. Hyndman, Mountain
Press, 1986] 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 the 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, This lake, which has
been called Lake Great Falls, gradually rose to a point where it could
spill over the ridge at the foot of the glacier.  The flow
quickly eroded part of the glacier, and in one great swoosh is thought
to have 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 that can be seen on hills throughout the
region, 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, andF-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.
However, it is mostly obliterated by later streams that crossed it and
eroded their own small valleys.  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; the laws of hydrodynamics suggest that something violent would
have happened when the rushing water hit the narrow entry to the lower
part of the Sag.  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 that pierced by
A-B, but lies just to the east of C.  There is a long ridgenamed
Frenchman's Ridgerunning from the mountains northward toward 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 slightly 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 breached in a catastrophic
flood unless there were already a pre-existing channel there.
This is not a fatal flaw in the theory, for the ridge appears to have
been catastrophically breaches at least once, forming the upper channel
and dry falls, F-G.  Unless there was a pre-existing breech near C,
the water rushing down the steep slope would have either continued to
excavate that channel or might have caused a collapse of part of the
ice dam that was blocking its exit.
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 valley of
Flat Creek.  The elevation irregularities 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 negligible 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 in
one catastrophic event we might expect the gradient to be rather
irregular, with many ledges where rapids or falls 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 igneous intrusion many millions of
years ago; and there appear to be no active faults in the vicinity.
The igneous activity was finished long before the onset of the ice
ages.  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 cross section 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
traces are not labelled, but are in order red, orange, green, blue,
gray; the lowest points give an idea of the descent of the river.  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 mixture of poorly consolidated crud.
Now look at the cross sections of the Shonkin Sag.  The locations
of the profiles are indicated by thin orange lines on the map
above.  The traces on the diagram are in the same sequence as
the preceding diagram: red, orange, green, blue, gray. Below the
overflow channel, A-B, this is an rather abnormal valley.  If
the valley had been 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 excavated
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 must 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 from the overflow of Lake Missoula,
spreading over a broad expanse at the foot of the Highwood Mountains,
carrying away ice, boulders, and everything in its path.
The continued flow of the Missouri River during the Ice Ages
We know now that the Ice Ages consisted of many cold periods interspersed
with warmer periods.  Some of the warm episodes may have resulted
in summers nearly as warm as the present.  Though it was previously
thought that many of the rivers stopped flowing entirely, it is now
believed that major rivers, such as the Missouri, continued flowing
throughout most of the Ice Ages.  The flow was probably greatly
reduced during the winters, but seldom stopped completely.
Therefore the notion that Lake Great Falls was formed only when the
ice began to melt must be modified.
This may seem a minor pointthe timing of the overflow of Lake Great
Fallsbut the continued flow of the Missouri River must have caused a
lake to begin rising any time its path was blocked by ice.  Immense
lakes must have formed downstream and overflowed through several channels
toward the east.
FOR THE FORMATION OF THE SHONKIN SAG
I have suggested that the formation of the Shonkin Sag must be much more
complex than the simple picture of catastrophic overflows from a single
vast lake.  With the knowledge that the Missouri River may have
continued flowing throughout most of the Ice Ages, we can infer the
sequence of events from the topography of the land and rivers.
A more complete explanation for the Shonkin Sag is that it was formed
over immensely long periods, 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 has been sufficient time for a river to gradually 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 lower part of 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,
and because there has been little erosion since the river returned to its
An 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 relatively high
ground running between the Judith Mountains and the Bears Paw
A period of glacial advance in which an ice
dam causes the Missouri River to overflow to the east.
As the Ice Age glaciers advanced they eventually reached the Bears
Paw Mountains and blocked the Missouri River, causing a lake to rise
behind the ice dam.  Eventually the lake began to overflow the
high ground to the eastthe only way around the ice.  The
water would have quickly breached the ridge and begun to drain the
lake; establishing the present course of the Missouri through the
Further glacial advance in which the water
is diverted to the Shonkin Sag.  At some point the ice
front would have advanced far enough to cut off the newly created
overflow channel.  Despite the greatly reduced flow of the
Missouri River at the most severe phases of the Ice Ages, a lake
centered near the site of the town of Fort Benton may have risen
further until it overflowed and began to excavate the divide
through which the Shonkin Sag passes.  The river may have
been diverted through the Sag for thousands of years in order to
create the present deep valley.
Filling of Lake Great Falls and spill over
the high ridge at the foot of the Highwood Mountains.  As
the ice front reached the foot of the Highwood Mountains this cut off
all the channels to the east and northeast, allowing the lake to rise
another 500 to 800 feet.  Because the source of the Missouri is
far to the south of the front of the ice, it may have continued to
flow during most of the Ice Ages, but at a very reduced rate.
The vast lake may not have filled completely until the ice began to
melt.  That lake is now known as Lake Great Falls because the
deepest part lay near the site of the city of Great Falls.
Whatever the sequence of events, the lake eventually began to spill
over the ridge to the north side of the Highwood Mountains.
Beginning of ice melting, adding to flow
through Sag.  The initial overflow of Lake Great Falls was
probably rather moderate, amounting to no more than the reduced flow
of the ice-age Missouri River.  But, as the spillway channel was
eroded the flow became catastrophic, carrying huge rocks and blocks
of ice that gouged away the lower part of the Shonkin Sag.  As
the ice began to melt the outflow probably increased  The
tremendous 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
lakes disappear.  As the ice rapidly retreated the lakes
finally disappeared.  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 modern rivers are
established.  From the time when the Ice Ages were over
until the present there have been minor adjustments of streams. Many
of the rivers, both great and small have and they cut deeply into their