|CLIMATE HISTORY AND GEOLOGY
2c.  THE SHONKIN SAG
DETAILS OF OLD CHANNELS
Adapted from Delorme topographic maps
The simple picture of the Shonkin Sag fails to account for many relics
of the Ice Age and the pre-glacial rivers.  This map shows several
features which are pertinent to the formation of the Shonkin Sag.
First note that there is a structural ridge, the Frenchman's Ridge,
running northward from the Highwood Mountains; the crest of the ridge
is is indicated as a brown line.  The ridge forms the ledge over
which the overflow plunged to form the dry falls (in the center of
the map).  Water fell 250 feet (76 m) from those falls,
which indicates that the ridge, though now quite eroded, presented
a substantial barrier to any river.  Yet the ridge is penetrated
by the Sag, just 3 miles north of the falls.
- Pre Glacial Rivers
Some traces of pre-glacial rivers are shown as dashed lines.  There
are many small streams originating in the mountains, but these have
been mostly obliterated by later events.  At the far left is the
valley of Flat Creek, which flows toward the north.
However, the overflow of Glacial Lake Great Falls and the
Missouri River took over part of the Flat Creek Valley; that
flow was toward the south, to meet the valley of Arrow
Creek.  This is a clear indication that the Flat Creek Valley
was blocked by ice or debris when the Sag was formed; otherwise
any slight rise in the water level would have sent the water down
There is another trace of a channel running eastward from Big Lake,
just to the right of the map center.  This channel has been largely
erased by glacial action, but it, too, could have made an escape route
for the overflow water in the Sag.  The stream that ran here must
have been the stream that first excavated a small valley in which the
waters of the Sag flowed south of Big Lake.
Running northward along the center of the map is an ancient river
channel that appears to cut across the Frenchman's Ridge.  This
channel pre-dates the Sag, for both the high (dotted) and low (solid)
channels of the Sag cut across it.  After the Sag was formed the
stream never returned to this channel.  What is important about
this stream is that it had somehow pierced the Frenchman's Ridge, which
provided a gap through which the water of the Sag could flow.
This creek, too, must have been blocked by ice, so the overflow water
had to take a new route at a 20 degree angle to the existing stream
channel; thereby permanently cutting off that stream.  That the
older stream had cut a deep gap in the ridge is demonstrated by the
fact that the abandoned channel is only 200 ft (60 m) higher than
the floor of the Sag.
A visitor will readily note that the section where the Sag pierces
the Frenchman Ridge has the oldest appearance of any part of the Sag.
Here the valley walls are eroded, and suggest a rather old valley;
whereas elsewhere the walls of the Sag are abrupt and show little erosion.
- Overflow Channels
There are traces of a very old channel indicated by a dotted line.
This lies at elevations of 3770 ft (1150 m) near Highwood to 3600 ft (1095 m)
at the brink of the dry falls.  This is clearly the oldest trace, for
it has been intersected by several streams that have erased the channel
over several miles.  After plunging over the falls, the overflow must
have followed an existing valley running southward from Big Lake.
This channel must have have carried the overflow from one or several
catastrophic overflow events caused by rapidly rising waters in Lake
Great Falls, when the glaciers were melting rapidly.  The relative
preservation of the separate channels suggests that this overflow
event occurred in a previous Ice Age, long before the event that
created the later overflow channel near the town of Highwood.
The high overflow channel is not entirely consistent with the highest
shorelines of Lake Great Falls, at about 3850 ft (1175 m). We
should expect that the overflow channel would be opened quite
rapidly, and that the lake would reach a long-time equilibrium
at about 3770 feet.  The discrepancy might be explained by
rebound of the land surface, which was depressed by a lake
containing many cubic miles of water.
The more pronounced western end of the Shonkin Sag was formed by
another catastrophic breach of a ridge containing Lake Great Falls.
The ridge above the town of Highwood was at an elevation of about 3694 ft (1125 m);
this was breached, and a channel cut to an elvation of 3457 ft
(1054 m).  Again we have a discrepancy of about 100 feet
between the channel elevation and prominent shorelines seen on
hills north of the City of Great Falls.  Nonethess, if we accept
the possibility of land rebound by about 100 feet, the two
overflow channels correspond the two most prominent lake levels.
- Sequence of Formation Events
We do not know with certainty the sequence of events that formed the
Sag.  We aren't even sure in which of the last several Ice
Ages Lake Great Falls rose high enough to overflow through
the channel at Highwood.  Our most crucial uncertainty is in
not knowing how much of the valley that comprises the Sag east of the
Frenchman's Ridge existed at the time of the first overflow event
that formed the high channel and the dry falls.  Prior to the
formation of the Sag there was a gap at an elevation of 3380 ft
(1030 m); and any event that dammed the Missouri to the north
of the Ridge would have overflowed there when the impounded lake
reached an elevation of 3380 ft.  The lake would have backed
up the valley now occupied by Shonkin Creek until it reached the
overflow point.  This is not a recipe for a catastrophic
overflow, but rather the formation of a natural spillway for a
slowly rising lake at a time when the glaciers were advancing,
and the river flow may have been reduced.
Where the water flowed after breaching the Frenchman's Ridge would
depend on whether the existing creeks were blocked by ice.
The light blue line shows were we find indications that the ice
had advanced at least this far.  It does seem likely that ice
blocked the flow of the Missouri for very long times, and the Shonkin
Sag became the natural course of the river for many thousands of years.
In one plausible sequence of events, the lower reaches of the Sag
were initiated in an early Ice Age, when Lake Great Falls never rose
high enough for a catastrophic overflow event.  Then the lower
portion of the Sag was created over many thousands of years, and
perhaps several Ice Ages, as a normal V-shaped valley.  Then
the glaciers advanced far enough to block the overflow, and the
Lake rose to nearly 3900 feet, whereupon it overflowed through
the high channel, creating the dry falls and enlarging the lower
part of the Sag.  The more recent overflow channel at Highwood
must have been created in one of the most recent Ice
Agesprobably the last one.
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Photographs by G.  Davidson