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. 

• Topography
  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 Flat Creek.
  
  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 Ages—probably the last one.

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Photographs by G.  Davidson