CLIMATE HISTORY AND GEOLOGY


THE EVOLUTION OF GLACIAL LAKE GREAT FALLS
AND THE FORMATION OF THE SHONKIN SAG

G.T. Davidson, PhD.

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.

Overview of Sag

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 reaches—one 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.

Local Map

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—particularly those within the Sag—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:

A-B:  5.14 mi (8.3 km)
A relatively straight section with no breaks in the walls nor small streams entering from the side.  The elevations of the floor are 3380 – 3470 ft (1030 – 1058 m) above sea level, and the walls are about 250 ft (76 m) high.

B-C:  8.65 mi (13.9 km)
A relatively steeply descending section with elevations ranging from 3430 ft (1045 m) at the western end to 3125 ft (953 m) at the eastern end.  The descent is very abrupt for a large, straight river, averaging as much as 30 ft/mi (0.57 %).  The sides of the Sag here are rather low and irregular.

C-D:  33.77 mi (54.3 km)
The main body of the Sag, with a relatively level floor at elevations of 3100 – 3200 ft (945 – 975 m).  Some parts of the floor have clearly experienced differential erosion, but the channel would seem quite flat to anyone traveling along its length.  The channel is quite sinuous; which may be due to deeper structures rather than ordinary meanders formed by hydraulic processes.  The channel cuts through some rather hard rocks; at one place it bisects a prominent laccolith.  Most of the large, shallow lakes lie in this section.

The secondary channel running northward from point C is not generally considered part of the Sag.  Its sides are relatively low and gentle, and its shape is more like that of most of the other minor valleys in the region.  It is now occupied by the small stream, Shonkin Creek, that flows northward from the Highwood Mountains to join the Missouri.

D-E:  9.07 mi (14.6 km)
A broad, nearly straight section, connecting the valleys of Flat Creek (which runs toward the north) and Arrow Creek (which runs in a generally northeasterly direction).  Elevations are 2970 – 3150 ft (905 – 960 m).

F-G:  7.8 mi (12.6 km)
An isolated section, lying well above most of the Sag at elevations as high as 3698 ft (1127 m).  Along this section is a spectacular dry fall (at the west end of the long, narrow lake), with a drop of more than 250 ft (76 m) to the lake bottom.  At the low end of this channel it runs into the main channel (C-D)

See another map, showing the topography and the stream channels connected to the Shonkin Sag.


Take a virtual field trip exploring the Shonkin Sag





THE STANDARD EXPLANATION

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:





AN ALTERNATIVE EXPLANATION
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 ice—a 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 present course.

An alternate model for the formation of the Shonkin Sag involves several distinct steps:

  1. 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 Mountains.

  2. 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 east—the 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 Missouri Breaks.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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 present channels.

The first step: the pre-glacial landscape


Site Map



This web site has been created and maintained by Gerald Davidson, PhD.
http://www.onemain.com/~gdavids/index.htm.
Most recent revision March 2010

I apologize if I have not provided the extensive citations that the reader expects. However most of the work on the Shonkin Sag was done long ago, and the Shonkin Sag and the shonkinite volcanic intrusions have been neglected for the past half century My speculations are based entirely on a reading of the landscape and topography; and are intended mainly to re-awaken interest in this fascinating region.

Please send comments, suggestions, and corrections to
Gerald Davidson.
P.O. Box 1466
Red Lodge, MT 59068