Abstract:

The Shields River-Yellowstone River drainage divide area in the Crazy Mountains discussed here is located in south central Montana, USA and this essay interprets landform origins using topographic map evidence. The Crazy Mountains are an isolated and high mountain range located in southern Montana along the Great Plains western edge. The Shields River originates in the Crazy Mountains and flows in a west and southwest direction before turning to flow in south and south-southeast direction along the Crazy Mountains’ west flank to join the Yellowstone River. Multiple southwest oriented tributaries originating in the high Crazy Mountains join the south-southeast oriented Shields River segment. The Yellowstone River flows in an east and east-northeast direction along the Crazy Mountains south flank and is joined by south and southeast oriented tributaries also originating in the high Crazy Mountains. The Shields River-Yellowstone River drainage divide area discussed here includes the Crazy Mountains’ south flank and southern regions of the high Crazy Mountains. Through valleys, which to many observers might better be described as mountain passes, link headwaters of southwest oriented Shields River tributaries with headwaters of southeast oriented Yellowstone River tributaries. These through valleys provide evidence of former flood flow channels eroded into and across the Crazy Mountains when the Crazy Mountains were still being uplifted. Floodwaters were derived from a rapidly melting thick North American ice sheet. Crustal warping responsible for the Crazy Mountains uplift is interpreted to have been caused by the thick ice sheet presence, with melt water from the rapidly melting thick ice sheet being responsible for crustal unloading that further contributed to Crazy Mountains uplift. Floodwaters also deeply eroded regions surrounding the rising Crazy Mountains, which contributed to the Crazy Mountains emergence. Subsequent to the Crazy Mountains emergence as a high mountain range valley glaciers formed in several Shields River and Yellowstone River tributary valley head regions.

Preface

The following interpretation of detailed topographic map evidence is one of a series of essays describing similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored deep glacial erosion paradigm, which is fundamentally different from most commonly accepted North American glacial history interpretations. Project essays are listed on the sidebar category list under their appropriate Missouri River tributary drainage basin, Missouri River segment drainage basin (by state), and/or state in which the Missouri River drainage basin is located.

Introduction

The purpose of this essay is to use topographic map interpretation methods to explore the Shields River-Yellowstone River drainage divide area landform origins in the southern Crazy Mountains, Montana, USA. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions and/or to propose alternate solutions that better explain evidence and are also consistent with adjacent map area and big-picture evidence. Readers may do so either by making comments here or by writing and publishing their own essays and then by leaving a link to those essays in a comment here.

This essay is also exploring a new geomorphology paradigm in which erosional landforms are interpreted as evidence left by immense glacial melt water floods. Implied in that interpretation is the immense floods were derived from a thick North American ice sheet that created a deep “hole” in the North American continent and also melted fast. The previously unexplored paradigm being tested in this and other Missouri River drainage basin landform origins research project essays is a thick North American ice sheet, comparable in thickness to the Antarctic ice sheet, occupied the North American region usually recognized to have been glaciated, and through its weight and erosive actions created a deep North American “hole”. The southwestern rim of that deep “hole” is today preserved in the high Rocky Mountains. The ice sheet through its weight and deep erosion (and perhaps deposition along major south-oriented melt water flow routes) caused significant crustal warping and tectonic change, through its action of melting fast produced immense floods that flowed across the continent, and through its action of melting fast systematically opened up space in the ice sheet created “hole” so headward erosion of newly developed north-oriented drainage systems captured immense south-oriented melt water floods and diverted immense melt water floods north into space the ice sheet had once occupied.

If this previously unexplored paradigm is correct the geographic region explored by this essay should contain evidence of immense floods that were captured by headward erosion of new valley systems so as to cause the floods to flow in a different direction. Ability of this previously unexplored paradigm to explain Shields River-Yellowstone River drainage divide area landform evidence in the southern Crazy Mountains, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Shields River-Yellowstone River drainage divide area location map

Figure 1: Shields River-Yellowstone River drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software. 

Figure 1 provides a location map for the Shields River-Yellowstone drainage divide in the southern Crazy Mountains and illustrates a region of south-central Montana with a strip of northwest Wyoming along the figure 1 south edge. The Crazy Mountains are located slightly west of the figure 1 center. The Yellowstone River flows from the Yellowstone National Park area in northwest Wyoming in a northwest and north-northeast direction to Livingston, Montana. From Livingston the Yellowstone River flows in an east-northeast direction to Big Timber before turning to flow in an east-southeast direction to Greycliff and Columbus and then turning to flow in a northeast direction to Laurel and Billings before reaching the figure 1 east center edge. East of figure 1 the Yellowstone River continues to flow in a northeast direction and joins the east oriented Missouri River at the North Dakota border. North of the Crazy Mountains the Musselshell River is formed near Martinsdale at the confluence of its southeast oriented North Fork and northeast oriented South Fork and then flows to Harlowton, Lavina, Roundup and Delphia before reaching the figure 1 east edge (north of center). East of figure 1 the Musselshell River turns to flow in a north direction to join the east oriented Missouri River. The Shields River is an unlabeled stream shown as originating near Loco Mountain (in the Crazy Mountains) and flowing in a southwest direction to Wilsall and then in a south-southeast direction through Clyde Park to join the Yellowstone River. An unnamed southwest oriented tributary (Cottonwood Creek on more detailed maps) flows from near Crazy Peak to join the Shields River near Clyde Park. East of the high Crazy Mountains ridges Sweet Grass Creek flows in a northeast direction and then southeast and south direction to join the Yellowstone River near Greycliff. South of Sweet Grass Creek an unnamed stream (Big Timber Creek on more detailed maps) flows from near Crazy Peak to join the Yellowstone River near Big Timber.

Looking at the big picture erosion history of the figure 1 map area the drainage routes shown developed as immense south and southeast oriented melt water floods flowed across the region and later as the deep Yellowstone River valley and subsequently the deep Missouri River-Musselshell River valley eroded headward from a deep “hole” in which a large North American ice sheet was rapidly melting. The deep “hole” was located north and east of the figure 1 map area, which is located along the deep “hole’s” deeply eroded southwest wall. The east and northeast oriented Yellowstone River and the Musselshell River-Missouri River valleys eroded headward from the deep “hole” to capture immense south and southeast oriented ice marginal floods flowing from western Canada across Montana. At that time mountain ranges in the figure 1 map area, including the Crazy Mountains, did not stand high above the surrounding regions and floodwaters could freely flow across the entire figure 1 map area. Over time however uplift of mountain ranges channeled the huge south and southeast oriented melt water floods into valleys or basins between the rising mountains. One such flood flow channel was between the Big Belt, Little Belt, Crazy, Bridger (not identified on figure 1), Gallatin, and Absaroka mountain ranges along valleys now used by the north-northwest oriented Smith River the south-southeast oriented Shields River, and the northwest and north-northeast oriented Yellowstone River. These flood flow channels were next dismembered by continued crustal warping and by headward erosion of deep northeast and east oriented valleys from the deep “hole” the melting ice sheet had once occupied to capture the immense south and southeast oriented ice sheet marginal melt water floods. The east and northeast oriented Yellowstone River valley was the first deep valley to erode headward across the figure 1 map area and southeast and south oriented Yellowstone River tributary valleys began to erode headward from newly eroded Yellowstone River valley north wall. The southeast and south oriented Sweet Grass Creek valley was one of these tributary valleys eroding headward from the newly eroded Yellowstone River valley to capture southeast oriented flood flow moving across the present day Crazy Mountains region.

For a time as the Crazy Mountains were being uplifted the south and southeast oriented floodwaters flowed across them and carved deep valleys into the rising mountain mass, although gradually they were captured by the developing south-southeast oriented channel along the Crazy Mountains west flank. This channeling process took place from the south to the north as southwest oriented valleys eroded headward into the rising mountain mass to capture southeast oriented flood flow. At the same time the deep south-southeast oriented Shields River flood flow channel was being eroded an east-northeast and east-southeast oriented channel was being eroded between the rising Crazy Mountains and Absaroka Mountains along the present day Yellowstone River route. Deep southeast oriented valleys eroded headward from actively eroding early Yellowstone River valley while deep southwest oriented valleys eroded headward from the actively eroding south-southeast oriented Shields River flood flow channel. Each southeast and southwest oriented valley captured flood flow moving to what was then the newly eroded valley immediately to the south. Crustal warping of the Yellowstone Plateau area combined with headward erosion of the deep northeast and east oriented Yellowstone River valley from the ice sheet’s deep “hole” reversed south oriented flood flow between the Gallatin and Absaroka Ranges to create the present day northwest and north-northeast Yellowstone River south of Livingston. Subsequently Missouri River valley headward erosion (north of the figure 1 map area) beheaded and reversed southeast oriented flood flow moving to the newly eroded south-southeast oriented Shields River valley to create the northwest oriented Smith River seen just west of White Sulphur Springs.

Detailed location map for Shields River-Yellowstone River drainage divide area

Figure 2: Detailed location map for Shields River-Yellowstone River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 2 provides a more detailed location map for the Shields River-Yellowstone River drainage divide area in the southern Crazy Mountains. County boundaries are shown and Sweet Grass County in the figure 2 east half is labeled. Park County is west of Sweet Grass County of Sweet Grass County and the county west of Park County is Gallatin County. Big Timber is a town in Sweet Grass County and the Yellowstone River flows in a northeast direction from south of Livingston (in Park County) to Big Timber and then flows in a southeast direction. The green shaded areas show regions of National Forest land, which are generally located in mountainous regions. The figure 2 north-center region green shaded area shows National Forest lands in the Crazy Mountains. The Shields River flows in a southwest and south direction from near the north-center edge to near Wilsall and then in a south-southeast direction to join the Yellowstone River near Mission. Note northwest oriented Shields River tributaries (near north-center edge) in the Crazy Mountains region, although most Shields River tributaries from the east or Crazy Mountains are oriented in southwest directions. Cottonwood Creek, Tobin Creek, and Adair Creek are labeled southwest oriented Shields River tributaries originating in the Crazy Mountains. Note Sheep Mountain between Adair Creek and the Yellowstone River. Sweet Grass Creek originates in the Crazy Mountains and flows in a northeast direction to near Porcupine Butte (in Sweet Grass County northwest corner area). Near Porcupine Butte Sweet Grass Creek turns to flow in a southeast and south direction to join the Yellowstone River near Greycliff. Big Timber Creek is located south of Sweet Grass Creek and also originates in the Crazy Mountains and flows in an east and southeast direction to join the Yellowstone River near Big Timber. Swamp is a labeled southeast oriented Big Timber Creek tributary. Little Timber Creek is a southeast oriented Yellowstone River tributary paralleling Swamp Creek. Duck Creek and its West Fork drain the Crazy Mountains south flank to the Yellowstone River and are labeled.

Adair Creek-Lowell Creek drainage divide area along the Crazy Mountains south flank

Figure 3: Adair Creek-Lowell Creek drainage divide area along the Crazy Mountains south flank. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 provides a topographic map of the Adair Creek-Lowell Creek drainage divide area between Sheep Mountain and the Crazy Mountains (north of figure 3).  Sheep Mountain is labeled and is in the south-center of figure 3. South of Sheep Mountain is the Yellowstone River valley with east oriented Yellowstone River visible along the figure 3 southwest quadrant south edge and also near Springdale  in the southeast corner region of figure 3. The south-southeast oriented Shields River flows from the west center edge of figure 3 to join the Yellowstone River near the south edge of figure 3. Adair Creek is a southwest oriented Shields River tributary originating near Antelope Butte and with a north and west oriented tributaries drain regions on the Sheep Mountain upland including regions between Antelope Butte and Sheep Mountain. Falls Creek is a southwest oriented stream draining from the Crazy Mountains north of figure 3 to join the Shields River a short distance upstream from Adair Creek.  East of the Adair Creek drainage basin in the region between Antelope Butte and Sheep Mountain are north and east oriented Lowell Creek headwaters. Lowell Creek flows to southeast oriented Hole in the Rock Creek, which joins southeast oriented West Fork Duck Creek with water eventually reaching the Yellowstone River just east of figure 3.  The figure 3 map contour interval is 50 meters and Sheep Mountain reaches an elevation of 1924 meters. The Antelope Butte high elevation is between 1850 and 1900 meters and higher elevations are found north of figure 3. Elevations along the drainage divide between Adair Creek and Lowell Creek in the region between Sheep Mountain and Antelope Butte are between 1600 and 1650 meters indicating the presence of a deep west to east oriented through valley between Sheep Mountain and Antelope Butte. The through valley is a water eroded valley and was eroded by east oriented floodwaters at a time when the deep Yellowstone River valley to the south was just being eroded as a parallel and probably much larger and deeper flood flow channel. Headward erosion of the deep Shields River valley (probably aided by Crazy Mountains uplift occurring to the north) beheaded the east oriented Adair Creek-Lowell Creek flood flow channel. Flood waters on the west end of the beheaded flood flow channel reversed flow direction to erode the west and southwest oriented Adair Creek drainage basin.

Detailed map of Adair Creek-Lowell Creek drainage divide area along the Crazy Mountains south flank

Figure 4: Detailed map of Adair Creek-Lowell Creek drainage divide area along the Crazy Mountains south flank. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 provides a detailed topographic map of the Adair Creek-Lowell Creek drainage divide area between Antelope Butte and Sheep Mountain. Antelope Butte can be seen straddling the figure 4 north edge (east of center) and has a point of 6190 feet marked (the figure 4 map contour interval is 20 feet). Sheep Mountain is located south of figure 4, although the north facing slopes are seen in the figure 4 south-center. Elevations on Sheep Mountain reach 6200 feet and at some points are more than 6290 feet. Adair Creek is labeled, drains the figure 4 west half and flows in a south direction from the Antelope Butte south flank and then near the figure 4 center turns to flow in a west-southwest direction to the figure 4 west edge (south of center).  Lowell Creek is also labeled and drains in a north direction from near the figure 4 south edge (east of figure 4 south center edge) and flows in a north and then east direction to the figure 4 east center edge. A through valley in the section 28 north half (near figure 4 center) links the east oriented Lowell Creek valley with the west oriented Adair Creek valley. The through valley floor elevation at the drainage divide is between 5280 and 5300 feet, indicating the through valley is at least 900 feet deep. This 900-foot deep (or deeper) through valley is a water eroded feature and as stated in the figure 3 discussion was eroded by east oriented flood water before headward erosion of the Shields River flood flow channel beheaded and reversed the east oriented flood flow route to erode the west-southwest oriented Adair Creek drainage basin.  Headward erosion of the deep Shields River flood flow channel was probably aided by Crazy Mountains uplift, which was occurring at the time floodwaters were flowing across the region.

Falls Creek-West Fork Duck Creek drainage divide area

Figure 5: Falls Creek-West Fork Duck Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the Falls Creek-West Fork Duck Creek drainage divide area north of Antelope Butte and includes overlap areas with figure 3. Antelope Butte can be seen near the figure 5 south edge (west of center). North of Antelope Butte is Choke to Death Butte. Further north, near the figure 5 north edge and east of the Park County-Sweet Grass County line, is Fairview Peak, which is in the high Crazy Mountains. Much of figure 5 illustrates the Crazy Mountains south-facing slope. West of Antelope Butte and Antelope Butte is southwest oriented Falls Creek which flows to the figure 5 south edge  (west half) and then to Shields River. North and west of Falls Creek is Tobin Creek, which flows to the figure 5 west edge (north of southwest corner). North and west of Tobin Creek is Rock Creek, which near the figure 5 north edge (and in the high Crazy Mountains) is oriented in a south direction. East of Choke to Death Butte is southeast oriented Hole-in-the-Rock Creek, which flows to southeast oriented West Fork Duck Creek. West Fork Duck Creek originates just west of Fairview Peak and flows in a southwest, south, south-southwest, and southeast direction down the Crazy Mountains south flank. Note how in the region between Choke to Death Butte and Fairview Peak several through valleys link southwest oriented Falls Creek with southeast oriented Hole-in-the-Rock Creek and with West Fork Duck Creek. The figure 5 contour interval is 50 meters and Choke to Death Butte reaches an elevation of 1924 meters. Just north of Choke to Death Butte a trail crossing the Falls Creek-Hole-in-the-Rock Creek drainage divide reaches an elevation of between 1800 and 1850 meters (and from the map evidence the elevation appears probably to be closer to 1800 meters it is to 1850 meters). The west to east oriented through valley north of Choke to Death Butte is approximately 100 meters deep and is one of several water-eroded through valleys on what is today the Crazy Mountains south flank. These through valleys were probably eroded by south and southeast oriented floodwaters at the time the Crazy Mountains uplift was occurring, which was probably approximately the same time as the deep Shields River flood flow channel was eroding headward along the Crazy Mountains west flank.

Detailed map of Falls Creek-Hole in the Rock Creek drainage divide area

Figure 6: Detailed map of Falls Creek-Hole in the Rock Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 provides a detailed topographic map of the Falls Creek-Hole-in-the-Rock Creek drainage divide area north of Choke to Death Butte, seen in less detail in figure 5. Choke to Death Butte is located in section 4 near the figure 6 south center edge. Falls Creek is labeled and flows in a south-southwest direction from the figure 6 north edge (east half) to the figure 6 south edge (west of Choke to Death Butte). West Fork Duck Creek flows in a south-southwest and then southeast direction from the figure 6 north edge (near northeast corner) to the figure 6 east edge (south half). Hole-in-the-Rock Creek originates in section 27 (figure 6 northeast quadrant) and flows in a south-southeast direction to the figure 6 south edge (east half). The map contour interval is 20 feet except in the northeast quadrant where the contour interval is 40 feet. The regular spacing of contour lines on the slopes suggests the presence of steeply dipping sedimentary rock layers, which have been eroded by the various streams. Near the corner of sections 3, 4, 33, and 34 is a through valley linking the Hole-in-the-Rock Creek valley with the Falls Creek valley. The through valley floor elevation at the drainage divide is between 5900 and 5920 feet. Choke to Death Butte rises to 6313 feet, suggesting the through valley is approximately 400 feet deep. The through valley was eroded by east and southeast oriented flood flow moving to what was at that time the actively eroding and deep Yellowstone River valley. Probably at that time Crazy Mountains uplift had not created the steep slopes seen today and floodwaters were flowing across a rising mountain mass. Headward erosion of the deep Shields River flood flow channel, aided by the ongoing Crazy Mountains uplift, enabled the southwest oriented Falls Creek valley to erode headward into the figure 6 map area and to behead flood flow moving to the Hole-in-the-Rock valley.

Big Timber Creek-Rock Creek drainage divide area

Figure 7: Big Timber Creek-Rock Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 has been enlarged to better depict details and illustrates the Big Timber Creek-Rock Creek drainage divide area north of figure 5 map area and shows a region in the high Crazy Mountains. Fairview Peak seen in figure 5 is a short distance south of figure 7. Big Timber Peak and Crazy Peak are the labeled high peaks seen in Sweet Grass County and Granite Peak and Iddings Peak are the labeled high peaks in Park County. Big Timber Creek originates west of Twin Lakes (near Granite Peak) and flows in an east direction north of Granite and Big Timber Peaks to the east edge of figure 7 (north half). East of figure 7 Big Timber Creek turns to flow in a southeast direction to join the Yellowstone River. Rock Creek originates at Rock Lake (north of Iddings Peak) and flows in a south direction to the figure 7 south edge. South of figure 7 Rock Creek turns to flow in a southwest direction to join the Shields River. Smelter Creek is a southwest oriented Rock Creek tributary originating at Smelter Lake (southeast from Iddings Peak). Cottonwood Creek, seen better in figure 8, originates at Cottonwood Lake and flows to west edge of figure 7 (north of center). Grasshopper Glacier located near Cottonwood Lake is a remnant of larger glaciers that once eroded the figure 7 valley heads to produce the present day lake basins. While alpine glacier eroded this high mountain region, the valleys those glaciers eroded existed prior to the glaciation event or events and through valleys across some of the drainage divides provide evidence of routes floodwaters used to erode the valleys. Floodwaters eroded the valleys as Crazy Mountain uplift was occurring and as the deep Shields River flood flow channel was being eroded along the Crazy Mountains west flank. The Crazy Mountains emerged as crustal warping raised the mountain range and as erosion caused by Yellowstone River-Shields River valley headward erosion deeply eroded surrounding areas. An example of a north to south oriented through valley that was probably initially eroded by south oriented floodwaters can be seen north of Smelter Lake and east of Iddings Peak. Headward erosion of the deep Big Timber Creek valley and its northeast oriented tributary valley (the Pear Lake valley) beheaded the south oriented flood flow.

Detailed map of Cottonwood Creek-Big Timber Creek drainage divide area

Figure 8: Detailed map of Cottonwood Creek-Big Timber drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 uses an enlarged map to better illustrate the Cottonwood Creek-Big Timber creek drainage divide area seen in figure 7. Cottonwood Creek originates at Cottonwood Lake and flows in a southwest and west direction to the west edge of figure 8 (south half). West of figure 8 Cottonwood Creek flows in a southwest direction to join the south-southeast oriented Shields River. Big Timber Creek originates on the east side of the ridge just east of Cottonwood Lake and flows ion an east direction to the east edge of figure 8 (near center). Note how the ridge between the Cottonwood Creek and the Big Timber Creek headwaters is much at that point than either to the north or to the south. The map contour interval is 50 meters and Conical Peak to the north of the through valley (or mountain pass) east of Cottonwood Lake rises to 3276 meters while Iddings Peak to the south rises to 3333 meters. The through valley at the Cottonwood Creek-Big Timber Creek drainage divide has an elevation of between 2900 and 2950 meters suggesting a 300 meter or deeper valley once was eroded across the region. The deep valley was eroded into what was at that time the rising Crazy Mountains by east oriented floodwaters moving to what was at that time the actively eroding and deep Yellowstone River valley. Another interesting through valley is located between Cottonwood Lake and Glacier Lake (just north of Cottonwood Lake). Today the north oriented South Fork Sweet Grass Creek drains from Glacier Lake and north of figure 8 joins northeast oriented Sweet Grass Creek, which east of the Crazy Mountains turns to flow in a southeast and south direction to the Yellowstone River valley. The through valley floor elevation is also between 2900 and 2950 meters suggesting it also was once a 300 meter deep flood water eroded valley and supplied floodwaters to both the actively eroding Big Timber Creek valley and the actively eroding Cottonwood Creek valley, which was beheaded and reversed by headward erosion of the deep Shields River flood flow channel. Crazy Mountains uplift eventually ended all flood flow across the region.

Shields River-Sweet Grass Creek drainage divide area

Figure 9: Shields River-Sweet Grass Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 illustrates the Shields River-Sweet Grass Creek drainage divide area north and west of figure 8 and includes overlap areas with figure 8. The Shields River is labeled and flows in a west direction near the north edge of the figure 9 northwest quadrant. West of figure 9 the Shields River turns to flow in a south-southeast direction along the Crazy Mountains west flank. Note north and northwest oriented Shields River tributaries in the north center region of figure 9. These tributaries include Buck Creek, Bennett Creek, Deep Creek and its Sunlight Creek tributary, Mill Creek, and the South Fork of the Shields River. Sweet Grass Creek flows in an east-northeast direction to the east center edge of figure 9 and has an east and southeast oriented North Fork, a north oriented South Fork, and a northeast oriented Middle Fork. Note how the North Fork and Middle Fork of Sweet Grass Creek valleys are linked by through valleys (many people might say the through valleys are high mountain passes or even notches in high mountain ridges) with the north and northwest oriented Shields River tributary valleys. The through valleys (or mountain passes or notches) and the north and northwest oriented Shields River tributary valley orientations are what remain of former south and southeast oriented flood flow channels eroded into the what was at that time a rising Crazy Mountains mountain mass. Floodwaters were flowing to what was then the newly eroded east oriented Sweet Grass Creek valley, which had eroded headward into the region. Probably at that time the Sweet Grass Creek valley was a northeast oriented valley diverting south and southeast oriented floodwaters toward the deep “hole” where the ice sheet was melting. Subsequently headward erosion of the deep Yellowstone River valley and the southeast and south oriented Sweet Grass Creek valley captured the northeast oriented flow to create the Sweet Grass Creek route seen today. Headward erosion of the west oriented Shields River valley, probably aided by ongoing Crazy Mountains uplift,  from the south-southeast oriented flood flow channel along the Crazy Mountains west flank beheaded and reversed flood flow channels to the Sweet Grass Creek valley and created the north and northwest oriented Shields River tributaries.

Detailed map of Sunlight Creek-North Fork Sweet Grass Creek drainage divide area

Figure 10: Detailed map of Sunlight Creek-North Fork Sweet Grass Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 provides a detailed topographic map of the Sunlight Creek-North Fork Sweet Grass Creek drainage divide area seen in less detail in figure 9 above. The North Fork Sweet Grass Creek originates at Sunlight Lake in section 8 (near south center edge) and flows in an east-northeast and southeast direction to the south edge of figure 10 (near southeast corner). Sunlight Creek originates in section 7 (west of Sunlight Lake) and flows in a north-northwest direction to the north edge of figure 10 (near northwest corner) and north of figure 10 joins the west and south-southeast oriented Shields River. Note the through valley (or notch in the high mountain ridge) between a northwest oriented Sunlight Creek tributary and Sunlight Lake (a trail crosses the ridge at that mountain pass). The figure 10 contour interval is 80 feet and the through valley (or pass) elevation at the drainage divide is between 9200 and 9280 feet.  Sunlight Peal to the south reaches 10090 feet while the peak to the north reaches 9822 feet. In other words the through valley is approximately 600 feet deep. This through valley was eroded by southeast oriented flood flow moving to what was once the actively eroding North Fork Sweet Grass Creek valley. Crazy Mountains uplift combined with headward erosion of the deep Shields River valley resulted in the flood flow route being beheaded and reversed to create the northwest oriented tributary and Sunlight Creek route. Similar through valleys cross the high mountain ridge that today serves as the drainage divide and provide evidence of multiple south and southeast oriented flood flow channels that were beheaded and reversed.

Additional information and sources of maps studied

This essay has provided only a sample of the detailed topographic map evidence supporting the flood erosion interpretation. Many additional illustrations could be provided. Readers are encouraged to look at mosaics of detailed topographic maps to see the abundance of available data. Maps used in this study were created and published by the United States Geologic Survey and can be obtained directly from the United States Geological Survey and/or from dealers offering United States Geological Survey maps. Hard copy maps can also be observed at United States Geological Survey map depositories, which are located throughout the United States and elsewhere. Illustrations used here were created using National Geographic Society TOPO software and digital map data. TOPO software and map data can be obtained from the National Geographic Society and/or dealers offering National Geographic Society digital map data.