Topographic map interpretation methods are used to determine landform origins in the Sixteenmile Creek-Gallatin River drainage divide area located in Gallatin County, Montana. Sixteenmile Creek is a southwest, northwest, and west oriented stream flowing between the Big Belt Mountains to the north and the Bridger Range to the south and flows to the north-oriented Missouri River. The Gallatin River flows in a north and northwest direction in the region west of the Bridger Range and joins the Jefferson and Madison River at Three Forks to form the north-oriented Missouri River. The Horseshoe Hills are an upland region east of the north-oriented Missouri River located between Sixteenmile Creek and the Gallatin River. Between the Horseshoe Hills and the Bridger Range is a large north-south oriented through valley drained in the south by south-oriented Dry Creek with water eventually reaching the Gallatin River. Through valleys cross present day study region drainage divides and orientations of all valleys, including valleys of secondary streams, are used to reconstruct drainage routes that existed prior to evolution of present day drainage system. Present day drainage routes evolved during massive south-oriented floods derived from a rapidly melting thick North American ice sheet located north of the study region. At the time flood waters first flowed across the study region the Bridger Range and Horseshoe Hills did not stand high above the surrounding valleys (or basins) and flood waters could freely flow across them. As the Bridger Range and Horseshoe Hills emerged as high regions flood waters moving in what was a giant south-oriented anastomosing channel complex carved deep canyons (or valleys) into the rising uplands until such time as the flood waters were channeled around the rising upland and mountain areas. Crustal warping was probably related to the ice sheet’s tremendous weight and eventually contributed to massive flood flow reversals that included reversals of flood flow that resulted in erosion of the north-oriented Missouri River and Gallatin River valleys and tributary valleys.
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 available on this site can be found by selecting the desired Missouri River tributaries and/or states from this essay’s sidebar category list.
- The purpose of this essay is to use topographic map interpretation methods to explore the Sixteenmile Creek-Gallatin River drainage divide area landform origins in Gallatin County, 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 providing a link to those essays in a comment at the end of this essay.
- 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 essays in the Missouri River drainage basin landform origins research project 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 Sixteenmile Creek-Gallatin River drainage divide area landform evidence in Gallatin County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm (see essay listed at header). This essay is included in the Missouri River drainage basin landform origins research project essay collection being posted on this WordPress site.
- Figure 1 provides a location map for the Sixteenmile Creek-Gallatin River drainage divide area and illustrates a region in south central and southwest Montana with the Wyoming northwest corner seen along the east half of the figure 1 south margin (and a bit of Idaho can be seen in the figure 1 southwest corner). The north section of Yellowstone National Park can be seen in the Wyoming northwest corner. The Gallatin River originates in the Yellowstone National Park northwest corner and flows in a north direction from the Wyoming northwest corner (and Yellowstone National Park northwest corner) to west of Bozeman and then turns to flow in a west-northwest direction to near Three Forks. At Three Forks the Gallatin River joins the north-oriented Madison River and the north-northeast and east oriented Jefferson River to form the Missouri River. From Three Forks the Missouri River flows in a north and north-northwest direction along the Big Belt Mountains western flank to Holter Lake (reservoir) and then turns to flow in a northeast direction to the figure 1 north edge. North of the figure 1 map area the Missouri River flows in a northeast direction before eventually turning to flow in more of an east direction across Montana to North Dakota. East of the Gallatin River headwaters in Yellowstone National Park is the Gallatin Range and then the northwest oriented Yellowstone River which once in Montana turns to flow in a north-northeast direction to near Livingston where it turns again to flow in more of an east direction to the figure 1 east edge (south of center). East of figure 1 the Yellowstone River turns again to flow in a northeast direction to join the Missouri River near the Montana-North Dakota border. Between the Big Belt Mountains and the Gallatin Range is the unnamed Bridger Range which extends in a north-south direction just east of Bozeman. The unlabeled south-southeast oriented stream located west of the Crazy Mountains and east of the Bridger Range and joining the Yellowstone River near Livingston is the Shields River. Ringling is a town near the Shields River headwaters and an unnamed stream flows in a west and southwest direction through Ringling to the north-oriented Missouri River near Toston. That unlabeled stream is Sixteenmile Creek and flows between the Big Belt Mountains to the north and the Bridger Range to the south. The Sixteenmile Creek-Gallatin River drainage divide area investigated here is located south of the Sixteenmile Creek, east of the Missouri River, north of the Gallatin River, and generally west of the Bridger Range. The Deep Creek-Sixteenmile Creek drainage divide area landform origins southern Big Belt Mountains, Montana essay illustrates and describes the region directly north of Sixteenmile Creek. Essays illustrating and describing other Missouri River drainage divide areas can be found by selecting desired states and Missouri River tributaries from category list found on the sidebar.
- Today the Sixteenmile Creek-Gallatin River drainage divide area is a mountainous region although to understand this essay it is necessary to think of the region at the time the mountains were just beginning to emerge. At that time immense south and southeast melt water floods were flowing across the entire figure 1 map area (and a much larger region) on an erosion surface that permitted flood waters to freely flow across Montana and into Wyoming. The melt water floods were coming from western Alberta and eastern British Columbia which was the location of the west rim of a deep “hole” in the North American continent in which a thick North American ice sheet was located. When the ice sheet formed there was no deep “hole”, however the deep “hole” developed as deep glacial erosion scoured the bedrock surface under the ice sheet and as crustal warping, caused by the ice sheet’s tremendous weight, caused mountain ranges and high plateau areas to gradually rise along the ice sheet’s west and southwest margins and elsewhere in the North American continent. Probably for a time the massive south and southeast-oriented melt water floods were able to erode the rising mountain masses although in time the mountain masses rose faster than the flood waters could erode them. However, the removal of significant bedrock materials from the rising mountain masses probably accelerated the uplift of the mountain ranges as immense south and southeast-oriented flood flow moved across them and deposited debris in adjacent valleys. As the present day mountain ranges began to emerge the south and southeast-oriented flood flow was moving in a gigantic south-oriented anastomosing channel complex consisting of diverging and converging flood flow channels located between rising mountain ranges and/or of flood flow channels carving deep canyons into the rising mountain masses.
- The Missouri River drainage basin in Montana and northern Wyoming is the deep “hole’s” deeply eroded southwest wall and was eroded by massive flood flow reversals that developed as deep east and northeast-oriented valleys eroded headward from space being opened up in the deep “hole” when melting of the thick ice sheet opened up deep “hole” space, especially along the southern margin. These deep east and northeast-oriented valleys eroded headward from the deep “hole” in sequence from the southeast to the northwest to capture the immense south and southeast-oriented melt water floods. In figure 1 the deep Yellowstone River valley eroded headward into the region first. North-oriented Yellowstone River tributary valleys, including the Yellowstone River headwaters valley south of Livingston, were eroded by massive reversal of flood flow on north ends of beheaded south and southeast-oriented flood flow channels (the northwest-oriented Yellowstone River segment is flowing in a valley that was initiated as a southeast-oriented flood flow channel). At the same time deep south and southeast-oriented tributary valleys eroded headward along the captured south and southeast-oriented flood flow channels. Headward erosion of the deep Missouri River valley next captured the immense south and southeast-oriented flood flow and beheaded south-oriented flood flow routes to the newly eroded Yellowstone River valley. Again flood water on the north and northwest ends of beheaded flood flow channels reversed flow direction to erode north and northwest-oriented Missouri River tributary valleys. North of Ringling in figure 1 are headwaters of the north-northwest oriented Smith River, which is located on the alignment of a former south-southeast oriented flood flow channel between what were the emerging Big Belt Mountains and the Little Belt Mountains and which was reversed by headward erosion of the deep northeast-oriented Missouri River valley. The north-northwest oriented Missouri River valley segment west of the Big Belt Mountains was also eroded by a reversal of flood flow the north end of a south-oriented flood flow channel that was moving flood water to the Three Forks area where diverging flood flow channels then moved the flood waters further south along routes now used by the north-oriented Gallatin, Madison, and Jefferson Rivers. Prior to being beheaded and reversed at least some of the flood waters may have been moving to the deep Snake River valley, which had probably eroded headward into the region south of the figure 1 south center edge. With this brief overview of the big picture regional erosion history this essay will now focus on the Sixteenmile Creek-Gallatin River drainage divide area.
Detailed location map for Sixteenmile Creek-Gallatin River drainage divide area
- Figure 2 provides a detailed location map for the Sixteenmile Creek-Gallatin River drainage divide area in Gallatin County. County boundaries are shown and Gallatin County is labeled. Green shaded areas are National Forest lands and are generally located in mountainous regions. The Bridger Range is the mountainous region north of Bozeman in the Gallatin County east half. The Gallatin River flows in a north, north-northwest, and west direction from the figure 2 south edge near Bozeman Hot Springs to Manhattan and Logan before joining the north-oriented Madison River and the east, southeast, and northeast oriented Jefferson River near Three Forks. The combined rivers then flow in a north-northeast direction as the Missouri River along the Gallatin County west border before turning to flow in a north-northwest direction to the figure 2 north edge (west of center). Ringling is a town located near the figure 2 northeast corner. Sixteenmile Creek flows through Ringling and then in a southwest, northwest, and west direction to join the Missouri River near the town of Lombard. Note the southwest and west oriented Middle Fork Sixteenmile Creek in the Gallatin County northeast corner. The unnamed north and north-northwest oriented Middle Fork tributary is the South Fork Sixteenmile Creek, which is seen in figures 9 and 10 below. The East Fork Gallatin River flows in a northwest and west direction from Bozeman to Logan where it joins the Gallatin River. An important East Fork Gallatin River tributary in this essay is south oriented Dry Creek, which is located between the Bridger Range and the Horseshoe Hills. Cottonwood Gulch is a southwest-oriented tributary draining from the south end of the Horseshoe Hills to the Gallatin River downstream from Logan. Note unnamed northwest oriented streams flowing from the Horseshoe Hills north end to the Missouri River and to Sixteenmile Creek. Orientations of these secondary streams is significant. The Missouri River is today a north-oriented drainage route yet many of its tributaries seen in figure 2 are oriented in south directions. These south oriented tributaries are flowing in valleys eroded when south-oriented flood water crossed the figure 2 map area before the massive flood flow reversal that resulted in the north-oriented Missouri River. Most if not all of the northwest and north-oriented streams in figure 2, including the Missouri, Gallatin, and Madison Rivers are flowing in valleys that were initiated as south- or southeast-oriented flood flow channels, which were subsequently beheaded and reversed to produce north- and northwest-oriented valleys. Flood flow reversals that occurred in the figure 2 map region were probably significantly aided by crustal warping that was raising mountains ranges and other upland areas to the south as flood waters flowed across them.
Roy Gulch-Polson Hollow drainage divide area
- Figure 3 illustrates the Roy Gulch-Polson Hollow drainage divide area in the northern Horseshoe Hills. The Horseshoe Hills are labeled and are a series of forested ridges extending in a north-south direction from the figure 3 north edge to the south edge. The Missouri River flows in a north direction near the figure 3 west edge. Sixteenmile Creek flows in a west-southwest, west-northwest, and west-southwest direction near the figure 3 north edge and has cut a deep canyon across the Horseshoe Hills north end. Maudlow is a small town located on Sixteenmile Creek in the figure 3 northeast corner area. Brammer Creek is the north-oriented tributary joining Sixteenmile Creek at Maudlow. South of the Brammer Creek headwaters is south-southwest oriented Blacktail Creek which near the figure 3 southeast corner joins other streams to form south-oriented Dry Creek, which south of figure 3 flows to the East Fork Gallatin River. Polson Hollow is a southeast, east, and southeast-oriented Dry Creek tributary located in the figure 3 southeast quadrant and originating in the Horseshoe Hills. Just north of the Polson Hollow headwaters in the Horseshoe Hills are north-oriented headwaters of north, west, north, west, and northwest oriented Roy Gulch, which drains to Sixteenmile Creek in the figure 3 northwest quadrant. South of Roy Gulch is northwest and west-northwest oriented Garden Gulch and south of Garden Gulch is northwest, southwest, and northwest oriented Lone Pine Gulch. South of Lone Pine Gulch is northwest and west oriented Pole Gulch. The northwest-oriented valleys seen today in figure 3 were eroded by reversals of flood flow on northwest ends of southeast-oriented flood flow channels, which once crossed the figure 3 map area. At the time the flood flow channels first formed the Horseshoe Hills had not yet emerged as high ridges and flood waters could freely flow across the region on an erosion surface equivalent in elevation to the highest figure 3 elevations today. The Horseshoe Hills emerged as high ridges probably due to a combination of crustal warping and of deep erosion of deeper north-south oriented flood flow channels on either side. The west-northwest oriented Sixteenmile Creek canyon across the Horseshoe Hills north end was probably initiated by an east-southeast oriented diverging flood flow channel from a south-oriented flood flow channel on the present day Missouri River valley alignment which converged with a southwest-oriented flood channel on the Sixteenmile Creek alignment upstream from Maudlow to form a south-oriented flood flow channel on the present day Dry Creek alignment (between what were then the emerging Horseshoe Hills and Bridger Range). South-oriented flood flow to the figure 3 map area ended when headward erosion of the deep northeast-oriented Missouri River valley (north of Big Belt Mountains) beheaded the south-oriented flood flow channels and flood waters on north ends of the beheaded flood flow channels reversed flow direction to erode north-oriented valleys (in figure 3 the north-oriented Missouri River valley).
Detailed map Roy Gulch-Polson Hollow drainage divide area
- Figure 4 provides a detailed topographic map of the Roy Gulch-Polson Hollow drainage divide area seen in less detail in figure 3 above. Linear ridges are the Horseshoe Hills. Roy Gulch originates in the Horseshoe Hills in section 33 (near figure 4 center) and drains in a north direction to section 29 where it turns to drain in a southwest and then northwest direction to the figure 4 north edge (west of center) and as seen in figure 3 eventually drains to Sixteenmile Creek, which then flows to the north-oriented Missouri River. South of the north-oriented Roy Gulch headwaters in section 33 is section 4 where south-oriented Polson Hollow headwaters originate and drain to the west edge of section 4 where Polson Hollow turns to drain in an east and southeast direction to the figure 4 southeast corner. South and east of figure 4 Polson Hollow drains to south-oriented Dry Creek, which flows to the Gallatin River. Note how in the south half of section 33 north-oriented Roy Gulch headwaters are linked by a north-south oriented through valley with south-oriented Polson Hollow headwaters. The figure 4 map contour interval is 40 feet and the through valley floor elevation at the drainage divide is between 6200 and 6240 feet. To the east the drainage divide rises to more than 6400 feet while to the west it rises even higher. In other words the through valley is at least 160 feet deep and may be even deeper. The through valley provides evidence of a former southeast-oriented flood flow channel that eroded headward from the south-oriented Dry Creek valley along the present day Polson Hollow alignment and then north along the Roy Gulch alignment. Headward erosion of the southwest-oriented Sixteenmile Creek valley then beheaded and reversed the southeast-oriented flood flow channel and flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Roy Gulch valley. A close look at the figure 4 map reveals several other less obvious through valleys. For example in section 5 an erosional residual is surrounded by a through valley linking the south-oriented Polson Hollow headwaters valley with a southeast-oriented Polson Hollow tributary valley. This through valley (and others like it) provides evidence of multiple flood flow channels. The figure 4 map region drainage history is much more complex than what has been described here, although the processes described here for deciphering the various drainage orientations and the through valleys linking the present day drainage routes can be used to work out the much more complicated details of the flood eroded Horseshoe Hills regions.
Sixteenmile Creek-Dry Creek drainage divide area
- Figure 5 illustrates the Sixteenmile Creek-Dry Creek drainage divide area east of the figure 3 map area and includes overlap areas with figure 3. The Horseshoe Hills are labeled and are located in the figure 5 west half. The northwest end of the Bridger Range is the forested mountain area in the figure 5 southeast quadrant. Sixteenmile Creek flows in a west, west-southwest, northwest, and west direction from the figure 5 east edge (near northeast corner) to the figure 5 northwest corner. West of figure 5 Sixteenmile Creek flows to the north-oriented Missouri River. Maudlow is the small town located on Sixteenmile Creek near the figure 5 north center. Brammer Creek (Bremmer Creek on more detailed topographic maps) is the labeled north-oriented stream joining Sixteenmile Creek at Maudlow (note the unlabeled north-oriented tributary joining Brammer Creek just south of Maudlow). Blacktail Creek originates north of Blacktail Mountain (a high point in the Bridger Range) and flows in a west and southwest direction north and west of Blacktail Mountain to join east and southeast-oriented Polson Hollow in the large valley or basin between the Horseshoe Hills and Bridger Range to form south-oriented Dry Creek, which then flows to the figure 5 south center edge. South of figure 5 Dry Creek flows to the Gallatin River. Note how a large north-south oriented through valley between the Horseshoe Hills and the Bridger Range links the west-oriented Sixteenmile Creek valley with the Gallatin River valley to the south. The figure 5 map contour interval is 50 meters and through valley elevations along the drainage divide are in the 1600 to 1650 meter range. Blacktail Mountain to the east rises to 2555 meters while a high point of 2174 meters is shown in the Horseshoe Hills. These high points suggest the Sixteenmile Creek-Dry Creek through valley may be as much as 500 meters deep, if not deeper. The north-oriented Brammer Creek headwaters are linked by a “shallow” channel eroded into the floor of the much broader through valley with the south-oriented Blacktail Creek valley and the unnamed north-oriented Brammer Creek tributary valley is also linked by a “shallow” channel eroded into the floor of the much broader through valley with a south-oriented Polson Hollow tributary valley. These “shallow” channels provide evidence of diverging and converging flood flow channels eroded into the floor of the much broader and deeper through valley. Headward erosion of the deep west-oriented Sixteenmile Creek valley beheaded the south-oriented flood flow channels and flood waters on north ends of the beheaded flood flow channels reversed flow direction to erode the north-oriented Brammer Creek valley and the unnamed Brammer Creek tributary valley.
Detailed map of Bremmer Creek-Blacktail Creek drainage divide area
- Figure 6 provides a detailed topographic map of the Bremmer Creek-Blacktail Creek drainage divide area seen in less detail in figure 5 above. The eastern edge of the Horseshoe Hills can be seen along the figure 6 west edge. The northwest slope of the Bridger Range can be seen in the figure 6 southeast corner and Blacktail Mountain is located a short distance east of the figure 6 southeast corner. Blacktail Creek flows in a west, southwest, and south-southwest direction from the 6 east center edge to the figure 6 south edge (just east of center). South of figure 6 Blacktail Creek joins Polson Hollow to form south-oriented Dry Creek. Bremmer Creek (Brammer Creek in figure 5) originates in section 31 and flows in a north-northwest and north direction to the figure 6 north center edge and then joins west-oriented Sixteenmile Creek north of the figure 6 map area. Note the shallow through valley in section 31 linking the north-northwest oriented Bremmer Creek headwaters valley with the southwest-oriented Blacktail Creek valley. The figure 6 map contour interval is 40 feet and the through valley floor elevation at the drainage divide is between 5280 and 5320 feet. Elevations in the southeast corner of section 36 immediately to the west rise to more than 5440 feet while elevations to the east rise much higher. In other words this south-oriented flood flow channel is at least 120 feet deep and may be much deeper. Further west in the figure 6 southwest quadrant in sections 2 and 3 there is an east and south-oriented stream flowing from the Horseshoe Hills (along the figure 6 west edge) to the figure 6 south edge (west half). South of figure 6 this stream flows to southeast-oriented Polson Hollow, which then joins Blacktail Creek to form south-oriented Dry Creek. Note in the section 2 northeast corner a through valley linking the south-oriented valley segment with a north-northeast oriented stream valley. The north-northeast oriented stream flows to the figure 6 north edge (just west of center) and joins Bremmer Creek north of figure 6. The section 2 northeast corner through valley has an elevation at the drainage divide of between 5160 and 5200 feet. Elevations greater than 6400 feet are seen in the Horseshoe Hills along the figure 6 west edge (and an elevation of 7133 feet is located west of the figure 6 map area). Elevations greater than 5440 feet are located along the divide with Blacktail Creek and elevations rise higher than 7000 feet in the figure 6 southeast corner (Blacktail Mountain just east of the figure 6 southeast corner rises to 8375 feet). These elevations provide some measures of the through valley depth. The section 2 (northeast corner) channel eroded into the broader through valley floor is at least 200 feet deep, although the much broader north-south oriented through valley is at least 1900 feet deep and may be even deeper. .
Missouri River-Dry Creek drainage divide area
- Figure 7 illustrates the Missouri River-Dry Creek drainage divide area south of the figure 3 map area and includes overlap areas with figure 3. The Gallatin River flows in a northwest direction across the figure 7 southwest corner and joins the Madison and Jefferson Rivers a short distance west of figure 7 to form the north-oriented Missouri River, which then flows in a north-northeast direction near the figure 7 west edge. The west-oriented East Gallatin River can be seen flowing along the figure 7 south edge. Cottonwood Gulch is a southwest-oriented Gallatin River tributary originating near the figure 7 center. North of the Cottonwood Creek headwaters are the forested Horseshoe Hills. Southwest and south-southwest oriented Nixon Gulch originates west of the Cottonwood Gulch headwaters and drains to the East Gallatin River near the figure 7 south center edge. Note how southwest-oriented Nixon Gulch headwaters are located in what appears to be a northeast-facing basin. Big Davis Gulch is a southwest, northwest, and west oriented Missouri River tributary originating in the Horseshoe Hills (north of the Cottonwood Gulch headwaters). South-oriented Dry Creek is located near the figure 7 east edge and joins the East Gallatin River south of the figure 7 map area. East and southeast-oriented Polson Hollow drains to Dry Creek near the figure 7 northeast corner. Chipmunk Gulch is an east-southeast oriented Dry Creek tributary originating as a south-oriented stream in the forested Horseshoe Hills just east of the southwest-oriented Big Davis Gulch headwaters. Horseshoe Creek and Little Horseshoe Creek are northeast-oriented Chipmunk Gulch tributaries draining the northeast end of the northeast-facing basin drained in the southwest by the southwest-oriented Nixon Gulch headwaters.
- The Missouri River valley was initiated as a south-oriented flood flow channel with flood waters probably flowing along the present day Madison River alignment to the actively eroding Snake River valley in the region south and maybe west of present day Yellowstone National Park. The south-oriented Dry Creek valley also originated as a south-oriented flood flow channel with flood waters continuing to move south along the present day Gallatin River alignment. When these south-oriented flood flow channels were first beginning to form there were many diverging and converging flood flow channels moving flood waters across the figure 7 map area. Initially there were southeast-oriented flood flow channels moving flood waters to the Dry Creek-Gallatin River flood flow channel. Later deeper southwest-oriented flood flow channels eroding headward from the Missouri River-Madison River flood flow channel captured the southeast-oriented flood flow and at that time headward erosion of the deep Sixteenmile Creek valley north of figure 7 beheaded the south-oriented Dry Creek flood flow channel although a diverging flood flow channel in the figure 7 southwest corner continued to move flood waters to the south-oriented Gallatin River alignment. Finally headward erosion of the deep northeast-oriented Missouri River north of the Big Belt Mountains beheaded and reversed flood flow on the Missouri River alignment and in the process reversed flood flow in the Gallatin and Madison River valleys. The flood flow reversal was probably greatly aided by crustal warping that raised elevations south of figure 7.
Detailed map of Big Davis Gulch-Cottonwood Gulch drainage divide area
- Figure 8 provides a detailed topographic map of the Big Davis Gulch-Cottonwood Gulch drainage divide area seen in less detail in figure 7. Big Davis Gulch is labeled and drains in a southwest direction from the figure 8 north center edge and then at an elbow of capture (in section 2 northeast quadrant) turns to drain in a northwest direction to the figure 8 northwest area and west of figure 8 joins the Missouri River. Cottonwood Gulch originates in section 36 and drains in a southwest direction to the figure 8 south edge (near southwest corner). Note the northwest-southeast oriented through valley linking a northwest-oriented Big Davis Gulch tributary valley draining to the Big Davis Gulch elbow of capture with a southeast-oriented Cottonwood Gulch tributary valley (an unimproved road is located in the through valley). The figure 8 map contour interval is 40 feet and the through valley floor elevation at the drainage divide is given as 5777 feet. The ridge immediately to the southwest rises to 6041 feet while to the northeast an elevation of 6147 feet is nearby (and elevations get higher further to the northeast). The through valley is at least 270 feet deep and may be even deeper. The through valley (or wind gap) is a water eroded feature and was eroded by southeast-oriented flood flow moving to what was then the actively eroding southwest-oriented Cottonwood Gulch valley. The Cottonwood Gulch valley was probably eroding headward from a south-oriented flood flow channel on the present day north-oriented Madison River alignment. Prior to headward erosion of the deep Cottonwood Creek valley the southeast-oriented flood flow was probably moving to a south-oriented flood flow channel on the Dry Creek-Gallatin River alignment. Headward erosion of the deep south-oriented Missouri River-Madison River flood flow channel beheaded and reversed the southeast-oriented flood flow channel moving across the present day Big Davis Gulch-Cottonwood Gulch drainage divide. The reversal of flood flow eroded the northwest-oriented Big Davis Gulch valley and its northwest-oriented tributary valley. Southeast-oriented flood waters from north of the actively eroding south-oriented Madison River-Missouri River flood flow channel valley head was captured by the reversed flow in the Big Davis Gulch valley and moved in a southwest direction to the Big Davis Gulch valley and eroded the southwest-oriented Big Davis Gulch valley segment. Many additional complexities have been omitted, but the processes involved are similar. Study of the figure 8 map region reveals evidence for additional food flow channels, each of which has a history that can be reconstructed, although to do so would require much more detail than is appropriate for this illustrative essay.
South Fork Sixteenmile Creek-Pass Creek drainage divide area
- Figure 9 illustrates the South Fork Sixteenmile Creek-Pass Creek drainage divide area in the northern Bridger Range and is located south and east of the figure 5 map area and includes overlap areas with figure 5. The Bridger Range extends in a south-southeast direction from the figure 9 center area to the figure 9 south edge. Blacktail Mountain is located near the figure 9 center. Menard is the small town located near the figure 9 southwest corner and Dry Creek flows in a south direction through Menard to the figure 9 south edge. Polson Hollow drains in an east and southeast direction from the figure 9 west center edge to join west, southwest, and south-southwest oriented Blacktail Creek (located north and west of Blacktail Mountain) to form south-oriented Dry Creek. The southwest, west, and southwest oriented stream flowing from Flathead Pass (the deep valley crossing the Bridger Range) to join Dry Creek south of the figure 9 map area is Pass Creek. North-oriented Brammer Creek (or Bremmer Creek) and its unnamed tributary can be seen flowing to the figure 9 north edge (near northwest corner) and north of figure 9 flow to west-oriented Sixteenmile Creek. The northwest-oriented stream originating in the figure 9 east center region (east of the high Bridger Range) is the South Fork Sixteenmile Creek and joins Sixteenmile Creek north of the figure 9 map area. A northeast-oriented South Fork Sixteenmile Creek tributary drains the east side of Flathead Pass. The southeast oriented North Fork and east-oriented South Fork of Flathead Creek can be seen near the figure 9 southeast corner with Flathead Creek flowing to the south-oriented Shields River, which then flows to the east-oriented Yellowstone River. Flathead Pass is a remarkable landform that deserves an explanation. The figure 9 map contour interval is 50 meters. The drainage divide elevation at Flathead Pass is somewhat difficult to determine, but appears to be between 2050 and 2100 meters. Bridger Range elevations to the north rise to 2655 meters at Blacktail Mountain while Bridger Range elevations to the south rise even higher. In other words Flathead Pass is 550 meter deep through valley eroded across the Bridge Range. The deep through valley was probably eroded by a southwest-oriented flood flow channel diverging from a southeast-oriented flood flow channel on the present day northwest-oriented South Fork Sixteenmile Creek alignment. The southeast-oriented flood flow may have been moving to the south-oriented Shields River valley (or flood flow channel) and then to the actively eroding Yellowstone River valley. The southwest-oriented diverging flood flow channel was moving flood water to the south-oriented flood flow channel on the Dry Creek-Gallatin River alignment. The fact the southwest-oriented flood flow channel was able to cross the Bridger Range means the Bridger Range was not an obstacle at that time and that Bridger Range uplift occurred as the flood waters flowed across it. Enough flood water was flowing across the Bridger Range to erode a 550 meter deep valley before headward erosion of the deeper Sixteenmile Creek valley (and/or continuing Bridger Range uplift) beheaded the southeast- and southwest-oriented flood flow movement and forced a flood flow reversal that eroded the north and northwest-oriented South Fork Sixteenmile Creek valley seen today.
Detailed map of South Fork Sixteenmile Creek-Pass Creek drainage divide area
- Figure 10 provides a detailed topographic map of the South Fork Sixteenmile Creek-Pass Creek drainage divide area seen in less detail in figure 9 above. Flathead Pass is located in the northeast quadrant of section 30 in the figure 10 southwest quadrant and is labeled. Southwest-oriented Pass Creek originates at Flathead Pass and flows to the figure 10 southwest corner, and south and west of figure 10 joins south-oriented Dry Creek. The South Fork Sixteenmile Creek originates just south of the figure 10 south edge (east half) and flows in a generally north direction to the figure 10 north edge (east half). Just east of the South Fork Sixteenmile Creek headwaters are headwaters of the east-oriented South Fork Flathead Creek, which can just barely be seen near the figure 10 southeast corner. While not included in this essay’s study region there is a remarkable through valley in the figure 10 southeast corner region linking the north and northwest oriented South Fork Sixteenmile Creek valley with the east-oriented Flathead Creek valley. This through valley provides evidence of a southeast-oriented flood flow channel that moved flood water to the south-oriented Shields Creek valley. The focus here is on the diverging flood flow channel that moved flood water in a southwest direction across the Bridger Range at Flathead Pass to the south-oriented Dry Creek valley. A north and northeast-oriented South Fork Sixteenmile Creek tributary flows through section 29 (just east of Flathead Pass) and drains the east slope of Flathead Pass. The figure 10 map contour interval is 40 feet and the Flathead Pass elevation at the drainage divide is shown as being 6922 feet. Elevations greater than 8400 feet can be seen along the Bridger Range crest south of Flathead Pass and there are mountain peaks higher than 9500 feet south of the figure 10 map area. Likewise elevations greater than 8400 feet can be seen in figure 10 along the Bridger Range crest north of Flathead Pass. These elevations suggest Flathead Pass is approximately 1500 feet deep. Flathead Pass is a water eroded feature and was eroded by southwest-oriented flood flow which first moved across the Bridger Range at a time when the Bridger Range was just beginning to emerge. The deep Flathead Pass through valley (or wind gap) is evidence the Bridger Range emerged at a time when massive south-oriented flood flow was moving across the figure 10 map region. Apparently southwest-oriented flood flow across the Bridger Range was ended when Bridger Range uplift proceeded faster than the flood flow could erode the Flathead Pass valley. Southeast-oriented flood flow then continued along the South Fork Sixteenmile Creek alignment until headward erosion of the deeper Sixteenmile Creek valley north of figure 10 beheaded and reversed that flow to create the South Fork Sixteenmile Creek valley.