A geomorphic history based on topographic map evidence

The Fish Creek-Big Coulee Creek drainage divide area discussed here is located in Montana’s Musselshell River drainage basin, USA. Although detailed topographic maps of the Fish Creek-Big Coulee Creek drainage divide area have been available for more than fifty years detailed map evidence has not previously been used to interpret the region’s geomorphic history. The interpretation provided here is based entirely on topographic map evidence. The Fish Creek-Big Coulee Creek drainage divide area is interpreted to have been eroded during immense southeast-oriented flood events, the first of which flowed on a topographic surface at least as high as the highest points in the present-day drainage divide area. Flood erosion across the drainage divide ended when headward erosion of the deep Musselshell River valley and tributary valleys captured all southeast-oriented flood flow.

Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore Fish Creek-Big Coulee Creek drainage divide area landform origins in Montana’s Musselshell River drainage basin. 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 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 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 similar essays is a thick North American ice sheet, comparable in thickness to the present day Antarctic ice sheet, occupied approximately the North American region usually recognized to have been glaciated and through its weight and erosive actions created a “deep” North American “hole”, 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 Fish Creek-Big Coulee Creek drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Fish Creek-Big Coulee Creek drainage divide area location map

Figure 1: Fish Creek-Big Coulee Creek 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 Fish Creek-Big Coulee Creek drainage divide area location map and illustrates a region in Montana. The Yellowstone River flows northeast from the figure 1 southwest corner area to Livingston and continues to Big Timber, Columbus, Laurel, Billings, and Pompeys Pillar near the figure 1 east edge. The Musselshell River is located north of the Yellowstone River and flows from Martinsdale, which is located south of the Little Belt Mountains, to Harlowton, Ryegate, Lavina, Roundup, and M(elstone), which is located near the figure 1 east edge. At Melstone the Musselshell River turns to flow north-northwest to the figure 1 north edge. Fish Creek is the unnamed east-northeast oriented Musselshell River tributary joining the Musselshell River at Ryegate. Big Coulee Creek is the unnamed northeast-oriented stream joining the Musselshell River at Lavina. Based on evidence from hundreds of Missouri River drainage basin landform origins research project essays published on this website landform evidence illustrated here is interpreted in the context of an immense southeast-oriented flood flowing across the figure 1 map area and which was systematically captured and diverted northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today. The Musselshell River valley was a deep valley that eroded southwest to capture southeast-oriented flood water and to divert flood flow to the northeast. Prior to Musselshell River valley headward erosion (although not much before) the deep Yellowstone River valley eroded southwest to capture southeast-oriented flood waters and to divert flood waters northeast. Flood waters on northwest ends of beheaded flood flow routes reversed flow direction to flow northwest to the newly eroded Yellowstone River valley. By doing so reversed flood flow eroded northwest-oriented tributary valleys. The deep Musselshell River valley then eroded south, southwest, and west-northwest to capture the southeast-oriented flood flow and to divert flood waters further to the northeast (to the Missouri River valley, which is located north of the figure 1 map area). Again flood waters on the northwest ends of beheaded flood flow routes reversed flow direction to flow northwest to the newly eroded Musselshell River valley and its northeast-oriented tributary valleys. Detailed maps below provide evidence supporting this interpretation. The Painted Robe Creek-Yellowstone River drainage divide area essay, the Musselshell River-Yellowstone River drainage divide area in Musselshell and Yellowstone Counties essay, and the Musselshell River-Yellowstone River drainage divide in Rosebud and Treasure Counties essay describe drainage divides located southeast of the drainage divide area discussed here and can be found under Musselshell River on the sidebar category list.

Fish Creek-Big Coulee Creek drainage divide area detailed location map

Figure 2: Fish Creek-Big Coulee Creek drainage divide area detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 2 illustrates a somewhat more detailed map of the Fish Creek-Big Coulee Creek drainage divide area discussed here. Sweet Grass, Wheatland, and Golden Valley Counties are located in Montana. The unnamed county in which Lake Basin is located is Stillwater County. The Musselshell River flows from Twodot, in the figure 2 northwest corner, to Harlowton along the figure 2 north edge and then to Ryegate and Lavina. Big Coulee Creek is the northeast-oriented stream joining the Musselshell River at Lavina. Fish Creek is the east-northeast oriented stream immediately north of Big Coulee Creek and joins the Musselshell River east of Ryegate. Figure 2 evidence is interpreted in the context of an immense southeast-oriented flood first captured by headward erosion of the northeast-oriented Big Coulee Creek valley and subsequently captured by headward erosion of the east-northeast oriented Fish Creek valley. Prior to headward erosion of the Musselshell River-Big Coulee Creek valley flood waters flowed southeast across the figure 2 region to what was then the newly eroded and deep northeast-oriented Yellowstone River valley. The Painted Robe Creek-Yellowstone River drainage divide area essay presented evidence that a major southeast-oriented flood flow route to the Yellowstone River valley was through the present day Lake Basin of northern Stillwater County. Headward erosion of the Big Coulee Creek valley  captured flood waters moving along that Lake Basin flood flow route and diverted the flood waters northeast to what was then the newly eroded Musselshell River valley. Subsequently headward erosion of the Musselshell River valley and the east-northeast oriented Fish Creek valley captured southeast-oriented flood flow to the newly eroded Big Coulee Creek valley. Detailed maps below start with the Fish Creek-Big Coulee Creek drainage divide area near Ryegate and then proceed southwest along the drainage divide. At the west end of the drainage divide northwest-southeast oriented through valleys linking the presented day east-northeast Fish Creek valley with the northeast-oriented Big Coulee Creek valley and also with headwaters of present day southeast-oriented drainage routes to the Lake Basin area are illustrated and discussed.

Fish Creek-Big Coulee Creek drainage divide area south of Ryegate

Figure 3: Fish Creek-Big Coulee Creek drainage divide area south of Ryegate. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the Fish Creek-Big Coulee Creek drainage divide area south of Ryegate. The east-south, northeast, and east oriented Musselshell River is located near the figure 3 north edge. Fish Creek flows northeast from the figure 3 west edge to join the Musselshell River east of Ryegate. Big Coulee Creek flows northeast from the figure 3 south edge to the figure 3 east edge and joins the Musselshell River east of the figure 3 map area. Immediately south of the Musselshell River between Fish Creek and Big Coulee Creek is what appears to be a northeast dipping hogback. The hogback presence suggests the present day topographic surface has been eroded across a significant geologic structure. Other figures in this essay provide further evidence supporting this interpretation. Immediately southwest of the hogback there are southeast-oriented Big Coulee Creek tributaries and northwest-oriented Fish Creek tributaries. This northwest-southeast orientation of tributaries to the northeast-oriented valleys provides evidence the region was eroded by southeast-oriented flood flow, which was captured first by headward erosion of the northeast-oriented Big Coulee Creek valley and subsequently by headward erosion of the northeast-oriented Fish Creek valley. The northwest-oriented tributary valleys were eroded by reversals of flood flow on the northwest ends of beheaded southeast-oriented flood flow routes. Prior to headward erosion of the Big Coulee Creek and Fish Creek valleys flood southeast-oriented waters were flowing on a topographic surface at least as high as the highest figure 3 elevations today. The Musselshell River then eroded a deep valley into the figure 3 map area to capture flood waters and to divert the flood waters northeast and north. The Big Coulee Creek valley eroded southwest from what was then the actively eroding Musselshell River valley head, which was located east of the figure 3 map area. When the actively eroding Musselshell River valley head reached the Ryegate area the northeast-oriented Fish Creek valley eroded southwest to capture southeast-oriented flood water moving to what was then the newly eroded Big Coulee Creek valley. Headward erosion of the deep Musselshell River valley and its tributary valleys subsequently beheaded all flood flow routes to what was then the newly eroded Fish Creek valley.

Fish Creek-Rock Creek-Big Coulee Creek drainage divide area

Figure 4: Fish Creek-Rock Creek-Big Coulee Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates Fish Creek-Rock Creek, Rock Creek-Big Coulee Creek, and Fish Creek-Big Coulee Creek drainage divide areas southwest of the figure 3 map area and includes overlap areas with figure 3. Northeast-oriented Big Coulee Creek is located in the figure 4 southeast corner area. Fish Creek flows east, northeast, from the figure 4 west edge to the figure 4 north center area and then flows east-northeast along the figure 4 north edge. Rock Creek originates in the figure 4 center area and flows northeast through the figure 4 northeast quadrant to the figure 4 north edge. The north-northeast oriented  “Creek” in the figure 4 west center area is Simmons Creek, which is illustrated and discussed in figures 6, 7 and 8 below. Note southeast-oriented Fish Creek and Big Coulee Creek tributaries and also northwest-oriented tributaries to all major northeast-oriented figure 4 streams. The northwest-southeast oriented drainage alignments of these tributary valleys is further evidence of the southeast-oriented flood flow captured by headward erosion of the northeast-oriented valleys. Flood waters initially flowed on a topographic surface at least as high the highest figure 4 elevations today. The valley eroded across O’Brien Hill in the figure 4 south center area provides evidence of flood flow moving on a topographic surface at least that high. Figure 5 below illustrates the O’Brien Hill area in more detail. The Big Coulee Creek valley eroded headward into the figure 4 map area first. Rock Creek headward erosion was second and beheaded and reversed southeast-oriented flood flow routes to the newly eroded Big Coulee Creek valley, which explains the northwest-oriented Rock Creek tributaries. Headward erosion of the Fish Creek valley followed and beheaded and reversed southeast-oriented flood flow routes to the newly eroded Rock Creek and Big Coulee Creek valleys.

Fish Creek-Big Coulee Creek drainage divide area at O’Brien Hill

Figure 5: Fish Creek-Big Coulee Creek drainage divide area at O’Brien Hill. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the Fish Creek-Big Coulee Creek drainage divide area at O’Brien Hill in more detail than figure 4 above. Northwest-oriented streams in the figure 5 west half flow to north-northeast oriented Simmons Creek, which then flows to northeast-oriented Fish Creek. East and southeast-oriented streams in the figure 5 southeast quadrant flow to northeast-oriented Big Coulee Creek. Northeast-oriented Rock Creek headwaters are located in the figure 5 northeast quadrant. Note how the through valley eroded across O’Brien Hill is linked to the head of a northwest-oriented Simmons Creek  tributary valley and also to the head of an east-oriented Big Coulee Creek valley. The through valley and those linkages provide evidence the through valley was eroded by southeast-oriented flood water flowing on a topographic surface at least as high as the through valley floor to what was then the newly eroded northeast-oriented Big Coulee Creek valley. At that time the northeast-oriented Fish Creek (and Simmons Creek) valley did not exist. Headward erosion of the northeast-oriented Rock Creek valley then reached the figure 5 map area, but did not interrupt southeast-oriented flood flow over the present day O’Brien Hill. Subsequently headward erosion of the northeast-oriented Fish Creek-Simmons Creek valley beheaded and reversed the southeast-oriented flood flow route moving water over the present day O’Brien Hill. Reversed flow on the present day northwest-oriented Simmons Creek tributary valley alignment then began to move northwest to the newly eroded and deeper northeast-oriented Simmons Creek-Fish Creek valley. Because flood waters were moving in anastomosing (or interconnected) channels and because the Fish Creek-Simmons Creek valley was eroded headward from the northeast to the southeast, the reversed flood flow was able to capture significant amounts of yet to be beheaded (by Simmons Creek valley headward erosion) southeast-oriented flood flow from flood flow routes further to the southwest. That captured flood water moved north in the valley now occupied by the small lake and helped erode the northwest-oriented Fish Creek tributary valley. Subsequently headward erosion of the Fish Creek-Simmons Creek valley beheaded and reversed the flood flow routes further to the southwest and the resulting reversed flood flow eroded northwest-oriented Simmons Creek tributary valleys seen in figure 4.

Fish Creek-Cherry Creek drainage divide area

Figure 6: Fish Creek-Cherry Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates the Fish Creek-Cherry Creek and the Mud Creek-Fish Creek drainage divides area west and north of the figure 4 map area and includes overlap areas with figure 4. Fish Creek flows east and southeast in the figure 6 northwest quadrant to the county line and the flows east to the town of Progress, where it turns to flow northeast to the figure 6 east edge. North of the east and southeast oriented Fish Creek valley segments (in the figure 6 northwest quadrant) is east-northeast oriented Mud Creek, which flows independently to join the Musselshell River northeast of the figure 6 map area. North and north-northeast oriented Simmons Creek joins Fish Creek east of the Chinamans Hat (located east of Progress) and is joined by northeast and east-oriented Cherry Creek in the figure 6 south center area (figure 7 below shows the  Simmons Creek headwaters). Ridges in the figure 6 map area, especially in the western half, are probably hogback ridges related to underlying geologic structures. However, valleys between those ridges were eroded by water and can best be explained in the context of southeast oriented anastomosing flood flow channels that were subsequently captured by headward erosion of the Fish Creek valley and its various tributary valleys. Numerous northwest-southeast and north-south-south oriented through valleys link the present day northeast-oriented drainage routes. Prior to erosion of those through valleys southeast oriented flood waters moved across the figure 6 map area on a topographic surface at least as high as the highest figure 6 elevations today. Southeast-oriented flood flow in the large north-south figure 6 central through valley was first captured by headward erosion of he Simmons Creek-Cherry Creek valley and diverted to flow north-northeast to what was then the deep and actively eroding Fish Creek valley head. The north-oriented Simmons Creek valley segment (south of Cherry Creek) was formed by reversed flow on the north end of a beheaded south flood flow channel. Headward erosion of the Fish Creek valley next captured south-oriented flood flow in that figure 6 central north-south oriented through valley and headward erosion of the Mud Creek valley subsequently beheaded south-oriented flood flow to the newly eroded Fish Creek valley. An interesting question is whether or not these captures of southeast- and south-oriented were aided by regional uplift that was occurring at the same time? Initial flood flow movements are inconsistent with present day topography, and while deep flood water erosion accounts for many of the topography changes, some topography changes are difficult to explain unless regional and local uplifts were occurring as the immense southeast-oriented flood progressed. Such regional and local uplifts during an immense southeast-oriented flood could occur if flood waters were derived from rapid melt of a thick North American ice sheet. Ice sheet weight along with crustal unloading caused by deep flood erosion could trigger regional and local uplifts at locations where deep flood erosion was taking place.

Simmons Creek-North Fork Big Coulee Creek drainage divide area

Figure 7: Simmons Creek-North Fork Big Coulee Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the Simmons Creek-North Fork Big Coulee Creek drainage divide south of the figure 6 map area and includes overlap areas with figure 6. Simmons Creek flows northeast from the figure 7 southwest corner area to the figure 7 center south area and flows north and north-northeast to the figure 7 north edge (and joins Fish Creek north of the figure 7 map area). Cherry Creek flows northwest from the figure 7 center west area to join north-oriented Simmons Creek in the figure 7 center north area. South of Simmons Creek the North Fork Big Coulee Creek flows northeast across the figure 7 south center area to the figure 7 east edge. The northeast-oriented South Fork Big Coulee Creek flows across the figure 7 southeast corner and is located southeast of the northeast-oriented North Fork Big Coulee Creek. Note the large northwest-southeast oriented through valley linking the present day north and northeast oriented Simmons Creek valley, the northeast-oriented North Fork Big Coulee Creek valley, and the northeast-oriented South Fork Big Coulee Creek valley. While probably related to the underlying geologic structure the through valley is a water eroded feature, eroded by southeast-oriented flood flow moving to the Lake Basin area illustrated and discussed in the Painted Robe Creek-Yellowstone River drainage divide area essay. Based on evidence presented in that essay large volumes of southeast-oriented flood water flowed across he figure 7 map area and helped erode the large present day figure 7 through valley. Headward erosion of the northeast-oriented South Fork Big Coulee Creek valley captured the southeast-oriented flood flow first, although headward erosion of the northeast-oriented North Fork Big Coulee Creek valley probably occurred very soon thereafter. Next headward erosion of the Fish Creek-Simmons Creek valley captured southeast-oriented flood flow to the newly eroded Big Coulee Creek valley and reversed flood flow on the north end of the beheaded flood flow channel to create the north-oriented Simmons Creek valley segment. Figure 6 evidence illustrates how headward erosion of the Fish Creek valley and Mud Creek valley subsequently captured the south-oriented flood flow and diverted the flood waters northeast.

Detailed map of Simmons Creek-North Fork Big Coulee Creek drainage divide area

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

Figure 8 illustrates a detailed map of the Simmons Creek-North Fork Big Coulee Coulee Creek drainage divide area seen in less detail in figure 7 above. The North Fork Big Coulee Creek flows east across the figure 8 south center area and then turns north and northeast to flow to the figure 8 east edge. Simmons Creek flows northeast and north from the figure 8 west center edge to the figure 8 north edge (west half). As previously described the large northwest-southeast oriented through valley linking the present day Simmons Creek and North Fork Big Coulee Creek valleys was eroded by southeast-oriented flood flow moving across the figure 8 map area to Lake Basin located southeast of the figure 8 map area and then flowing to what was then the newly eroded and deep northeast-oriented Yellowstone River valley. The southeast-oriented flood water in the figure 8 map area was captured by headward erosion of the Musselshell River-Big Coulee Creek valley and was subsequently captured by headward erosion of the Musselshell River-Fish Creek-Simmons Creek valley. Figure 8 evidence suggests the time difference between the North Fork Big Coulee Creek capture and the Simmons Creek capture was short. In other words, erosion of the figure 8 map area occurred rapidly. The source of the southeast-oriented flood waters cannot be determined from evidence presented in this essay. However, essays in this Missouri River drainage basin landform origins research project series when taken as a group can be used to trace flood waters both up flood to source areas and down flood to see where flood waters were going. A logical flood water source would be rapid melting of a thick North American ice sheet located in a deep “hole” occupying approximately the North American location usually recognized to have been glaciated. The deep “hole” would have been created by deep glacial erosion and by crustal warping caused by the ice sheet weight. Such a flood water source would not only explain the immense southeast-oriented floods this essay series describes, but would also explain why deep valleys were eroding headward to capture the southeast-oriented flood waters and diverting the flood waters further and further to the northeast and north into space in the deep “hole” the rapidly melting thick ice sheet had once occupied. In addition, such a flood water source may explain uplift of the mountains regions during an immense southeast-oriented flood. A thick North American ice sheet in deep “hole” created in part due to the ice sheet’s weight would probably create crustal warping elsewhere on the continent, especially along ice sheet margins. Further, rapid erosion of overlying material might trigger localized uplifts and uplifts of geologic structures in areas such as the Fish Creek-Big Coulee Creek drainage divide area.

South Fork Big Coulee Creek-Sixshooter Creek drainage divide area

Figure 9 illustrates the South Fork Big Coulee Creek-Sixshooter Creek drainage divide south and east of the figure 7 map area. The South Fork Big Coulee Creek flows northeast from the figure 9 west center edge area to the figure 9 north center edge. Sixshooter Creek originates in the figure 9 west center area and flows east and southeast to the figure 9 southeast corner area. Note how topography in the figure 9 southeast quadrant has been eroded into what appears to be a relatively flat plain. That area is Lake Basin, which is better illustrated and described in the Painted Robe Creek-Yellowstone River drainage divide essay. That essay describes erosion of the Lake Basin surface by an immense southeast-oriented flood. Note how the northeast-oriented South Fork Big Coulee Creek is linked by a through valley with headwaters of east and southeast-oriented Sixshooter Creek. While that through valley is evidence water once flowed south from the present day South Fork Big Coulee Creek valley and the Sixshooter Creek valley it may also have been used by flood flow routes moving into the Lake Basin northwest end and then north to the newly eroded northeast-oriented South Fork Big Coulee Creek valley. For example, note how Whitney Creek flows northeast, north, and northeast to join Sixshooter Creek in northwest Sweet Grass County. Headward erosion of the Whitney Creek valley could have captured southeast-oriented flood flow southwest of the actively eroding South Fork Big Coulee Creek valley head and diverted water north to the newly eroded South Fork Big Coulee Creek valley. Subsequently headward erosion of the South Fork Big Coulee Creek valley could have beheaded southeast-oriented flood flow routes to the Whitney Creek valley. Through valleys located on the much higher level South Fork Big Coulee Creek-Sixshooter Creek drainage divide located just east of the deep through valley provide evidence most southeast-oriented flood flow to the Lake Basin area moved across a much higher topographic surface, probably equivalent to or higher than the elevation of the present day South Fork Big Coulee Creek-Sixshooter Creek drainage divide. Figure 10 below provides a detailed look at some higher level through valleys.

Detailed map of South Fork Big Coulee Creek-Sixshooter Creek through valley

Figure 10 illustrates through valleys crossing the South Fork Big Coulee Creek-Sixshooter Creek drainage divide shown in less detail in figure 9 above. The South Fork Big Coulee Creek flows northeast from the figure 10 west center edge area to the figure 10 north center edge area. Sixshooter Creek headwaters are located in the figure 10 map south half and northeast-oriented Whitney Creek joins Sixshooter Creek in the figure 10 south center area. Note multiple through valleys crossing the present day South Fork Big Coulee Creek-Sixshooter Creek drainage divide. These higher level through valleys suggest they were eroded by large volumes of southeast-oriented flood flow that was captured by headward erosion of the deep Sixshooter Creek valley and Whitney Creek valley and subsequently captured by headward erosion of the deep northeast-oriented South Fork Big Coulee Creek valley. Elevations along the present day South Fork Big Coulee Creek-Sixshooter Creek drainage divide suggest flood waters were flowing on a topographic surface that may have been 400 feet higher (or more) than floors of the present day valleys. In other words the deep Sixshooter Creek valley when it eroded headward into the region was eroding a 400 feet deep or deeper valley and the deep northeast-oriented South Fork Big Coulee Creek valley was of comparable depth as it eroded headward into the region. Between the time the Sixshooter Creek valley eroded into the region and the time the South Fork Big Coulee Creek valley eroded into the region flood waters lowered the present day drainage divide. As mentioned in the figure 9 discussion it is possible there was reversed flood flow movement, especially through the deep valley along the figure 10 west edge. However, the vast majority of the drainage divide erosion was done by southeast-oriented flood waters moving to what was then the newly eroded Yellowstone River valley.

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.