Ponca Creek-Keya Paha River drainage divide area landform origins, South Dakota and Nebraska, USA

Authors

A geomorphic history based on topographic evidence

Abstract:

The Ponca Creek-Keya Paha (and Niobrara) River drainage divide area discussed here is located along the South Dakota-Nebraska border, USA. Although detailed topographic maps of the Ponca Creek-Keya Paha (and Niobrara) River 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 Ponca Creek-Keya Paha (and Niobrara) River 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 ended when headward erosion of the deep southeast-oriented Missouri River valley and deep east-oriented White River valley captured all southeast-oriented flood flow.

Preface:

The following interpretation of detailed topographic map evidence is provided as evidence in the Missouri River drainage basin landform origins research project, which is compiling similar evidence for all major drainage divides contained within the Missouri River drainage basin and for all major drainage divides with and within certain adjacent drainage basins. The research project is interpreting evidence in the context of a previously unexplored geomorphology paradigm, which is briefly described in the introduction below. 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 South Dakota and Nebraska Ponca Creek-Keya Paha (and Niobrara) River drainage divide area landform origins. 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.
  • 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 South Dakota and Nebraska Ponca Creek-Keya Paha (and Niobrara) River drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Ponca Creek-Keya Paha (and Niobrara) River drainage divide location map

Figure 1: Ponca Creek-Keya Paha (and Niobrara) River drainage basin general 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 Ponca Creek-Keya Paha (and Niobrara) River drainage divide area. Nebraska is the state south of the west-to-east yellow boundary line and South Dakota is north of the west-to-east red boundary line. The east-oriented Niobrara River flows from the figure 1 west edge through northern Nebraska to reach the Missouri River near Niobrara, Nebraska, located in the figure 1 southeast corner. The Keya Paha River is a southeast oriented Niobrara River tributary, which begins near Hidden Timber, South Dakota and then flows southeast across the state line near Wewela, South Dakota to reach the Niobrara River. Ponca Creek is the unnamed Missouri River tributary in figure 1 immediately north of  southeast oriented Keya Paha River, which flows southeast from near Colome, South Dakota parallel to US Highway 18 and state highway 12 close to the towns of Dallas, Gregory, Burke, and Herrick, South Dakota and Butte, Spencer, Lynch and Verdel, Nebraska. Ponca Creek joins the Missouri River near Verdel. The unnamed stream flowing from northeast of Dallas (near Lucas) South Dakota to the Missouri River is Whetstone Creek and will be discussed here as will be north-oriented Bull Creek and unnamed north-oriented Dog Ear Creek, both of which flow north from the Winner, South Dakota area to the east-oriented White River. Landform evidence illustrated here will be interpreted in the context of an immense southeast-oriented flood flowing across the entire figure 1 map area and which was systematically captured and diverted east and perhaps even northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today. In the figure 1 map region headward erosion of the Niobrara River valley first captured the southeast-oriented flood flow and then the southeast-oriented Keya Paha River valley eroded headward along a southeast-oriented flood flow route as a tributary to the actively eroding Niobrara River valley. At approximately the same time the southeast-oriented Ponca Creek valley eroded headward along a parallel flood flow route, suggesting the presence of a large southeast-oriented anastomosing channel complex. Headward erosion of the much deeper southeast- and east-oriented Missouri River valley next captured flood flow moving in the newly eroded Niobrara River and Ponca Creek valleys and then eroded northwest, where a deep east-oriented White River valley eroded west and captured all southeast-oriented flood flow moving to what were then the actively eroding Keya Paha River and Ponca Creek valleys. Detailed maps will provide evidence supporting this interpretation.

Ponca Creek-Keya Paha (and Niobrara) River drainage divide area detailed location map

Figure 2: Ponca Creek-Keya Paha (and Niobrara) River drainage divide area detailed location map. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 2 provides a detailed location map for the Ponca Creek-Keya Paha (and Niobrara) River drainage divide area. Keya Paha and Boyd Counties are located in Nebraska and Gregory and Charles Mix Counties are located in South Dakota. Ponca Creek is named in figure 2 and flows from the Colome, South Dakota area (figure 2 northwest corner) in a southeast direction to join the southeast-oriented Missouri River near Verdel, Nebraska in the figure 2 southeast quadrant. The Keya Paha River flows southeast from just north of the state line (figure 2 west center) to join the Niobrara River in Nebraska (just south of the “B” in Boyd County). Note how in Boyd County, South Dakota the Niobrara River, Ponca Creek and the Missouri River area all flowing in parallel southeast-oriented valleys and how there are other southeast-oriented streams located throughout the figure 2 region south and west of the Missouri River. These parallel southeast-oriented valleys were probably initiated as interconnected channels in a large southeast-oriented anastomosing channel complex. Detailed map evidence presented below will demonstrate connections between the some southeast-oriented valleys to further support this interpretation. This essay begins by providing detailed map evidence of the Ponca Creek headwaters area and then proceed southeast along the Ponca Creek-Keya Paha (and Niobrara) River drainage divide. In addition to looking at the Ponca Creek-Keya Paha (and Niobrara) River drainage divide the detailed maps also illustrate Ponca Creek-Missouri River drainage divide evidence from South Fork Whetstone Creek (an east-oriented Missouri River tributary located in Gregory County, South Dakota east of the town of Gregory) southeast to Verdel, Nebraska, where Ponca Creek joins the Missouri River and also illustrate how headward erosion of north-oriented White River tributary valleys beheaded southeast-oriented flood flow that had been actively eroding the Ponca Creek valley. The White River-Keya Paha River drainage divide area essay addresses landform origins west of Ponca Creek and can be found under White River or Niobrara River on the sidebar category list.

Keya Paha River-Ponca Creek-White River drainage divides at Ponca Creek drainage basin west end

Figure 3: Keya Paha River-Ponca Creek-White River drainage divides at Ponca Creek drainage basin west end. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the Ponca Creek headwaters area near Colome, South Dakota. Colome is located in the figure 3 center. Winner is northeast of Colome along the figure 3 north edge and Dallas, South Dakota is the town east of Colome. Ponca Creek is the east-southeast oriented stream located south of the highway between Colome and Dallas. A southeast- and then northeast-oriented tributary flows to Ponca Creek just northeast of the word “Pleasant View” (located south of Colome). The southeast-oriented headwaters suggest the northeast-oriented tributary captured what was once a southeast-oriented flood flow route, probably another channel in what had been a southeast-oriented anastomosing channel complex. However, perhaps even more impressive is the large northeast-oriented through valley in which that northeast-oriented tributary flows. Further southwest along that through valley is a southwest-oriented stream flowing to Rahn Dam (located along the figure 3 south edge center) and then continuing south of figure 3 to join the southeast-oriented Keya Paha River as a barbed tributary. That northeast-oriented through valley is evidence that at one time flood flow moving southeast along what is now the Keya Paha River valley split southwest of the Rahn Dam area (south of figure 3) with some water continuing southeast along the present day Keya Paha River valley and a significant amount of water flowing northeast to what is now the east-southeast oriented Ponca Creek valley located southeast of Colome. In other words, the northeast-oriented through valley is an anastomosing channel complex relic from prior to beheading of flood flow routes to the Keya Paha River and Ponca Creek drainage basin region. Northeast of Colome are headwaters of north-oriented Bull Creek, which flows to the east-oriented White River. Immediately northwest of Colome and flowing parallel to the highway between Colome and Winner are northwest-oriented headwaters of north-oriented Thunder Creek, also flowing to the White River. When the Thunder Creek valley eroded south to capture southeast-oriented flood flow it beheaded flow routes to the actively eroding Ponca Creek valley and flood waters on the northwest ends of the beheaded flow routes reversed flow direction to create northwest-oriented drainage routes.  Further west, along the figure 3 west edge is north-oriented Dog Ear Creek. Headward erosion of the Dog Ear Creek valley, which occurred after headward erosion of the Bull Creek and Thunder Creek valleys) beheaded all southeast-oriented flood flow to the Thunder Creek valley and the Ponca Creek valley area.

White River-Missouri River-Ponca Creek drainage divide in Gregory, South Dakota area

Figure 4: White River-Missouri River-Ponca Creek drainage divide in Gregory, South Dakota area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the region east of the figure 3 map area and provides overlap with figure 3. Dallas, South Dakota is located by the highway along the figure 4 west edge. Ponca Creek flows southeast across the figure 4 southwest corner. Southeast-oriented South Fork Whetstone Creek is located north of Burke, South Dakota and southeast-oriented North Fork Whetstone Creek is located in the figure 4 to the north of Lucas in the figure 4 northeast quadrant. Whetstone Creek flows to the southeast-oriented Missouri River (see figure 2). Northeast oriented Landing Creek flows to the figure 4 north edge center and continues north to the Missouri River. Note northwest-oriented tributaries to northeast-oriented Landing Creek. These tributaries were eroded by reversals of flood flow on northwest ends of beheaded southeast-oriented flood flow routes that had been eroding the North Fork Whetstone Creek valley. In the figure 4 northwest quadrant area are northwest-oriented tributaries to north oriented Bull Creek, which flows to the east-oriented White River. These northwest-oriented tributaries were initiated by reversals of flood flow on the northwest ends of beheaded flood flow routes that had been eroding the deep South Fork Whetstone Creek valley. Figure 4 evidence demonstrates at least three major southeast-oriented flood flow channels once moved across the figure 4 map area. The northernmost of the three channels was eroding the deep North Whetstone Creek valley and drainage basin northwest from the what must have been a newly eroded and deep Missouri River valley and it was beheaded by headward erosion of the north and northeast-oriented Landing Creek valley. The middle flood flow route was eroding the deep South Fork Whetstone Creek valley, also from the deep Missouri River valley and was beheaded by headward erosion of the deep east-oriented White River valley and its north-oriented tributary Bull Creek valley. The third and southernmost of the three southeast-oriented flood flow routes was the using what is today the Ponca Creek valley and it was beheaded further to the west and northwest (as illustrated in figure 3). These three major southeast-oriented flood flow routes flowing across the figure 4 map area provide further evidence of a large-scale southeast-oriented anastomosing channel complex.

Ponca Creek-Keya Paha River drainage divide area southwest of Burke, South Dakota

Figure 5: Ponca Creek-Keya Paha River drainage divide area southwest of Burke, South Dakota area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 5 illustrates the Ponca Creek-Keya Paha River drainage divide area southwest of Burke, South Dakota and is located south of the figure 4 map area (and includes overlap areas with figure 4). Burke is the town located next to the highway along the figure 5 north edge. Ponca Creek flows southeast from the figure 5 north edge to the figure 5 east edge. Masdon Creek is the north-northeast and northeast-oriented Ponca Creek tributary flowing through Dickens Township. Murphy Creek is the northeast-oriented Ponca Creek tributary near Carlock, South Dakota, which flows to Ponca Creek south of Burke. South-oriented drainage along the figure 5 south edge flows to the southeast-oriented Keya Paha River, which is located south of the figure 5 map area. These south-oriented valleys eroded north from what must have been an actively eroding Keya Paha River valley to capture southeast-oriented flood flow routes beheaded by headward erosion of north- and northeast-oriented Ponca Creek tributaries. Note how northeast-oriented Masdon Creek and Murphy Creek have southeast-oriented tributaries, suggesting both valleys eroded southwest from what must have been an actively eroding southeast-oriented Ponca Creek valley to capture adjacent southeast-oriented flood flow routes. Also note how headward erosion of north-northeast and northeast oriented Masdon Creek valley beheaded southeast-oriented flood flow to some of the south-oriented Keya Paha River tributaries. This evidence strongly suggests southeast-oriented flood flow was moving across what is today the Ponca Creek-Keya Paha drainage divide at the same time the deeper Keya Paha River and Ponca Creek valleys were being eroded and is further evidence supporting the anastomosing channel complex interpretation.

Ponca Creek-Keya Paha River drainage divide north of Brocksburg, Nebraska

Figure 6: Ponca Creek-Keya Paha River drainage divide area north of Brocksburg, Nebraska. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 6 illustrates the region south of the figure 5 map area and includes significant overlap area. The Keya Paha River flows southeast near Brocksburg, Nebraska and then to the figure 6 southeast corner. Ponca Creek flows southeast across the figure 6 northeast corner. Note in the figure 6 southeast corner the Keya Paha River is flowing in an easterly direction and the east-oriented valley segment has southeast-oriented tributaries. Elevation of the erosion surface, which today represents the Ponca Creek-Keya Paha River drainage divide is more than 100 meters higher than adjacent areas on the Keya Paha River valley floor and 50 to 60 or more meters higher than adjacent areas on the Ponca Creek valley floor. These elevations provide minimum depths for the southeast-oriented valleys eroded headward as southeast-oriented flood water eroded the parallel Keya Paha River and Ponca Creek valleys headward along and across multiple southeast-oriented flood flow routes. The deeper Keya Paha River valley eroded headward from the Niobrara River valley, which eroded headward (west) somewhat in advance of the Keya Paha River valley. The combined flow of the two different valleys. The Niobrara River valley was eroded west in advance of the Keya Paha River valley to capture southeast-oriented flood flow routes yet to be captured by Keya Paha River valley headward erosion. Likewise, the Keya Paha River valley eroded northwest slightly in advance of the Ponca Creek valley headward erosion, otherwise Ponca Creek valley headward erosion would have captured southeast-oriented flood flow to the Keya Paha River valley, which figure 6 evidence indicates did not immediately happen (but it did happen before south-oriented Keya Paha River tributaries could erode their valley too far north into the present day Ponca Creek-Keya Paha River drainage divide area. Ponca Creek valley headward erosion was slightly in advance of headward erosion of the much deeper southeast-oriented Missouri River valley (not yet seen in these detailed maps), but located north and east of the southeast-oriented Ponca Creek valley.

Missouri-River-Ponca Creek-Keya Paha River drainage divide area southwest of Bonesteel, South Dakota

Figure 7: Missouri River-Ponca Creek-Keya Paha River drainage divide area southwest of Bonesteel, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the region east of the figure 6 map area and there is overlap.  A northeast-southeast Keya Paha River valley jog is located along the figure 7 south edge (the Keya Paha River joins the southeast oriented Niobrara River a short distance southeast of the figure 7 map area). Ponca Creek flows from the figure 7 northwest corner to the figure 7 southeast quadrant. North-oriented drainage north of the highway along the figure 7 north edge flows to the southeast oriented Missouri River (to be seen in figures 8, 9, and 10). The northeast-oriented tributary valleys eroded southwest from what was then an actively eroding deep Missouri River valley to capture southeast oriented flood flow moving on what is today the Missouri River-Ponca Creek drainage divide. Note how the northeast-oriented Missouri River east of St. Charles beheaded southeast-oriented flood flow eroding the south-southeast oriented Ponca Creek tributary valley that originates near Bonesteel Lake, how the northeast-oriented Missouri River tributary northeast of Bonesteel has southeast-oriented headwaters, and how a northeast-oriented Missouri River tributary northeast of Fairfax (figure 7 northeast quadrant) has northwest-oriented headwaters. The southeast-oriented headwaters are evidence the northeast-oriented tributary valley eroded southwest to capture yet to be beheaded (by Missouri River valley headward erosion) southeast-oriented flood flow routes and the northwest-oriented headwaters originated as a reversal of flood flow on the northwest end of a beheaded southeast-oriented flood flow route. Also note how Keya Paha River tributaries are southeast-oriented suggesting their valleys were eroded headward along southeast-oriented flood flow routes from the north wall of what was then an actively eroding Keya Paha River valley. Further, note how headward erosion of northeast- and east-oriented Squaw Creek (south of Milks Camp and northwest of Naper) would have beheaded southeast-oriented flood flow to some of the southeast-oriented Keya Paha River tributary valleys and today northwest-oriented headwaters are evidence the Ponca Creek valley did indeed behead and reverse flow on the northwest ends of beheaded flood flow routes. Finally note the southeast- and north-oriented Ponca Creek tributary beginning just east of Naper, South Dakota. The southeast-oriented headwaters are evidence the north-oriented tributary valley eroded south from what was then the actively eroding Ponca Creek valley to capture southeast-oriented flood flow moving along what is today the Ponca Creek-Keya Paha River drainage divide.

Missouri River-Ponca Creek-Niobrara River drainage divide areas east of Fairfax, South Dakota area

Figure 8: Missouri River-Ponca Creek -Niobrara River drainage divide areas east of Fairfax, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the Missouri River-Ponca Creek-Niobrara River drainage divide region east of the figure 7 map area and includes overlap areas. The southeast-oriented Missouri River is located in the figure 8 northeast corner. Southeast-oriented Ponca Creek flows between Anoka and Butte, Nebraska and the southeast-oriented Niobrara River is located in the figure 8 southwest corner. Note how most Ponca Creek tributaries are either southeast-oriented or have southeast-oriented valley segments or tributaries. Also note how northeast-oriented Missouri River tributaries have southeast-oriented tributaries and/or northwest-oriented tributaries providing evidence the northeast-oriented tributary valleys were eroding southwest to capture southeast-oriented flood flow moving on what is today the Missouri River-Ponca Creek drainage divide. What is remarkable in the figure 8, 9 and 10 map areas is how three major southeast-oriented valleys are located almost adjacent to each other. As demonstrated in figure 3 and in other figures these adjacent valleys originated as channels in what must have been an immense southeast-oriented anastomosing channel complex. Probably when the channels were originated the present day Missouri River valley did not exist and the anastomosing channel complex was moving flood water further east and maybe even northeast, although evidence in this essay is not adequate to say where. Subsequently, as the deep Missouri River valley eroded west and northwest it significantly lowered base level and enabled deeper valleys to erode headward along flood flow routes carrying the greatest amounts of flood flow. All three valleys eroded headward at about the same time, although the Niobrara River valley was slightly in advance of the Ponca Creek valley and the Ponca Creek valley was slightly in advance of the Missouri River valley.

Missouri River-Ponca Creek-Niobrara River drainage basin in the Spencer, Nebraska area

Figure 9: Missouri River-Ponca Creek-Niobrara River drainage divide in the Spencer, Nebraska area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 illustrates the Missouri River-Ponca Creek-Niobrara River drainage divide areas east of Spencer, Nebraska and is located southeast of the figure 8 map area and includes an overlap area. The southeast-oriented Missouri River is located in the figure 9 northeast quadrant. Southeast-oriented Niobrara River flows across the figure 9 southwest corner. Ponca Creek flows southeast from the figure 9 west edge past Spencer, Bristow, and Lynch, Nebraska to the figure 9 east edge. Whisky Creek is the major southeast-oriented Ponca Creek tributary, which joins Ponca Creek near Lynch. North-oriented Missouri River tributary valleys (seen in figure 8) eroded south to capture southeast-oriented flood flow that had been eroding the southeast-oriented Whisky Creek valley northwest. Note how headward erosion of the Missouri River valley and/or of northeast-oriented Missouri River tributary valleys beheaded southeast-oriented flood flow routes that had been eroding south and southeast-oriented Ponca Creek tributary valleys east of the Whisky Creek valley. Further, note how many Ponca Creek tributaries originate almost on the Niobrara River valley rim and some have northwest or southeast-oriented headwaters. For example, just north of Spencer Dam (where the highway crosses the Niobrara River) is an unnamed Ponca Creek tributary with northwest-oriented headwaters. As another example is the Crooked Creek tributary south of Spencer, which flows southeast and then northeast to flow to east-oriented Crooked Creek and then to Ponca Creek. The southeast-oriented headwaters are evidence the Crooked Creek tributary valley eroded southwest from what must have been an actively eroding Ponca Creek valley to capture southeast-oriented flood flow almost on the rim of the present day Niobrara River valley. Further, south of Bristow is another unnamed Ponca Creek tributary with northwest-oriented tributary valleys. This evidence indicates at the time the Ponca Creek valley was being actively eroded southeast-oriented flood water was still moving on what is today the Ponca Creek-Niobrara River drainage divide.

Missouri River-Ponca Creek-Niobrara River drainage divide in the Verdel, Nebraska area

Figure 10: Missouri River-Ponca Creek-Niobrara River drainage divide in the Verdel, Nebraska area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 illustrates the Missouri River-Ponca Creek drainage divide east end and also the Ponca Creek-Niobrara River drainage divide east end and is located east of the figure 9 map area. The Missouri River flows from the figure 10 north center edge to the figure 10 west center edge, just north of Niobrara, Nebraska. East of the figure 10 map area the Missouri River turns northeast before turning east and southeast again. Ponca Creek flows east from the figure 10 west edge past Verdel, Nebraska to join the southeast oriented Missouri River. To the south the Niobrara River flows in a southeast direction to the figure 10 south edge and then southeast of figure 10 map area turns north to join the Missouri River in the figure 10 southeast corner. Ponca Creek tributaries from the north are generally southeast oriented suggesting they have been beheaded by headward erosion of the deep Missouri River valley. This evidence suggests the Ponca Creek valley may have been initiated before the present day Missouri River valley existed. Across the Missouri River from the Ponca Creek mouth is southwest-oriented Coffee Creek (unnamed in figure 10), which flows in a large northeast-southwest oriented valley eroded between higher areas on both sides. Other essays better address landform evidence north and east of the Missouri River, however what is today a southwest-oriented valley may have originated as a northeast-oriented valley, which eroded southwest to capture southeast oriented flood flow and may have captured southeast-oriented flood flow moving in the southeast- and east-oriented Ponca Creek valley shortly before deep Missouri River valley headward erosion. If so, southeast-oriented flow on the Missouri River valley alignment upstream from the Ponca Creek would also have been captured and flood water from all sources was being diverted somewhere to the northeast of the figure 10 map area.

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.

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