Abstract:

Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore western South Dakota Boxelder Creek-Rapid Creek 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 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 Boxelder Creek-Rapid Creek drainage divide area landform evidence will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Boxelder Creek-Rapid Creek drainage divide area general location map

Figure 1: Boxelder Creek-Rapid Creek drainage divide area 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 general location map for the Boxelder Creek-Rapid Creek drainage divide area. The map illustrates an area in western South Dakota. Immediately west of the figure 1 map area is the state of Wyoming. Boxelder Creek and Rapid Creek both originate in the Black Hills west of Rapid City, South Dakota and flow southeast to join the northeast-oriented Cheyenne River just west of the Badlands National Park. Boxelder Creek is labeled on figure 1. Rapid Creek is not labeled, but it is the unlabeled southeast-oriented stream immediately south of Boxelder Creek and flows through Rochford, Silver City, Rapid City, and Farmingdale, South Dakota. West and north of Rapid Creek and Boxelder Creek headwaters are north-oriented Spearfish Creek (which flows along the figure 1 west edge through Spearfish to join the Belle Fourche River near Belle Fourche in the figure 1 northwest corner), northeast-oriented Whitewood Creek (which flows northeast through Deadwood and Whitewood to join the Belle Fourche River), and northeast-oriented Bear Butte Creek (which flows northeast through Sturgis to join the Belle Fourche River). Boxelder Creek and Rapid Creek are two of several streams shown on figure 1 flowing southeast from the Black Hills to join the northeast-oriented Cheyenne River east of the Black Hills. North of Boxelder Creek is Elk Creek, which is labeled in figure 1. Other Black Hills region drainage divide essays can be found under Black Hills on the sidebar category list.

• This essay interprets Boxelder Creek-Rapid Creek drainage divide area landform origins in the context of an immense southeast oriented flood that flowed across the entire figure 1 map area and that was systematically captured by headward erosion of deep northeast-oriented valleys, which diverted flood waters further and further northeast and north. The source of the southeast-oriented flood water cannot be determined from evidence presented here. However, by using the numerous Missouri River drainage basin landform origins research project essays published on this website it can be shown the floods occurred before and/or while the Black Hills area was being uplifted and the flood waters can be traced headward to a North American ice sheet location. Rapid melting of a thick North American ice sheet located in a deep “hole” would explain the flood water source and also why deep valleys eroded west and southwest to capture southeast-oriented flood waters to divert flood waters further and further northeast and north into space in the deep “hole” the rapidly melting ice sheet had once occupied. In addition, presence of a thick North American ice sheet in a deep “hole” north and east of the Black Hills might explain crustal warping that uplifted the Black Hills dome during or even after an immense southeast-oriented flood. Uplift of the Black Hills dome may have been accelerated by crustal unloading as flood waters deeply eroded the Black Hills region and removed overlying sedimentary layers.

Boxelder Creek-Rapid Creek drainage divide area detailed location map

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

Figure 2 provides a slightly more detailed location map for the Boxelder Creek-Rapid Creek drainage divide area. Lawrence County, Custer County, and Pennington County are all located in western South Dakota. Green areas represent Black Hills National Forest lands, which are generally located in the Black Hills upland region. Southeast-oriented Boxelder Creek originates near Custer Peak located in southern Lawrence County north of the “H” in the words “Black Hills” located just north of the Lawrence County south border. Southeast-oriented North Fork Rapid Creek is located just west of the Boxelder Creek headwaters. Figure 2 does not provide enough detail to make observations about Boxelder Creek-Rapid Creek drainage divide landform origins in the Black Hills area and such observations need to be made from more detailed maps below. However, east of Rapid City figure 2 shows southeast oriented Rapid Creek tributaries flowing from the Boxelder Creek-Rapid Creek drainage divide. Further, figure 2 shows southeast oriented tributaries to the northeast oriented Cheyenne River and the Belle Fourche River along the figure 2 north edge. Also figure 2 shows numerous northwest-oriented tributaries to the northeast oriented Cheyenne River. This predominance of southeast and northwest-oriented tributaries to major streams on the plains east of the Black Hills is evidence major east, northeast and north-oriented stream valleys eroded headward across multiple southeast-oriented flood flow routes, such as might be found in a large-scale anastomosing channel complex. Headward erosion of deep northeast and east-oriented valleys (or of even a southeast-oriented valley) across such a complex of southeast-oriented flood flow channels would have captured the southeast-oriented flood waters and enabled southeast-oriented tributary valleys to erode headward from the newly eroded and deep stream valley wall along the captured southeast-oriented flow channel. At the same time capture of the southeast-oriented flood flow would have beheaded flood flow routes to the southeast and flood waters on the northwest ends of those beheaded flood flow channels would have reversed flow direction to flow northwest into the newly eroded and deeper stream valley. Because the channels were anastomosing (meaning they were interconnected) reversed flow on beheaded flood flow routes often captured flood waters from yet to be beheaded flood flow routes further to the south or southwest. Capture of water from such yet to be beheaded flood flow routes frequently provided sufficient water to erode significant northwest-oriented tributary valleys.

West end of Boxelder Creek-Rapid Creek drainage divide area in Black Hills

Figure 3 illustrates the Boxelder Creek-Rapid Creek drainage divide area west end area in the Black Hills. The North Fork Rapid Creek originates in the figure 3 northwest corner near the “H” in Hills and flows south-southeast past Nahant to join east-southeast oriented South Fork Rapid Creek and then flow east-southeast to the figure 3 south center edge. North Boxelder Creek originates near Custer Peak in the figure 3 northwest quadrant and flows southeast, northeast, southeast, northeast to Benchmark, where it joins southeast oriented Hay Creek and flows southeast to the figure 3 east edge. Middle Boxelder Creek originates south of the Custer Peak and flows southeast before turning east to flow north of Minnesota Ridge and then northeast to join North Boxelder Creek. The Rapid Creek-Boxelder Creek drainage divide south of Custer Peak is  a north-northwest to south-southeast oriented drainage divide and is located between south-southeast oriented North Fork Rapid Creek and headwaters of southeast and east-oriented North and Middle Boxelder Creeks. The drainage divide between Middle Boxelder Creek and Rapid Creek is a west-northwest to east-southeast oriented drainage divide in the Minnesota Ridge area. South of Minnesota Ridge are several south-southeast-oriented Rapid Creek tributaries, while north of Minnesota Ridge is the east-oriented Middle Boxelder Creek valley. The drainage divide orientation, the North Fork Rapid Creek south-southeast orientation, and the orientation of Rapid Creek tributaries south of Minnesota Ridge suggest the Middle Boxelder Creek valley was eroded headward across multiple south-southeast oriented flood flow routes or channels, such as might be found in an anastomosing channel complex, and diverted the flood flow east and northeast to what must have been the newly eroded Boxelder Creek valley. North Fork Rapid Creek headwaters are linked by through valleys in the figure 3 northwest corner to north-oriented Whitewood Creek headwaters and also by a northwest oriented through valley to northwest- and north-oriented East Spearfish Creek. These through valleys are better illustrated and described in the Spearfish Creek-Whitewood Creek and the Whitewood Creek-Bear Butte Creek drainage divide essay, although they provide further evidence of south- and southeast-oriented anastomosing flood channels captured by headward erosion of deep north- and northeast-oriented valleys. Detailed maps below illustrate the North Fork Rapid Creek-Middle Boxelder Creek drainage divide north of Nahant, the Middle Boxelder Creek-Rapid Creek drainage divide area at Minnesota Ridge, and the Jim Creek-Jenny Gulch drainage divide near Merritt (located in the figure 3 southeast quadrant near Pilot Knob).

North Fork Rapid Creek-Middle Boxelder Creek drainage divide north of Nahant, South Dakota

Figure 4: North Fork Rapid Creek-Middle Boxelder Creek drainage divide north of Nahant. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 illustrates the North Fork Rapid Creek-Middle Boxelder Creek drainage divide area north of Nahant. North Fork Rapid Creek flows south-southeast from the figure 4 northwest corner to the Bull Dog Ranch, Nahant School, and Nahant (at the figure 4 south edge). Middle Boxelder Creek flows south-southeast from the figure 4 north center and then turns east to flow to the figure 4 east edge (center). Gudat Gulch is a southwest-oriented North Fork Rapid Creek tributary located in the figure 4 northwest corner area. A north-northwest to south-southeast oriented through valley links Gudat Gulch with the valley of an unnamed southwest-oriented North Fork Rapid Creek tributary in the Nahant School area. Another similar through valley  (east of the North Fork Rapid Creek valley) continues south from the Nahant School area to the figure 4 south edge. These through valleys are parallel to the south-southeast oriented North Fork Rapid Creek valley and provide evidence the North Fork Rapid Creek valley was eroded by south-southeast oriented flood waters flowing in multiple anastomosing channels. Note other higher level through valleys located throughout the figure 4 map area. For example a higher level through valley links the Gudat Gulch valley with the south-southeast oriented Middle Boxelder Creek valley head. The unnamed southwest-oriented North Fork Rapid Creek tributary valley is also linked by a higher level through valley with the south-southeast oriented Middle Boxelder Creek valley head (the through valley to Gudat Gulch). Further south, near the French Ranch in the figure 4 southwest quadrant, an unnamed southwest-oriented North Fork Rapid Creek tributary is linked by a through valley to a northeast-oriented Middle Boxelder Creek tributary valley and also with headwaters of a south-southeast oriented Rapid Creek tributary. These and other similar high level through valleys located in the figure 4 map area provide evidence of higher level anastomosing channels that were dismembered as headward erosion of the deep Rapid Creek and Boxelder Creek valleys eroded into the figure 4 map area and captured south-southeast oriented flood flow and diverted floodwaters east and northeast.

Middle Boxelder Creek-Rapid Creek drainage divide area at Minnesota Ridge

Figure 5: Middle Boxelder Creek-Rapid Creek drainage divide area at Minnesota Ridge. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 5 illustrates the Middle Boxelder Creek-Rapid Creek drainage divide area at Minnesota Ridge and overlaps with areas on figure 4. Middle Boxelder Creek flows southeast into the figure 5 northwest quadrant, then east across the figure 5 north center before turning northeast. South Boxelder Creek (unlabeled on figure 5) originates in the figure 5 center (just above the word FOREST) and flows northeast before turning southeast in the figure 5 northeast corner. A northwest-southeast oriented through valley links the northeast-oriented Middle Boxelder Creek valley with the southeast-oriented South Boxelder Creek valley and is evidence headward erosion of the Middle Boxelder Creek valley beheaded southeast-oriented flood flow. South-southeast oriented drainage in the figure 5 south half flows to east-southeast oriented Rapid Creek. High level though valleys link headwaters of the south-southeast oriented Rapid Creek tributaries with the east-oriented Middle Boxelder Creek valley and the northeast-oriented South Boxelder Creek valley. These high level through valleys provide evidence the south-southeast oriented Rapid Creek valleys were eroded headward from what must have been an actively eroding and deep Rapid Creek valley along what must have been multiple south-southeast oriented flood flow channels. Headward erosion of what must have been a deep South Boxelder Creek valley next eroded headward along a southeast-oriented flood flow route, with northeast-oriented tributary valleys eroding southwest to capture flood waters further to the southwest. These tributary valleys eroded in sequence, with the southernmost eroding first and the northernmost last. Included in these tributary valleys were what are today the northeast-oriented South Boxelder Creek valley and the east- and northeast-oriented Middle Boxelder Creek valley segment located in figure 5. Headward erosion of what is today the northeast-oriented South Boxelder Creek valley beheaded flood flow to the eastern south-southeast-oriented Rapid Creek tributary valleys. Subsequently headward erosion of the Middle Boxelder Creek valley beheaded flood flow to the newly eroded northeast-oriented South Boxelder Creek valley, to what is today the southeast-oriented South Boxelder Creek valley, and to the south-southeast oriented Rapid Creek tributary valleys west of the northeast-oriented South Boxelder Creek valley head.

Jim Creek-Jenny Gulch drainage divide area near Merritt, South Dakota

Figure 6: Jim Creek-Jenny Gulch drainage divide area near Merritt, South Dakota. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 6 is a greatly reduced map which illustrates the Jim Creek-Jenny Gulch and the Jim Creek-Deer Creek drainage areas near Merritt, South Dakota. Merritt is located in the figure 3 southeast corner and is also located along the figure 7 west edge (north half). Jim Creek flows east along the figure 6 center top to Merritt and then northeast to the figure 6 north edge and eventually to southeast-oriented Boxelder Creek (see northwest corner of figure 7 below for Jim Creek route to Boxelder Creek). Deer Creek is the southeast stream originating east of Merritt Peak (south of Merritt) and following the highway to the figure 6 east edge (south). Jenny Gulch originates west of Merritt Peak and flows southeast to join Rapid Creek, which flows southeast in the figure 6 southwest corner and east along the figure 6 south edge to Silver City, South Dakota. East Gimlet Creek flows south and southwest along the figure 6 west edge and joins southeast-oriented Rapid Creek just west of the figure 6 map area. Well defined northwest-southeast oriented through valleys link the east-oriented Jim Creek valley with the southeast-oriented Deer Creek and Jenny Gulch valleys. Those through valleys are evidence the east-oriented Jim Creek valley eroded west across multiple southeast-oriented flood flow routes and beheaded southeast-oriented flood flow to the Deer Creek and Jenny Gulch valleys. Between Jenny Gulch and Deer Creek (just west of Merritt Peak) is south-southeast oriented Broad Gulch, which originates in a north-south through valley crossing what is today the Merritt Peak highland area. That through valley is evidence flood waters once flowed south and southeast across a topographic surface at least as high as the Merritt Peak elevation today. However, when flood waters crossed the figure 6 map area the Black Hills probably did not stand high above the surrounding plains region as it does today. Uplift of the Black Hills dome and deep erosion of the surrounding plains area were both still underway and perhaps only just beginning.

Boxelder Creek-Rapid Creek drainage divide area on Black Hills east slope

Figure 7: Boxelder Creek-Rapid Creek drainage divide area on Black Hills east slope. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 7 illustrates the Boxelder Creek-Rapid Creek drainage divide area on the Black Hills east slope northwest of Rapid City, South Dakota. Boxelder Creek flows southeast to the corner of Lawrence, Meade, and Pennington Counties and then southeast to South Canyon and the figure 7 east edge. Rapid Creek flows east-southeast in the figure 7 southwest quadrant to Pactola Reservoir and then through Dark Canyon before flowing to the figure 7 east edge and Rapid City. Jim Creek, which was discussed in figure 6 above, is located in southern Lawrence County, where it flows from Merritt near the figure 7 west edge to join Boxelder Creek near Steamboat Rock, located near the Lawrence County-Meade County border. North of Jim Creek is southeast and northeast-oriented Estes Creek, which also flows to the southeast-oriented Boxelder Creek. Note the Estes Creek-Jim Creek drainage divide area. Two parallel through valleys link the Estes Creek and Jim Creek valleys and continue south to the southeast-oriented Bogus Jim Creek valley, which also flows to Boxelder Creek. Figure 8 below illustrates these through valleys in more detail, although the through valleys are evidence headward erosion of the Jim Creek valley captured south-oriented flood flow to the Bogus Jim Creek valley and headward erosion of the northeast-oriented Estes Creek valley captured southeast flood flow to the Jim Creek valley. Figure 7 also illustrates southeast-oriented Deer Creek and Jenny Gulch (and Broad Gulch), which were seen earlier in figure 6, and illustrates their big picture relationship to the Rapid Creek valley and the Boxelder Creek-Jim Creek valley. As headward erosion of the deep Rapid Creek valley proceeded west to capture southeast-oriented flood flow routes, first the deep southeast-oriented Deer Creek and second the deep southeast-oriented Jenny Gulch valley eroded headward along southeast-oriented flood flow routes. Headward erosion of the deep Boxelder Creek-Bogus Jim Creek valley proceeded shortly after, with headward erosion of the deep Boxelder Creek-Jim Creek valley capturing southeast-oriented flood flow to the newly eroded Bogus Jim Creek valley and then capturing in sequence southeast-oriented flood flow to the newly eroded Deer Creek valley and the Jenny Gulch valley. At the same time headward erosion of the Boxelder Creek-Estes Creek valley beheaded southeast oriented flood flow to the Jim Creek valley.

Estes Creek-Jim Creek-Bogus Jim Creek drainage divide in Boxelder Creek drainage basin

Figure 8 illustrates the Estes Creek-Jim Creek-Bogus Jim Creek drainage divide areas illustrated in less detail in figure 7 above. Boxelder Creek flows southeast from the figure 8 north edge (northeast quadrant) to the figure 8 east edge (center). Estes Creek flows southeast in the figure 8 northwest corner and turns northeast to join southeast-oriented Boxelder Creek just north of the figure 8 north edge. Jim Creek flows from the figure 8 west center edge across the figure 8 center to join southeast-oriented Boxelder Creek south of Steamboat Rock. North Bogus Jim Creek flows south in the figure 8 south center and joins east-southeast oriented Middle Bogus Jim Creek to flow to the figure 8 southeast corner. Figure 8 illustrates in detail through valleys linking Estes Creek with Jim Creek, North Bogus Jim Creek, and Middle Bogus Jim Creek. These through valleys provide evidence of multiple south and southeast-oriented flood flow routes that were captured and diverted east and northeast by headward erosion of the Middle Bogus Jim Creek valley, North Bogus Jim Creek valley, Jim Creek valley, and the Estes Creek valley, in that order. The figure 8 region appears to be a region of east-dipping hogbacks and the valleys are probably eroded along less resistant geologic strata, however the valleys were eroded by running water and at one time that water was moving in multiple south and southeast-oriented flood flow routes as would be expected in a south and southeast-oriented anastomosing channel complex. Other through valleys link various valleys and are further evidence of an anastomosing channel complex that was dismembered by headward erosion of the deep Boxelder Creek valley. For example, southwest of Steamboat Rock (northeast quadrant) is a north-south through valley linking east-oriented segments of the Boxelder Creek valley and the Jim Creek valley. This north south through valley is evidence of multiple flood flow routes prior to Boxelder Creek valley erosion that captured all flood flow.

Boxelder Creek-Rapid Creek drainage divide area east of Rapid City airport

Figure 9: Boxelder Creek-Rapid Creek drainage divide area east of Rapid City airport. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 9 illustrates the Boxelder Creek-Rapid Creek drainage divide area east of Rapid City, where the Rapid City airport is located. East-oriented Boxelder Creek can be seen in the figure 9 northeast corner. Southeast oriented Rapid Creek is seen in the figure 9 southwest corner. The Rapid City airport is located in the figure 9 southwest quadrant. The Boxelder Creek-Rapid Creek drainage divide is the east-west ridge separating what are today the large east- and southeast-oriented Boxelder Creek valley from the large southeast-oriented Rapid Creek valley. Note how the Rapid Creek valley north wall is a relatively smooth south-oriented sloping erosion surface now drained by southeast-oriented valleys. In contrast the north facing Boxelder Creek valley wall has many shallow north oriented Boxelder Creek tributary valleys. What has happened is a large southeast-oriented Rapid Creek headcut eroded west into a topographic surface higher than any figure 9 elevations today to capture southeast-oriented flood flow moving across the figure 9 region. The southeast-oriented flood flow then began to erode the newly eroded and deep Rapid Creek valley north wall and eroded the relatively smooth southeast-oriented erosion surface. Shortly thereafter the large Boxelder Creek valley headcut eroded west just to the north of the newly eroded Rapid Creek valley and beheaded the southeast-oriented flood flow moving to the Rapid Creek valley. Headward erosion of the Boxelder Creek valley headcut occurred by eroding valleys headward from the actively eroding valley head to capture yet to be beheaded southeast-oriented flood flow. The south valley wall was eroded by north and northeast-valleys that eroded south and southwest to capture yet to be beheaded southeast-oriented flood flow. For that reason the north-facing Boxelder Creek valley wall was never eroded by southeast-oriented flood flow, but was instead eroded by captured flood flow moving north to the actively eroding Boxelder Creek valley head. The south-facing Boxelder Creek valley wall is a smooth southeast-sloping erosion surface similar to the south-facing Rapid Creek valley wall and is illustrated in the Elk Creek-Boxelder Creek drainage divide essay. The relatively smooth southeast-sloping erosion surfaces have sometimes been considered stream terraces, because valleys have been eroded into them. The deeper valleys eroded into the erosion surfaces are evidence that headward erosion of the large Rapid Creek and Boxelder Creek valley headcuts occurred shortly before headward erosion of the deep Cheyenne River valley. Prior to headward erosion of the deep Cheyenne River valley flood waters were flowing east across what is now the Badlands National Park area to what was then an actively eroding and deep White River valley. Headward erosion of the deep Cheyenne River valley captured the southeast-oriented flood flow and also was responsible for eroding the Rapid Creek valley deeper as Boxelder Creek valley headward erosion was still beheading the southeast-oriented flood flow.

Boxelder Creek-Rapid Creek drainage divide area west of Cheyenne River

Figure 10: Boxelder Creek-Rapid Creek drainage divide area west of Cheyenne River. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 10 illustrates using greatly reduced maps the Boxelder Creek-Rapid Creek drainage divide area immediately west of the Cheyenne River. The Cheyenne River is the large northeast oriented valley located in figure 10. Boxelder Creek is the southeast oriented valley flowing from the figure 10 northwest quadrant to the Cheyenne River. Rapid Creek flows southeast from the figure 10 west center edge to the Cheyenne River. The Cheyenne River-White River drainage divide essay describes the area east of the Cheyenne River valley and can be found under White River on the sidebar category list. The southeast oriented Boxelder Creek valley is headed almost directly toward what is today the Sage Creek Basin in Badlands National Park. The southeast-oriented Rapid Creek valley is headed almost directly toward what is today the Scenic Basin. The Sage Creek Basin and the Scenic Basin are today two large southeast-oriented abandoned headcuts carved into what was once the deep northeast and east oriented White River valley northwest wall. Headward erosion of the deep Cheyenne River valley captured southeast-oriented flood flow moving in what are today the Boxelder Creek valley and the Rapid Creek valley, beheading flood flow to what had been the actively eroding Sage Creek and Scenic Basin headcut faces. Reversed flood flow was responsible for eroding the northwest-oriented Cheyenne River tributary valleys.

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.