The Milk River originates in the Glacier National Park region west of Browning, Montana and flows in a northeast direction into Alberta before turning to flow in a southeast direction to Havre, Montana and then in an east and southeast direction to join the east-oriented Missouri River east of Glasgow, Montana. This overview essay provides highlights from more detailed essays, which use topographic map evidence to describe drainage divides between Milk River tributaries and with adjacent drainage basins. The detailed essays can be found under Milk River on this website’s sidebar category list. The topographic map evidence indicates a deep Milk River valley eroded headward to capture immense southeast-oriented floods moving across what is today the Milk River drainage basin. Flood waters were derived from a rapidly melting thick North American ice sheet, which was located in a deep ice sheet created “hole”, and which were flowing in a southeast direction along the decaying ice sheet’s southwest margin. The ice sheet had originally formed on a topographic surface at least as high as present day high-level Rocky Mountain erosion surfaces and headward erosion of the deep Missouri River and Milk River valleys in sequence from south to north captured the ice-marginal flood waters and diverted the flood flow into space in the deep “hole” being opened up as the thick ice sheet roots melted. The present day Bear Paw Mountains and Little Rocky Mountains emerged as flood waters deeply eroded regions surrounding them and/or as ice sheet-triggered crustal warping caused regional and/or local uplifts. Deep flood water erosion of the Milk River-Missouri River drainage divide area occurred as the deep Missouri River valley and subsequently the deep Milk River valley eroded headward to capture the southeast-oriented flood flow.
Figure 1: Milk River drainage route in northern Montana and southern Alberta. National Geographic Society map digitally presented using National Geographic Society TOPO software.
Milk River drainage basin drainage history
The Milk River originates along the east edge of the Glacier Nation Park region of northern Montana and near the east-west continental divide and flows in a northeast direction into southern Alberta before turning to flow in an east and then southeast direction to Havre, Montana. From Havre the Milk River flows in an east direction to near Saco, Montana where it turns to flow in a southeast direction to join the Missouri River east of Glasgow, Montana. The Missouri River then flows in an east and southeast direction to eventually join the south-oriented Mississippi River, which flows to the Gulf of Mexico. North of the Milk River drainage basin is the South Saskatchewan River drainage basin, which drains to the northeast-oriented Saskatchewan River with water eventually reaching Hudson Bay. The Saskatchewan River-Milk River drainage divide for its entire distance is the north-south continental divide and, which with the exception of a small area in and near Montana’s Glacier National Park, is located in southern Alberta and southern Saskatchewan.
- Figure 2 (below) provides a slightly more detailed regional map (than figure 1) showing the Milk River route from the Montana Glacier National Park area to Havre, Montana. Note how the northeast-oriented South and North Forks of the Milk River originate just east of the Glacier National Park boundary near St Mary, Montana and join in southern Alberta. North and west of the Milk River headwaters area is the northeast-oriented St Mary River, which is located in the Saskatchewan River drainage basin. South of the Milk River headwaters area is Cut Bank Creek, which flows to the east-southeast and southeast oriented Marias River, which joins the northeast-oriented Missouri River near Loma, Montana. The Missouri River north and east of Loma makes an abrupt turn to flow in a southeast and then east direction, while northeast-oriented Big Sandy Creek continues on the northeast-oriented Missouri River alignment to join the Milk River near Havre, Montana. A through valley illustrated and described in the Big Sandy Creek-Birch Creek drainage divide landform origins essay provides evidence water once flowed in a northeast direction from the northeast-oriented Missouri River to the northeast-oriented Big Sandy Creek valley and then to the east-oriented Milk River near Havre. This through valley has been interpreted as evidence the Missouri River once flowed in a northeast direction to the Havre region and then in an east direction along the present day Milk River valley route. The interpretation goes on to claim the Missouri River was diverted to its present day course by an ice sheet margin, which blocked the Milk River valley route. Topographic map evidence presented in the detailed essays suggests a very different drainage history, which is briefly presented in this Milk River drainage basin drainage history overview and in more detail in the detailed essays
Figure 2: Regional map showing Milk River route west of Havre, Montana. National Geographic Society map digitally presented using National Geographic Society TOPO software.
Understanding the Milk River drainage basin drainage history requires an understanding of other Montana and adjacent state and province drainage basin drainage histories. Readers wanting to fully understand Milk River drainage basin history are encouraged to read overview essays and detailed essays for drainage basins south and east of the Milk River drainage basin. These other essays can be found under their respective river names on this website’s sidebar category list. Essays under MT Missouri River, Musselshell River, Yellowstone River, Tongue River, Powder River, and Little Missouri River (among others) show detailed topographic map evidence for drainage divide areas south and east of the Missouri River in central and eastern Montana and in northeast Wyoming. Evidence illustrated in the several hundred topographic maps included in these and other Missouri River drainage basin landform origins essays suggests all Missouri River drainage basin valleys were formed by headward erosion of deep valleys across and along immense southeast and south-oriented floods and supports the interpretation presented in this Milk River drainage basin landform origins overview and detailed essays suggests the Milk River valley eroded headward from a decaying ice sheet margin to capture immense southeast-oriented ice marginal floods.
Figure 3: Saint Mary River-Milk River-Cut Bank Creek drainage divide near Saint Mary, Montana. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Topographic map evidence for the Milk River headwaters region supports the interpretation of headward erosion of deep valleys across immense southeast-oriented floods. Figure 3 illustrates the South Fork Milk River headwaters region near Saint Mary, Montana to show evidence water from the northwest once flowed across the present day Saint Mary River-South Fork Milk River drainage divide, which is today the north-south continental divide. Saint Mary Lake drains in a northeast direction to the northeast-oriented Saint Mary River, with water in the Saint Mary River eventually flowing to Hudson Bay. The northeast-oriented South Fork Milk River flows to the figure 3 east edge (north half) and is located north of the labeled Milk River [Ridge] and east of the Saint [Mary Ridge]. South of Milk River [Ridge] is northeast and southeast oriented North Fork Cut Bank Creek. The Milk River as seen in figures 1 and 2 flows into southern Alberta before flowing back into Montana and eventually joining the east and southeast-oriented Missouri River. Cut Bank Creek flows to Marias River, which flows to Missouri River near Loma. From Loma the Missouri River flows in a generally east direction south of the Milk River until the Milk and Missouri Rivers converge near Glasgow, Montana. What we are seeing in figure 3 is the divergence of two channels, which then converge further to the east. Such divergence and convergence of channels occurs in flood formed anastomosing channel complexes. Note how east-oriented Fox Creek, which joins the northeast-oriented South Fork Milk River near the figure 3 east edge, is linked by a through valley with the north-oriented Divide Creek valley, with Divide Creek flowing to the Saint Mary River. The west to east oriented through valley is located north of Divide Mountain and south of Saint [Mary Ridge]. The map contour interval is 50 meters and the through valley is at least 100 meters deep. The through valley provides evidence water once flowed from what is now the deep northeast-oriented Saint Mary River valley to the northeast-oriented South Fork Milk River valley. Other through valleys are also present in figure 3. For example a much deeper (and higher level) through valley is located between Divide Mountain and White Calf Mountain, which also links the Divide Creek valley with the South Fork Milk River valley. This Divide Mountain-White Calf Mountain through valley is more than 350 meters deep and also provides evidence of east-oriented drainage from the Saint Mary River valley to the South Fork Milk River valley. Looking at only the figure 3 map evidence these through valleys can perhaps be explained in the context of melt water from a valley glacier, which probably once filled the Saint Mary River valley. However, when viewed along with evidence from drainage divides elsewhere in the region, these through valleys are best explained in the context of immense southeast-oriented floods which once flowed across what are now high mountains in the Montana Glacier Park region.
Figure 4: Milk River-Missouri River drainage divide in high Bear Paw Mountains south of Havre, Montana. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Moving east to the Bear Paw Mountain area detailed essays illustrate and describe evidence seen on dozens of topographic maps. Figure 4 provides a topographic map of the Milk River-Missouri River drainage divide area near Mount Baldy in the high Bear Paw Mountains south of Havre, Montana. The northeast-oriented and north-northeast and northwest oriented West and East Forks of Beaver Creek converge just north of the figure 4 map area and Beaver Creek then flows in a north direction to join the east-oriented Milk River near Havre. Contours on the figure 4 map are given in feet and the Milk River valley elevation where Beaver Creek joins it is less than 2500 feet. Note how the northeast-oriented West Fork Beaver Creek is linked by a large through valley west of Baldy Mountain with south-southwest and southwest oriented Eagle Creek. West and south of the figure 4 map area Eagle Creek flows in a southwest and west direction to join a south-oriented Missouri River segment (see figure 2). The Missouri River elevation where Eagle Creek joins is slightly less than 2500 feet. The through valley linking the West Fork Beaver Creek valley with the Eagle Creek valley has an elevation of between 5160 feet and 5200 feet. The top of Baldy Mountain to the east is 6916 feet. Wellen Peak to the west of the through valley has an elevation of at least 6360 feet. In other words the through valley is more than 1100 feet deep. East of Baldy Mountain similar through valleys link the East Fork Beaver Creek with the south-oriented Birch Creek valley, where Birch Creek drains to the Missouri River. These deep through valleys are water eroded valleys and provide evidence multiple channels carrying large volumes of water once flowed across the figure 4 map area. At that time the deep Milk River valley north of the figure 4 map area did not exist. It is possible ice was present north of the Bear Paw Mountains and the south-oriented flood flow responsible for eroding the through valleys and south-oriented Missouri River tributary valleys flowed across a high level ice surface to reach the figure 4 map area, although it is also possible headward erosion of the deep Milk River valley removed easily eroded bedrock north of the Bear Paw Mountains and/or that the Bear Paw Mountains were uplifted as flood flowed south across the region. Which ever of these alternatives is considered, there can be no question about the evidence for massive south-oriented flood flow across the Bear Paw Mountains.
Figure 5: Clear Creek-Peoples Creek through valley in Bear Paw Mountains. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Figure 5 provides more clues about the south-oriented flood flow across the Bear Paw Mountains and how headward erosion of the deep Milk River valley and deep east-oriented Milk River tributary valleys captured that massive south-oriented flood flow. Figure 5 uses a reduced size topographic map to illustrate the Clear Creek-Peoples Creek through valley located east and slightly north of the figure 4 map area. Clear Creek is the north-northwest stream located in the figure 5 west half and north of the figure 5 map area flows to the east-oriented Milk River. Battle Creek is the west-oriented Clear Creek tributary located in the west half of the large west to east oriented through valley eroded across the figure 5 south center area. Note how Battle Creek originates as a southeast-oriented stream before making a U-turn to flow into the large through valley and then to flow in a west direction to join north-northwest oriented Clear Creek. Peoples Creek originates near the figure 5 center and flows in a southeast direction into the large through valley and then flows in an east and northeast direction to the figure 5 east edge. East of the figure 5 map area Peoples Creek flows in a generally east and northeast direction to eventually join the Milk River. The Battle Creek-Peoples Creek drainage divide in the through valley has an elevation of between 4520 and 4560 feet and Murphy Butte to the south has an elevation of 5272 feet and the peak north of the parallel Battle Creek and Peoples Creek headwaters rises to more than 5960 feet. In other words the Battle Creek-Peoples Creek through valley is at least 700 feet deep and may have been much deeper at one time. Note how the parallel southeast-oriented Battle Creek and Peoples Creek headwaters are linked by other through valleys and also note the high level through valley (or saddle) between the high points in section 30 north of the Battle Creek and Peoples Creek headwaters.
- Through valleys shown in figure 5 all provide evidence of south-oriented flood flow prior to headward erosion of the deep Milk River valley, located north of the figure 5 map area. The large west to east Battle Creek-Peoples Creek through valley was originally eroded as an east-oriented valley which eroded headward into the figure 5 map area to capture south and southeast-oriented flood flow. The north-northwest oriented Clear Creek valley was initiated as a south-southeast flood flow channel supplying flood water to what was then the actively eroding east-oriented Peoples Creek valley. At that time the deep Milk River valley to the north was just beginning to be eroded and flood waters were flowing across what is today the high ridge north of the west to east through valley. The high ridges seen in figure 5 emerged as the deep valleys (including the Milk River valley to the north) eroded headward into the region and perhaps as uplift raised the Bear Paw Mountains region. For a time the south-southeast Clear Creek and east-oriented Peoples Creek flood flow channel moved large quantities of flood water east to the Milk River valley (further to the east) and for a time the Milk River valley to the north and the Clear Creek-Peoples Creek valley were diverging and converging channels in a large east-oriented anastomosing channel complex. Headward erosion of the deep Milk River valley, perhaps aided by Bear Paw Mountains uplift, then beheaded the south-southeast oriented flood flow on the Clear Creek alignment. Flood waters on the north and west ends of the beheaded flood flow route reversed flow direction to erode the north-northwest oriented Clear Creek valley and the west-oriented Battle Creek valley.
- Why would immense southeast-oriented floods be flowing across what are now high mountain ranges and how could those southeast-oriented flood waters flow across what are today high mountain elevations? The Milk River drainage basin history began with development of a North American ice sheet comparable in size to the present day Antarctic Ice Sheet, if not larger. The ice sheet was thick, probably several kilometers thick, and was located in a deep “hole”, which the ice sheet had formed by a combination of deep glacial erosion and crustal warping caused by the ice sheet weight. When at its maximum size the ice sheet stood high above the pre-glacial surface, but also had roots that extended well below the pre-glacial surface, which no longer exists. Some, if not all of the present day Milk River drainage basin location was probably located south and west of the thick ice sheet’s southwest margin, although evidence for the ice sheet’s southwest margin has probably been removed by deep melt water flood erosion. The pre-glacial surface under the ice sheet was completely destroyed by deep glacial erosion and the pre-glacial surface adjacent to the ice sheet and elsewhere on the North American continent was deeply eroded by deep melt water flood erosion and was also probably significantly altered by crustal warping caused by the thick North American ice sheet presence.
- Events important to Milk River drainage basin history began as the ice sheet had melted to the point that in the south it no longer stood high above the surrounding non-glaciated surface, which had probably already been significantly lowered by deep melt water erosion. Immense melt water floods were flowing in a southeast direction along the ice sheet’s southwest margin and were just beginning to deeply erode the region between the Rocky Mountains and the ice sheet’s southwest margin, which at that time may have been located north and east of the Milk River drainage basin and/or may have been located within the present day Milk River drainage basin. At that time the Rocky Mountains did not stand high above the ice sheet surface, or for that matter above the surface located between the Rocky Mountains and the ice sheet southwest margin. Initially immense floods of melt water flowing from the rapidly melting ice sheet flowed across and along what is now the east-west continental divide. Flood waters flowing into northwest Montana were probably derived from immense southeast and south-oriented supra-glacial melt water rivers which carved giant ice-walled and ice-floored (later bedrock-floored) canyons into the decaying ice sheet surface and which flowed to the ice sheet’s southwest margin in present day Alberta and then across the east-west continental divide into a gigantic southeast-oriented river which was flowing along the alignment of the present day northwest-southeast oriented Rocky Mountain Trench. Until headward erosion of deep valleys from the Pacific Ocean systematically captured and eroded the Rocky Mountain Trench this immense southeast-oriented melt water river the deep Rocky Mountain Trench valley did not exist and flood waters flowed in a southeast direction across western Montana to the present day Yellowstone Plateau area and then in a southeast direction across Wyoming. The Milk River valley was probably just one of many deep northeast and east-oriented valleys which eroded headward from the deep “hole” the decaying ice sheet had once occupied to capture this gigantic southeast-oriented melt water river and to divert the flood waters into ice sheet’s deep “hole”.
- How did the deep “hole” the ice sheet had been occupying open up so as to permit headward erosion of the deep east-oriented Milk River valley and other deep northeast and east-oriented valleys? Remember the ice sheet was thick and had deep roots. The ice sheet roots may have extended more than a kilometer below the pre-glacial surface on which the ice sheet had formed. The deep “hole” had probably been formed by a combination of deep glacial erosion of the pre-glacial surface underlying the ice sheet and of crustal warping caused by the weight of an ice sheet several kilometers thick. The crustal warping, which almost certainly did not occur instantaneously, probably also affected regions elsewhere on the continent and may have contributed to uplift of the Rocky Mountains and Rocky Mountain outliers such as the Bear Paw Mountains and Little Rocky Mountains (and also other North American mountain ranges and high plateau areas) as flood waters flowed across them. In other words, not only was the rapidly decaying ice sheet located in a deep “hole” that was opening up as the ice sheet melted, but delayed crustal warping caused by the ice weight was raising mountain ranges and high plateaus regions south and west of the ice sheet margin. The combination of these two events created a situation where the gigantic southeast-oriented melt water river flowing across and along what is now the east-west continental divide was systematically captured by headward erosion of deep northeast and east-oriented valleys, which were eroding headward from the decaying ice sheet location.
Figure 6: East end of the Milk River-Missouri River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
Evidence for the massive southeast-oriented floods is perhaps best seen along the Milk River-Missouri River drainage divide east of the Rocky Mountain outliers (i.e. the Bear Paw Mountains and the Little Rocky Mountains). Figure 6 provides a map showing drainage routes in what is today the east end of the Milk River-Missouri River drainage divide area. The large lake in the figure 6 southeast quadrant is Fort Peck Lake or Reservoir, which floods the northeast-oriented Missouri River valley. The southeast-oriented river joining the Missouri River near the figure 6 east center edge is the Milk River, which flows in an east and northeast direction in the figure 6 northwest quadrant. Note how many Missouri River tributaries from the north and west are oriented in a southeast direction and join the northeast-oriented Missouri River as barbed tributaries. Also note how several Missouri River tributaries from the south and east are oriented in a northwest direction and also join the Missouri River as barbed tributaries. These northwest and southeast oriented tributaries provide evidence the deep northeast-oriented Missouri River valley eroded headward across multiple southeast-oriented flood flow channels such as might be found in a large southeast-oriented anastomosing channel complex. North-oriented Milk River tributaries also have southeast-oriented and northwest-oriented tributaries and/or headwaters. Again, this northwest-southeast orientation of tributary valleys is evidence the north-oriented Milk River tributary valleys eroded headward in sequence from east to west across multiple southeast-oriented flood flow channels. The figure 6 evidence disproves the hypothesis the Milk River valley was eroded prior to headward erosion of the deep Missouri River valley to the south. Headward erosion of the deep Milk River valley beheaded all southeast-oriented flood flow channels across the present day Milk River-Missouri River drainage divide. In other words, the deep northeast-oriented Missouri River valley eroded headward across the figure 6 map area first and the deep Milk River valley eroded headward across the figure 6 map area last, with the north-oriented Milk River tributary valleys being eroded headward in sequence from east to west between the time the deep Missouri River valley eroded headward and the time the deep Milk River valley eroded headward. North-oriented Milk River tributary valleys were eroded at the time Milk River valley beheaded and reversed south-oriented flood flow channels on their alignment. Headward erosion of these north-oriented Milk River tributary valleys enable reversed flood flow in these valleys to capture southeast-oriented flood flow channels located west of the actively eroding Milk River valley head.
Figure 7: Missouri River-Big Sandy Creek through valley west of Bear Paw Mountains. United States Geological Survey map digitally presented using National Geographic Society TOPO software.
To fully understand the Milk River-Missouri River drainage divide area between Big Sandy Creek and the Milk River-Missouri River confluence area seen in figure 6 it is necessary to see the through valley linking the northeast oriented Missouri River valley segment (south and west of the Bear Paw Mountains) with the northeast oriented Big Sandy Creek segment west of the Bear Paw Mountains. Figure 7 illustrates that through valley. The figure 7 map does not show contour lines for the east half, although green areas are forested areas in the high Bear Paw Mountains. Baldy Mountain is located near the figure 7 east center edge. The northeast and south-southeast oriented Missouri River elbow of capture can be seen in the figure 7 southwest corner area (see figures 1 and 2 for larger region maps). Big Sandy is the town located north of the Missouri River elbow of capture. The stream flowing north from Big Sandy is Big Sandy Creek. Big Sandy Creek originates in the high Bear Paw Mountains and flows in a southwest direction before turning to flow in a northeast and north direction to join the east-oriented Milk River north of the figure 7 map area. Note how the north-oriented Big Sandy Creek valley is linked by a through valley with the northeast oriented Missouri River valley in the figure 7 southwest corner. The through valley provides evidence of a major northeast oriented flood flow route which eroded headward along the Bear Paw Mountains west flank to capture southeast oriented flood flow moving across the figure 7 map area. The northeast oriented Missouri River valley may very well have been initiated by headward erosion of this northeast and north oriented Big Sandy Creek valley, which may have eroded headward from what was at that time the newly eroded Milk River valley. However, the south-southeast, east, and northeast-oriented Missouri River valley probably already existed in some form at the time the deep northeast- and north-oriented Big Sandy Creek valley eroded headward into the region. Probably headward erosion of the deep Big Sandy Creek valley did not behead all southeast oriented flood water moving south of the Bear Paw Mountains to the east-oriented Missouri River valley. Instead the Big Sandy Creek-Milk River valley served as one flood flow route while the southeast, east, and northeast oriented Missouri River valley served as another flood flow route. As is typical in anastomosing channel complexes these two major flood flow routes diverged in figure 7 and then converged further to the east in figure 6. The Milk River-Missouri River drainage divide area was an island between these two diverging and converging flood flow channels.
- At the time it was eroded the deep Milk River valley was probably near the decaying ice sheet’s southwest margin. Evidence described in detailed essays suggests a thin ice sheet formed very late during the thick ice sheet melt down history. Essays found under MT Missouri River illustrate a large abandoned valley extending in a northeast direction from the present day east-oriented Missouri River valley near Poplar, Montana (east of where the Milk River joins the Missouri River) to the North Dakota northwest corner. West of the Poplar, Montana area the Missouri River valley is wider than it is to the east, suggesting at one time the Missouri River flowed in a northeast direction from the Poplar area to the North Dakota northwest corner. Essays found under ND Missouri River illustrate how this abandoned valley once extended to the present day northeast-facing Missouri Escarpment, which is what remains of the southwest wall of what was once a gigantic southeast and south-oriented ice-walled and bedrock-floored canyon carved in the decaying ice sheet surface. This large abandoned northeast-oriented Missouri River valley eroded headward from an immense southeast and south-oriented river which once flowed in that giant ice-walled canyon and the northeast-oriented valley then eroded headward along the present day Missouri River-Musselshell River and Milk River valley alignments (in sequence from south to north) to capture southeast oriented ice-marginal flood flow. Late during the thick ice sheet melt down history deep ice-walled canyons were for all practical purposes chopping the decaying thick ice sheet up into a large number of isolated and small ice sheet masses. As the ice-walled canyons eroded headward they intersected with each other, which opened up new flood flow routes across what had been the ice sheet floor. These new flood flow routes systematically captured the immense southeast and south-oriented melt water river into which this northeast-oriented abandoned Missouri River valley had been draining and the flood waters were first diverted east to what is now the Saint Lawrence River drainage basin and later north to what is now the Hudson Bay area. This diversion of what had been huge south-oriented floods to become north-oriented floods had a profound impact on the North American climate, which halted the decaying ice sheet’s rapid melt down.
- The south-oriented flood waters had been flowing to the Gulf of Mexico where they displaced warm water which then moved north in the Atlantic Ocean to produce warm climates responsible for the ice sheet’s rapid melt down. However, when the giant floods were diverted north the warm Gulf of Mexico water was no longer displaced and instead cold North Atlantic water was displaced and pushed south. The result was a general cooling of North American climates which in turn caused the north-oriented flood waters to freeze on the former ice sheet floor. The frozen flood waters surrounded the former thick ice sheet remnants to create what was for all practical purposes a new thin ice sheet. Unlike the previous thick ice sheet the new thin ice sheet did not deeply erode the region, however the thin ice sheet did block the northeast-oriented flood flow routes such as the northeast-oriented valley from the Poplar, Montana area to northwest North Dakota. The blockage forced the remaining ice marginal drainage to flow in an east and southeast direction to the deep Yellowstone River valley and then to flow for a distance in that valley before spilling in a southeast and east direction along the ice sheet margin, which eroded the present day Missouri River valley east of the Poplar, Montana area. The thin ice sheet also did move some debris and some of that debris ended up in the former northeast-oriented valley (the abandoned valley extending northeast from Poplar, Montana to the North Dakota northwest corner). It is possible the thin ice sheet extended south into what was then the newly formed Milk River valley and perhaps even further south and it is possible this wet based thin ice sheet did move some surface materials.
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.