Montana Marias River drainage basin area landform origins, Montana, USA, overview essay

· Marias River, Montana, Overview essays
Authors

A geomorphic history based on topographic map evidence

Abstract:

This overview essay provides highlights from more detailed essays interpreting landform origins along drainage divides in and around the Montana Marias River drainage basin. The more detailed essays can be found under Marias River on the sidebar category list. The Marias River is a Missouri River tributary with headwaters along the east-west continental divide in northern Montana and flows in an east direction from the Rocky Mountains onto the Montana plains. Topographic map evidence illustrated in this overview essay and in the much more detailed essays documents how the Marias River valley and its major tributary valleys eroded headward across massive south and southeast-oriented flood waters at a time when the Rocky Mountains were still being uplifted. Flood waters were derived from a rapidly melting thick North American ice sheet, which was located in a deep “hole”, and were flowing in south and southeast-directions along the deep “hole’s” west and southwest rim, which is today the Rocky Mountain crest and roughly parallels the present day east-west continental divide. Deep Marias River tributary valleys eroded headward in sequence from south to north and from the southeast to the northwest to capture the immense south and southeast-oriented floods. At the same time the immense south and southeast-oriented flood waters deeply eroded all drainage divide areas, with drainage divide areas on the plains being lowered the most, and flood waters on north ends of beheaded flood flow channels often reversed flow direction to erode what are today north-oriented valleys. Through valleys crossing present day drainage divides, including the east-west continental divide, and valley orientations are just a few of the many topographic map landscape features supporting this deep flood erosion hypothesis for the Marias River drainage basin landform origins.

Introduction:

  • The Marias River drainage basin landform origins overview essay is one of more than 40 such overview essays included in the Missouri River drainage basin landform origins research project. Each overview essay provides highlights from a group of much more detailed essays illustrating and describing topographic map evidence within the overview essay’s geographic region. The project goal is to use topographic map evidence to interpret landform origins along all major Missouri River drainage basin drainage divides. Overview essays are being developed for drainage divides contained within and/or adjacent to each major Missouri River tributary drainage basin and/or for all areas draining to the Missouri River in each of the states the Missouri River passes through. At the time this introduction is being written overview essays are available for all major Missouri River tributaries including Missouri River segments in the states of Missouri, Iowa, Kansas, Nebraska, South Dakota (and Minnesota), and North Dakota. In addition essays for most eastern and central Montana and northeast Wyoming Missouri River tributaries have been completed while essays for other Montana, Wyoming, and Colorado Missouri River drainage basin areas are incomplete and/or have yet to be published. The incomplete overview essays will be completed and/or the yet to be published overview essays will be published as more detailed essays describing Missouri River drainage basin drainage divide areas in those regions are written and published, which should be completed by the end of 2012 or by early 2013.
  • Topographic map evidence has been ignored by the geologic community when developing interpretations of North American geologic history, especially in regard to the origins of erosional landforms. The Missouri River drainage basin landform origins research project is the first known attempt to systematically use topographic map evidence to study and interpret erosional landform evidence for a large drainage basin. Interpretations presented in the Missouri River drainage basin landform origins research project essays are based entirely on the topographic map evidence observed and no effort has been made to reconcile those interpretations with prevailing geologic community interpretations, which are based on completely different types of evidence. The topographic map evidence observed is best interpreted in the context of the rapid melt down of a thick North American ice sheet, which was located in a deep “hole.” The upper Missouri River drainage basin in Montana and northern Wyoming and the Saskatchewan River drainage basin in southwest Alberta represent the deeply eroded southwest and west wall of the deep “hole” and the northern Montana Rocky Mountain crests including the east-west continental divide represents a segment of the deep “hole’s” western rim. Massive ice-marginal melt water floods once flowed in a south and southeast direction along what are today the crests of high Rocky Mountain ranges from Canada into and across western Montana and then into and across Wyoming and Colorado along a routes roughly corresponding with the present day continental divide. At that time the Rocky Mountains did not stand high above surrounding regions as they do today and flood waters could freely flow in southeast and south directions. Uplift of the Rocky Mountains occurred as the immense south and southeast-oriented flood waters flowed across them and proceeded from the south to the north. Rocky Mountain uplift combined with headward erosion of deep valleys from both the east and west diverted flood waters to the southeast and/or southwest and created the continental divide from south to north. Evidence presented in the Montana Marias River drainage basin landform origins overview essay illustrates how massive south and southeast-oriented flood flow flowed along and across the continental divide and how the continental divide was created by headward erosion of deep valleys from both the east and the west.

Marias River drainage basin location map

Figure 1: Marias River drainage basin location map showing a region in north central Montana with southern Alberta and the Saskatchewan southwest corner to the north. National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 1 provides a location map for the Marias River drainage basin in northern Montana. The United States-Canada border is located near the figure 1 north edge and the southwest corner of Saskatchewan is visible in the figure 1 northeast corner while west of Saskatchewan is southern Alberta. Glacier National Park can be seen south of the international border in the figure 1 northwest quadrant while Waterton Lakes National Park is located north of the border in the figure 1 northwest corner. The Missouri River flows in a north and northeast direction from the Gates of the Rocky Mountains (along figure 1 south center edge) to Great Falls, Fort Benton, and Loma before turning to flow in a south-southeast and east-northeast direction to the figure 1 east edge (south of center). The Marias River is one of two east-oriented tributaries joining the Missouri River near Loma. The Marias River is formed at the confluence of Cut Bank Creek and the Two Medicine River south and slightly east of Cut Bank, Montana and then flows in an east-southeast direction to Lake Elwell (also known as Tiber Reservoir). From Tiber Dam the Marias River flows in an east and south direction to join the east-oriented Teton River and then the northeast-oriented Missouri River. The east-oriented Teton River drainage basin is located south of the Marias River while the east-oriented Milk River drainage basin is located north of the Marias River drainage basin (with the Milk River flowing in a northeast direction into and across southern Alberta before turning to flow back into Montana and then to join the Missouri River east of the figure 1 map area). Each of these rivers has headwaters located along and/or near the east-west continental divide which extends in a south-southeast direction from the figure 1 northwest corner along the crest of the Lewis and Clark Range to the figure 1 south edge (west half). West of the continental divide is the Flathead River drainage basin with the northwest-oriented Middle Fork Flathead River and north-northwest oriented South Fork Flathead River flowing to the south-oriented Flathead River (west of figure 1), which flows through Flathead Lake (seen along the figure 1 west edge) to the northwest-oriented Clark Fork (just barely seen at Missoula in the figure 1 southwest corner). The northwest-oriented Middle Fork Flathead River and the north-northwest oriented South Fork Flathead River are flowing in valleys initiated as southeast and south-southeast oriented flood flow channels prior to being beheaded and reversed by Flathead River valley headward erosion. Headward erosion of the deep Flathead River valley (west of the present day continental divide) combined with headward erosion of deep Marias River tributary valleys (east of the continental divide) was responsible for creating the continental divide seen today. Major Marias River tributaries seen in figure 1  from north to south are Cut Bank Creek, the Two Medicine River and its Badger Creek and Birch Creek tributaries, and Pondera Coulee. The unlabeled Birch Creek tributary flowing through the town of Dupuyer is Dupuyer Creek, which will be seen in topographic maps illustrated in this overview essay.

North Fork Cut Bank Creek-Dry Fork Two Medicine River drainage divide area

Figure 2: North Fork Cut Bank Creek-Dry Fork Two Medicine River drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

This summary of the detailed essays in the Marias River drainage basin landform origins essays (found under Marias River on the sidebar category list) begins with a look at the North Fork Cut Bank Creek-Dry Fork Two Medicine River drainage divide in Glacier National Park. The Flathead County-Glacier County boundary seen in the figure 2 west half is the location of the continental divide with drainage in Glacier County flowing to the Missouri River and eventually reaching the Gulf of Mexico and drainage in Flathead County flowing to the Flathead River and eventually reaching the Pacific Ocean. Figure 2 landscapes have been shaped by alpine glaciation, although prior to the alpine glaciers the valleys existed and the valley pre glaciation origins are emphasized in the detailed essays. Through valleys (today mountain passes or saddles crossing high mountain ridges) can be seen along many of the present day drainage divides. For example the North Fork Cut Bank Creek originates at Pitamakan Lake (in figure 2 southwest quadrant) and flows in a north and northeast direction to the figure 2 north center edge and then in an east-southeast and east direction to the figure 2 east edge (near northeast corner). South of Pitamakan Lake is Oldman Lake where the east-oriented Dry Fork Two Medicine River begins. The Two Medicine River flows in a northeast direction from Two Medicine Lake to Lower Two Medicine Lake where it turns and flows in southeast direction to the figure 2 south edge (east half). Note how there is a through valley (known as Pitamakan Pass) eroded into the high ridge which separates Pitamakan Lake from Oldman Lake. The figure 2 map contour interval is 50 meters and the through valley floor at the pass appears to be 2300 meters. Peaks on either side of the through valley or mountain pass rise to elevations greater than 2500 meters meaning the through valley is at least 250 meters deep. The through valley was eroded by a south-oriented flood flow channel prior to headward erosion of the deep east-oriented North Fork Cut Bank Creek valley. At that time there was no North Fork Cut Bank Creek valley and south-oriented flood waters were flowing to what was then the actively eroding Dry Fork Two Medicine River valley. Headward erosion of the deep east-oriented North Fork Cut Bank Creek valley beheaded the south-oriented flood flow channel. Flood waters on the north end of the beheaded flood flow channel reversed flow direction to erode the north-oriented North Fork Cut Bank Creek valley segment. Subsequently Rocky Mountain uplift, which was probably occurring as flood waters flowed across the region, and alpine glaciation further modified the figure 2 landscape. This interpretation requires the alpine glaciation to have occurred after the thick North American ice sheet rapid melt down had ended. The rapid melt down ended when immense south-oriented floods to the Gulf of Mexico were diverted to flow in northeast and north directions and significantly changed the northern hemisphere climate. This climate change combined with the newly uplifted and deeply eroded mountains seen in figure 2 created ideal conditions for alpine glacier development.

Two Medicine River-Mission Lake drainage divide area

Figure 3: Two Medicine River-Mission Lake drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

Proceeding east and north of the figure 2 map area onto the Montana plains we come to the figure 3 map area which illustrates the Two Medicine River-Mission Lake drainage divide area. The Two Medicine River flows from the figure 3 west edge (just north of southwest corner) in a northeast, east, and northeast direction before turning to flow in a southeast direction (with some jogs) to the figure 3 east edge (just north of the southeast corner). The east-northeast oriented stream joining the Two Medicine River in the figure 3 southeast corner is Badger Creek, which will be seen again south of the figure 3 map area in figure 4 and headwaters of which will be seen in figures 5 and 6. Cut Bank Creek can just barely be seen flowing in a northeast direction across the figure 3 northwest corner and then flows in an east direction north of the figure 3 map area before turning to flow in a south-southeast direction east of figure 3 (see figure 1). Mission Lake is a prominent feature in the figure 3 northeast quadrant and is drained by northeast-oriented Flat Coulee, which drains to east-oriented Spring Creek which in turn flows to south-southeast oriented Cut Bank Creek (east of the figure 3 map area). Note how the Two Medicine River valley and the Mission Lake valley are linked by well-defined northeast oriented through valleys. The figure 3 map contour interval is 20 meters and the deepest through valley is approximately 100 meters deep while the much broader through valley to the west and northwest is at least 20 meters deep and may be much deeper. These through valleys are water eroded features and were eroded by massive east-oriented flood flow moving to what was then the actively eroding south-southeast oriented Cut Bank Creek valley. At the time flood waters were flowing in a northeast direction from the Two Medicine River valley to the Mission Lake through valleys the flood waters were also flowing in a southeast direction along the present day Two Medicine River alignment. In time the present day Two Medicine River valley was eroded deeper than the diverging northeast-oriented flood flow channels (now through valleys) to the Mission Lake valley. First flood flow to the broader northeast-oriented through valley was beheaded and later flood flow to the narrower and deeper through valley was beheaded ending flood flow from the Two Medicine River valley to the Mission Lake valley. Also as headward erosion of the deep Two Medicine River valley was proceeding headward erosion of the Cut Bank Creek valley north of the figure 3 map area was occurring (although behind the Two Medicine River valley headward erosion). As seen in figure 2 eventually Cut Bank Creek headward erosion beheaded all southeast and south-oriented flood flow to the newly eroded Two Medicine River valley and what were then it’s actively eroding tributary valleys.

Badger Creek-Blacktail Creek drainage divide area on the Montana plains

Figure 4: Badger Creek-Blacktail Creek drainage divide area on the Montana plains. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

Before leaving the Montana plains let us look at Badger Creek-Blacktail Creek drainage divide area just south of the figure 3 map area. Badger Creek flows in a northeast direction from the figure 4 west center edge area to the figure 4 north edge (west of center) and north of the figure 4 map area joins the Two Medicine River as seen in figure 3. Blacktail Creek flows in a northeast direction from the figure 4 south center edge and in the figure 4 east center area turns to flow in an east direction to the figure 4 east center edge. East of figure 4 Blacktail Creek joins north-northeast oriented Birch Creek which flows to the Two Medicine River which in turn joins Cut Bank Creek to form the Marias River. Study of figure 4 reveals some remarkable through valleys crossing the Badger Creek-Blacktail Creek divide. One north-south oriented through valley is located along the county line seen in the figure 4 northeast quadrant. Perhaps the most interesting through valleys are associated with the Four Horse Lake basin seen near the figure 4 center. A large north-south oriented through valley links the Four Horse Lake basin with the northeast-oriented Badger Creek valley, The through valley north end is drained in the north direction by a north-oriented Badger Creek tributary and the south end is drained by a south-oriented stream, although there is no continuous drainage in the through valley today. Four Horse Lake today drains in a southeast direction along the Big Plum Coulee alignment to Blacktail Creek. North-northeast oriented Jackson Coulee is the major Four Horse Lake inlet from the south. Through valleys link the north-northeast oriented Jackson Coulee valley with the east-oriented Feather Coulee valley (near figure 4 south edge) which drains to northeast-oriented Blacktail Creek. Also through valleys can be see linking the north-northeast oriented Whitetail Creek valley (a Badger Creek tributary flowing from the figure 4 southwest corner) with the Four Horse Lake basin. These and many other through valleys seen in figure 4 provide evidence of multiple diverging and converging flood flow channels which once crossed the figure 4 map area. In general flood waters were flowing in south and east directions and were being captured by headward erosion of deep northeast-oriented valleys in sequence from the southeast to the northwest. At one time the Badger Creek-Four Horse Lake through valley was a south-oriented flood flow channel and the north-northeast oriented Jackson Coulee valley was also a south-oriented flood flow channel. Headward erosion of the deep northeast-oriented Badger Creek valley beheaded the south-oriented flood flow. Flood waters on the north end of the beheaded flood flow reversed flow direction to erode the north oriented valleys. Probably some type of earth movement blocked the low gradient north-oriented valley to create Four Horse Lake and to divert the new north-oriented water flow in a southeast direction along the Big Plum Coulee valley.

South Fork Two Medicine River-Badger Creek drainage divide area

Figure 5: South Fork Two Medicine River-Badger Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

Figure 5 illustrates the South Fork Two Medicine River drainage basin area south and slightly east of the figure 2 map area. The east-west continental divide is the dashed line crossing the figure 5 southwest quadrant from the west edge to the south edge. The South Fork Two Medicine River begins as a south-oriented stream (Whiterock Creek) just west of the figure 5 center and then turns to flow in a northwest direction to the figure 4 west edge (north half). West of figure 5 the South Fork Two Medicine River makes an abrupt turn to flow in a northeast direction and can just barely be seen crossing the figure 5 northwest corner. A northeast-oriented South Fork Two Medicine River tributary is located just north of the figure 5 northwest corner and originates at Marias Pass, which is a deep through valley eroded across the continental divide. Note how the northwest-oriented South Fork Two Medicine River flows in the northwest end of a large northwest-southeast oriented through valley which is drained at its southeast end by a southeast-oriented North Badger Creek segment. North Badger Creek flows in a north-northeast direction from the figure 5 south center edge into the northwest-southeast oriented through valley and just to the southeast of the South Fork Two Medicine River elbow of capture (where it changes from being south-oriented to being northwest-oriented) North Badger Creek turns abruptly to flow in a southeast direction and then after being joined by north and north-northwest oriented tributaries turns to flow in north-northeast and north directions. The figure 5 contour interval is 50 meters and the floor of the deep through valley linking the northwest-oriented South Fork Two Medicine River segment with the southeast-oriented North Badger Creek segment has an elevation of between 1700 and 1750 meters. Goat Mountain near the figure 5 south center edge rises to 2496 meters while Half Dome Crag rises to more than 2400 meters suggesting the through valley is at least 650 meters deep. There is no noticeable rise between the northwest-oriented South Fork Two Medicine River and a southeast-oriented North Badger Creek tributary. The through valley was initiated as a southeast-oriented flood flow channel and was captured by headward erosion of the deep Badger Creek valley, which beheaded and reversed a south-oriented flood flow channel on the present day north-oriented North Badger Creek alignment. The resulting reversal of flood flow eroded not only the north-oriented North Badger Creek valley but also valleys of its north and north-northwest oriented tributaries. Next the southeast-oriented flood flow channel was beheaded and reversed by headward erosion of the northeast-oriented South Fork Two Medicine River valley north of the figure 4 map area. This valley may have at one time been a southwest-oriented flood flow channel to the actively eroding Flathead River valley prior to being beheaded and reversed by Two Medicine River valley headward erosion to create the present day northeast oriented valley, which captured the northwest-oriented flow in the present day northwest-oriented South Fork Two Medicine River valley segment.

Muskrat and Badger Passes, two adjacent through valleys crossing the continental divide

Figure 6: Muskrat and Badger Pass, two adjacent through valleys crossing the continental divide.United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

 

Figure 6 illustrates the Muskrat Pass and Badger Pass region south and east of the figure 5 map area and shows two deep through valleys eroded across the east-west continental divide. The Pondera-Flathead County border in the figure 6 northwest quadrant and the Teton-Flathead County border in the figure 6 southeast quadrant are defined by the east-west continental divide. The south oriented stream flowing to the figure 6 south center edge is Strawberry Creek, which south of the figure 6 map area turns to flow in a west direction and joins other streams to form the northwest-oriented Middle Fork Flathead River (seen crossing the figure 6 southwest corner). The Middle Fork Flathead River flows to the south oriented Flathead River with water eventually reaching the Pacific Ocean. North of the Strawberry Creek headwaters are headwaters of northwest-oriented South Badger Creek, which flows to the figure 6 north edge (west half) and then to northeast-oriented Badger Creek. Note how a well-defined through valley (Badger Pass) links the northwest-oriented South Badger Creek headwaters with the south oriented Strawberry Creek headwaters. The through valley is a water eroded feature and was eroded by south oriented flood flow prior to headward erosion of the northeast-oriented Badger Creek valley, which beheaded and reversed the flood flow to erode the northwest-oriented South Badger Creek valley and to create the continental drainage divide. The figure 6 map contour interval is 50 meters and the through valley floor elevation at the continental divide is between 1900 and 1950 meters. Cap Mountain to the west rises to more than 2250 meters and Mount Field to the east rises even higher. In other words the through valley is at least 300 meters deep. Just to the west of Badger Pass is another even deeper through valley (Muskrat Pass) crossing the continental divide. Muskrat Creek flows in a northwest direction from this second through valley and joins northeast-oriented Elbow Creek (near figure 6 northwest corner) which then joins South Badger Creek. South of Muskrat Pass are headwaters of south and southwest oriented Cox Creek which joins the northwest-oriented Middle Fork Flathead River near the figure 6 southwest corner. The Muskrat Pass elevation at the continental divide appears to be between 1800 and 1850 meters with elevations greater than 2250 meters being found on both sides of the valley. This 400-meter deep through valley was also eroded by south oriented flood flow which was first captured by headward erosion of the deep northwest-oriented Middle Fork Flathead River valley. The northwest-oriented Middle Fork Flathead River valley was eroded by a reversal of a southeast-oriented flood flow channel which was beheaded and reversed by headward erosion of the deep south-oriented Flathead River valley. Headward erosion of the deep northeast-oriented Elbow Creek valley subsequently beheaded south-oriented flood flow in the Muskrat Pass through valley. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Muskrat Creek valley. The multiple south-oriented through valleys crossing the continental divide suggests these were channels in a much larger south- and southeast-oriented anastomosing channel complex, which once moved flood waters across a much larger region (than figure 6).

Dupuyer Creek-Blacktail Creek drainage divide area along the mountain front

Figure 7: Dupuyer Creek-Blacktail Creek drainage divide area along the mountain front. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

 

Figure 7 illustrates the Dupuyer Creek-Blacktail Creek drainage divide area along the mountain front south and east of the figure 6 map area (the Blacktail Creek in figure 7 and the Blacktail Creek in figure 4 are different streams and are located in different regions). The North Fork Teton River can be seen flowing in a south-southeast direction in the figure 7 southwest corner and is linked by a through valley with a north-northwest oriented Birch Creek tributary (not seen in maps illustrated in this overview essay). Dupuyer Creek is a north-northeast and northeast oriented Birch Creek tributary and flows to the figure 7 north center edge and is formed at the confluence of its South, Middle, and North Forks near the southeast corner of the figure 7 northwest quadrant. North and east of the figure 7 map area Dupuyer Creek eventually flows to Birch Creek, which then joins the Two Medicine River, which then joins Cut Bank Creek to form the Marias River. The South Fork Dupuyer Creek flows in a north, east and north-northeast direction west and north of Mount Frazier (in the figure 7 southwest quadrant). The Middle Fork Dupuyer Creek originates east of Old Man in the Hills (a mountain north of Mount Frazier) while the North Fork Dupuyer Creek originates in the figure 7 west center edge area and flows in an east, northeast, and east direction. Note how North Fork Dupuyer Creek tributaries from the north are oriented in south-southeast directions while tributaries from the south begin as north-northwest oriented streams. These valley orientations suggest the deep North Fork Dupuyer Creek valley eroded headward across south-southeast oriented flood flow channels and flood waters on north ends of the beheaded flood flow channels reversed flow direction to erode the north-northwest oriented valleys. Blacktail Creek flows in an east and northeast direction through Blackleaf Canyon to the figure 6 south center region and then turns to flow in an east-southeast direction to the figure 7 east edge (near southeast corner). East of the figure 7 map area Blacktail Creek eventually flows to the Teton River. Note the southeast-oriented Blacktail Creek tributaries in the figure 7 south center area. These tributaries include Gansman Coulee, Toms Coulee, Pings Coulee, and Hoy Coulee and they originate in a large northwest-southeast oriented through valley linking the north-northeast oriented South Fork Dupuyer Creek valley with the east-oriented Blacktail Creek valley. The large through valley is evidence of massive southeast-oriented flood flow from the mountain region to the west and northwest to what was at one time the actively eroding and deep east-oriented Blacktail Creek valley. At that time the deep Dupuyer Creek valley and its deep North, Middle, and South Fork valleys did not exist, nor did other deep east and northeast oriented valleys exist in the region to the northwest. Flood waters were freely moving in a southeast direction across a region now characterized by high mountain ridges. The mountains were deeply eroded as deep northeast and east-oriented valleys eroded headward into the region to capture the southeast-oriented flood flow. The multiple southeast-oriented coulee valleys on the through valley floor today probably are relics of a southeast-oriented anastomosing channel complex that once existed on the through valley floor, although study of the region in the more detailed essays reveals evidence for a much larger scale anastomosing channel complex in the region. The topographic maps illustrated in this Marias River drainage basin landform origins overview essay provide only a sample of the evidence illustrated in the more detailed essays which are listed under Marias River on the sidebar category list. And the detailed essays only provide a sample of the topographic map evidence available to anyone willing to take the time to study detailed topographic maps of the Marias River drainage basin.

Additional information and sources of maps studied

This overview 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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