Montana Sun River drainage basin landform origins, overview essay

A geomorphic history based on topographic map evidence

Abstract:

This essay is an overview of six essays describing drainage divide areas in and surrounding the Montana Sun River drainage basin. Essays are based on Sun River drainage basin topographic map evidence and are listed under the Sun River category (see sidebar category list). The Sun River is formed at the confluence of the south-southeast oriented North Fork Sun River and the north-oriented South Fork Sun River, both of which originate along the east-west continental divide in the Lewis and Clark Range, and flows in an east direction onto the plains to join the Missouri River at Great Falls, Montana. Through valleys cross all major drainage divides, including the continental divide, and provide evidence of multiple south and southeast-oriented flood flow channels, such as might be found in a large anastomosing channel complex, which once crossed the Sun River drainage basin (including areas that today are high mountain regions). Erosional landforms in the Sun River drainage basin are interpreted to have been eroded as the much deeper Sun River valley eroded headward to capture massive south and southeast-oriented flood flow. Flood waters on north ends of beheaded flood flow routes reversed flow direction to erode north-oriented Sun River valley tributary valleys. Headward erosion of the east-oriented Teton River north of the east-oriented Sun River valley subsequently beheaded south and southeast-oriented flood flow to the newly eroded Sun River valley while headward erosion of the much deeper south- and west-oriented Flathead River valley in the west beheaded southeast-oriented flood flow from west of the present day continental divide, which had been moving to the actively eroding south-southeast oriented North Fork Sun River valley. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode the northwest-oriented Middle Fork Flathead River valley and to create the drainage divide at Sun River Pass across the east-west continental divide, which is today the Flathead River-Sun River drainage divide. Based on topographic map evidence illustrated and discussed in hundreds of other Missouri River drainage basin landform origins research project essays flood waters are interpreted to have been derived from a rapidly melting thick North American ice sheet.

Introduction

This essay provides a summary of six detailed essays illustrating and describing interpretations of Montana Sun River drainage basin landform origins based on topographic map evidence. Essays in this Sun River drainage basin landform origins essay collection and this overview essay are components of the Missouri River drainage basin landform origins research project in which essays illustrating and describing landform origins for all Missouri River drainage basin drainage divide areas are being written. At the time this essay is being written more than 390 essays illustrating and describing Missouri River drainage divide areas in the states of Missouri, Kansas, Nebraska, Iowa, Minnesota, South Dakota, North Dakota, eastern and central Montana, and northeast Wyoming have been written. Essays illustrating and discussing remaining Missouri River drainage basin drainage divide areas in Montana, Wyoming, and Colorado are planned and the project should be completed some time in late 2012 or early 2013. Topographic map evidence illustrated in the previously written essays has demonstrated all Missouri River drainage basin erosional landforms were created during immense south and southeast-oriented melt water floods from a rapidly melting thick North American ice sheet. This thick North American ice sheet was located in a deep “hole” with the Missouri River drainage basin in Montana and northern Wyoming represents the deep “hole’s” deeply eroded southwest wall. The Canadian Rocky Mountains are located along what was once the deep “hole’s” western rim and high Montana and Wyoming mountain ranges are located along what was once the deep “hole’s” southwestern rim. Immense south and southeast-oriented meltwater floods flowed along the deep “hole’s” western and southwestern rim at a time when the present day mountain ranges were being uplifted. Uplift of the mountains was probably caused by crustal warping related to the thick North American ice sheet’s tremendous weight. As mountain uplift was occurring huge melt water floods flowed along routes roughly corresponding with the present day east-west continental divide. The continental divide further to the south in New Mexico, Colorado, and southern Wyoming was carved as deep valleys from both the east and the west eroded headward into the present day mountain ranges (in sequence from south to north) to capture the massive south-oriented melt water floods.

As ice sheet melting and as uplift of Montana and Wyoming mountain areas progressed the melt water flood flow pattern changed. Large supra-glacial melt water rivers carved giant south-oriented ice-walled canyons into the decaying ice sheet’s surface and supplied tremendous volumes of water to the rapidly developing Mississippi River drainage system in the central United States. The floors of these giant ice-walled canyons over time became significantly lower in elevation than areas along the deep “hole’s” southwestern rim. Of particular importance to Montana and northern Wyoming was a giant southeast and south-oriented ice-walled canyon in present day Saskatchewan, North Dakota, and South Dakota, which over time became an ice-walled and bedrock-floored canyon and which detached the ice sheet’s southwest margin. Today the northeast and east-facing Missouri Escarpment in Saskatchewan, North Dakota, and South Dakota is what remains of this giant ice-walled and bedrock-floored canyon’s southwest and west wall. Large northeast and east-oriented valleys eroded headward from this giant southeast and south-oriented ice-walled canyon into Montana and northern Wyoming to capture the immense southeast oriented ice-marginal melt water floods flowing at much higher elevations. These valleys eroded headward in sequence from the southeast to the northwest, with each valley beheading flood flow routes to newly eroded valleys to the southeast. The Missouri River valley and tributary valleys eroded headward from this giant ice-walled canyon and beheaded flood flow routes to the newly eroded Yellowstone River valley (Missouri River tributary valleys were also eroded in sequence from south to north). These deep northeast and east-oriented valleys eroding headward from the giant ice-walled canyon were much deeper than the southeast and south-oriented flood flow channels they captured. As a result flood waters on north and northwest ends of beheaded flood flow channels often reversed flow direction to erode what are today north and northwest-oriented tributary valleys.

The Sun River is an east-oriented Missouri River tributary and originates along the east-west continental divide in the high Lewis and Clark Mountain Range and then flows onto the Montana plains where it joins the Missouri River at Great Falls. Topographic map evidence demonstrates Sun River valley headward erosion beheaded south and southeast-oriented flood flow routes to the Dearborn River and to Dearborn River tributaries (the Dearborn River is a Missouri River tributary south of the Sun River). Topographic map evidence also demonstrates headward erosion of the Teton River valley and Teton River tributary valleys north of the Sun River drainage basin beheaded south and southeast-oriented flood flow routes to the Sun River valley and its tributary valleys. Perhaps even more intriguing is topographic map evidence demonstrating that for a time significant flood flow to the Sun River drainage basin came from west of the east-west continental divide. In fact topographic map evidence demonstrates prior to headward erosion of the present day deep valleys (both east and west of the continental divide) the Sun River headwaters region east of the continental divide and the adjacent Middle and South Fork Flathead River drainage area west of the continental divide were crossed by numerous diverging and converging south-oriented anastomosing flood flow channels, which were systematically dismembered as headward erosion of deep valleys from both directions carved the continental divide. The topographic map evidence also demonstrates massive flood flow reversals occurred as headward erosion of much deeper valleys beheaded the south-oriented flood flow channels. This topographic map evidence is readily available where anyone who is willing to look can see it, but to my knowledge the evidence has been completely ignored and has never been explained. Essays in the Missouri River drainage basin landform origins research project are to my knowledge the first attempt to systematically study and explain detailed topographic map evidence for a large drainage basin.

Montana Sun River drainage basin location map

Figure 1: Montana Sun River drainage basin location (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 1 provides a location map for the Montana Sun River drainage basin and illustrates a region in northern Montana. The Canadian border is located along the figure 1 north edge and Glacier National Park is the green area located in the figure 1 northwest corner. The Missouri River flows in a north-northwest direction from the figure 1 south center edge (just east of Helena) to Wolf Creek and then turns to flow in a northeast direction to Great Falls, Fort Benton, and Loma. South of Big Sandy the Missouri River makes an abrupt turn to flow in south-southeast and east direction to the figure 1 east center edge. The east-west continental divide (not labeled in figure 1) extends in a south-southeast direction from the figure 1 northwest corner through Logan Pass and Marias Pass (in Glacier National Park) and then along or near the Lewis and Clark Range crest to the figure 1 south edge just west of Helena. The Sun River (as shown in figure 1) originates along the east-west continental divide near the north end of the Lewis and Clark Range and flows in a south-southeast direction to Gibson Reservoir where it turns to flow in an east direction to join the Missouri River at Great Falls. East of the Sun River headwaters are headwaters of the Teton River (as shown in figure 1) and the Teton River flows in an east direction to join the Missouri River near Loma. Drainage divide areas in the Teton River drainage basin are illustrated and described in essays listed under the Teton River category (see sidebar category list). North of the Teton River is the east- and south-oriented Marias River, which also joins the Missouri River (and Teton River) near Loma. Drainage divide areas in the Marias River drainage basin are illustrated and described in the under the Marias River category. Marias River and Teton River tributaries and/or headwaters also originate along the east-west continental divide. South of the Sun River is the Dearborn River drainage basin, with the Dearborn River also originating along the continental divide and flowing in a southeast, east-northeast, and southeast direction to join the northeast-oriented Missouri River. Drainage divide areas in the Dearborn River drainage basin are listed under the Dearborn River category. West of the Sun River drainage basin (and the continental divide) is the north-northwest oriented South Fork Flathead River, which flows to the south-oriented Flathead River west of the figure 1 map area. North of the Sun River headwaters in figure 1 are headwaters of the northwest-oriented Middle Fork Flathead River, which also west of the figure 1 map area joins the south-oriented Flathead River.

Sun River Pass across the east-west continental divide

Figure 2: Sun River Pass across the east-west continental divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 2 illustrates the Sun River Pass region at the northwest end of the Sun River drainage basin. The east-west continental divide is labeled and extends in a southwest direction from the figure 2 northeast corner to Sun River Pass and then to Kevan Mountain where it then follows the high ridge in a south direction to the figure 2 south edge (west of center). The south-oriented stream originating near Sun River Pass is Fool Creek, which joins east-oriented Open Creek and south-southwest oriented McDonald Creek to form the North Fork Sun River (not labeled in figure 2) which flows the figure 2 south edge (east half). The northwest-oriented stream flowing from north of Sun River Pass to the figure 2 north edge (west of center) is Bowl Creek, which north of figure 2 flows to the northwest-oriented Middle Fork Flathead River. North of Bowl Creek in the through valley extending north from Sun River Pass is a north-northwest oriented segment of South Fork Trail Creek, which flows to north-northwest oriented Trail Creek, which in turn flows to south-oriented Strawberry Creek, which then turns to flow in a west direction to join the northwest-oriented Middle Fork Flathead River. I point these northern valleys out to emphasize Sun River Pass is a north-south oriented through valley linking other north-south oriented through valleys and is also directly linked to the northwest-oriented Middle Fork Flathead River valley. The figure 2 map contour interval is 50 meters and the Sun River Pass through valley is defined by six or more contour lines depending on where the valley walls are defined. This 300 meter or deeper through valley crossing the continental divide is a significant erosional landform and needs to be explained. The Sun River Pass through valley was eroded as a south-oriented flood flow channel which was fed by two converging flood flow channels from the north. One of the converging flood flow channels was on the alignment of present day northwest-oriented Bowl Creek and its northwest extension along the northwest-oriented Middle Fork Flathead River alignment. The other converging flood flow channel was on the alignment of present day south-oriented Strawberry Creek and the north-northwest oriented Trail Creek and South Fork Trail Creek segments. The south-oriented flood flow was moving to what was then the newly eroded and deep east-oriented Sun River valley south of the figure 2 map area. The deep Sun River valley knick point eroded headward along what is now the south-southeast oriented North Fork Sun River-Fool Creek valley and then north across Sun River Pass and northward along each of the two converging flood flow channels. Headward erosion of the deep south-oriented Flathead River valley west and north of the figure 2 map area beheaded the southeast-oriented flood flow channel on the Middle Fork Flathead River and Bowl Creek alignment. A massive flood flow reversal took place and flood waters on the northwest end of the beheaded flood flow channel reversed flow direction as the deep Flathead River valley knick point eroded headward along what became the northwest-oriented Middle Fork Flathead River-Bowl Creek valley. This deep northwest-oriented valley next captured south-oriented flood flow on the Strawberry Creek alignment (north of figure 2) and headward erosion of the deep knick point along the Strawberry Creek valley beheaded and reversed south-oriented flood flow on the Trail Creek and South Fork Trail Creek alignment to erode the north-oriented Trail Creek and South Fork Trail Creek valley segments north of Sun River Pass. Remember the water that eroded the south-southeast oriented North Fork Sun River valley came from west of the continental divide.

Confluence of North and South Forks Sun River near Medicine Springs

Figure 3: Confluence of North and South Forks Sun River near Medicine Springs. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 3 illustrates the North Fork Sun River-South Fork Sun River confluence area near Medicine Springs and is located south and slightly east of the figure 2 map area (there is a significant gap between figures 2 and 3). Medicine Springs is located near the figure 3 center. The Sun River flows in a southeast direction from Medicine Springs to Gibson Dam, which has flooded the southeast-oriented Sun River valley with Gibson Reservoir. East of Gibson Dam the Sun River flows in an east-northeast direction to the figure 3 east center edge. The North Fork Sun River flows in a south-southeast direction from the figure 3 north edge (west of center) to join the north-northeast oriented South Fork Sun River at Medicine Springs. Just west of Medicine Springs is Sun Butte and just west of Sun Butte is a north-south-oriented through valley linking the south-oriented North Fork Sun River valley with the north-oriented South Fork Sun River valley. The figure 3 map contour interval is 50 meters and Sun Butte by three contour lines. This north-south oriented through valley is a water eroded valley and is a continuation of the through valley at Sun River Pass seen in figure 2. What we are seeing in figure 3 is where the deep east-oriented Sun River valley eroded headward into the figure 3 map area to capture immense south-oriented flood waters including south-oriented flood flow channel on the present day North Fork Sun River and South Fork Sun River alignments. At that time flood waters were flowing on a high level erosion surface, probably as high as the figure 2 mountain ridges seen today. Headward erosion of the much deeper east-oriented Sun River valley captured the south-oriented flood flow and deep south-oriented valleys eroded headward from the newly eroded and deep Sun River valley north wall. At the same flood waters on north ends of beheaded flood flow routes reversed flow direction to erode north-oriented Sun River tributary valleys. These flood flow reversals occurred in sequence from east to west meaning a newly reversed flood flow channel could capture significant yet to be beheaded flood flow from south-oriented flood flow routes further to the west. The deep north- and south-oriented Sun River tributary valleys east of Medicine Springs as well as the deep North and South Fork Sun River valleys were eroded by the processes described here. Water that eroded the large and deep south-oriented North Fork Sun River valley came from west and north of the present day continental divide and eroded a large valley. The north-oriented South Fork Sun River valley is somewhat narrower, but after being beheaded and reversed by Sun River valley headward erosion it was able to capture south-oriented flood flow from further to the west and today has a number of south-oriented tributaries from the west, some of which originate along the continental divide and which provide evidence a major flood flow reversal took place.

Straight Creek Pass between South Fork Sun River and Dearborn River drainage basins

Figure 4: Straight Creek Pass between South Fork Sun River and Dearborn River drainage basins. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Figure 4 is located south and east of the figure 3 map area and illustrates Straight Creek Pass, which is a northwest-southeast oriented through valley linking the northwest-oriented Straight Creek valley with the southeast-oriented Dearborn River headwaters valley. Straight Creek Pass is located near the figure 4 center just north of Red Slide Mountain and Straight Creek is the stream flowing in a northwest direction from Straight Creek Pass to the figure 4 north edge (west half). North of figure 4 Straight Creek joins the north-oriented South Fork Sun River, which can be seen flowing in a north direction near the west margin of the figure 4 northwest quadrant. The east-west continental divide is located a short distance west of the figure 4 west edge. Welcome Creek flows in a southeast direction from Straight Creek Pass and joins the southeast-oriented Dearborn River, which flows to the figure 4 south edge (east half). The Dearborn River originates along the northeast face of Scapegoat Mountain and flows in a northeast, southeast, and northeast direction to join southeast-oriented Welcome Creek and then to flow in a southeast direction. Note how between Halfmoon Peak there is a northeast oriented Straight Creek tributary, which is linked by a northwest-southeast oriented through valley with the northeast, southeast, and northeast oriented Dearborn River headwaters valley. That through valley is approximately the same elevation than the Straight Creek Pass through valley to the northeast. The figure 4 map contour interval is 50 meters and the two through valley floors have elevations of between 2000 and 2050 meters. The two through valleys were eroded by diverging and converging southeast-oriented flood flow channels, which eroded headward from the deep Dearborn River valley, and which had eroded headward into what was probably a rising mountain mass to capture immense south and southeast-oriented flood flow. These two through valleys provide evidence of a continuous north-northwest to south-southeast oriented flood flow channel extending from the northwest-oriented Middle Fork Flathead River valley to the southeast-oriented Dearborn River valley. This flood flow channel was dismembered first by headward erosion of the much deeper east-oriented Sun River valley and later by headward erosion of the still deeper south-oriented Flathead River valley. Note Welcome Pass just north of where Welcome Creek joins the Dearborn River. Welcome Pass is a through valley linking the northeast-oriented Dearborn River headwaters with north-northeast and east oriented Moudess Creek, which flows to north-northeast oriented Smith Creek, which flows to the east-oriented Sun River. The north-northeast oriented Smith Creek and Moudess Creek valley segments were probably eroded by reversals of flood flow on north ends of flood flow routes beheaded by headward erosion of the deep east-oriented Sun River valley and its northeast- and east-oriented Smith Creek tributary valley (both north of the figure 4 map area). For a time this reversal of flood flow captured some of the southeast-oriented flood flow moving to the actively eroding Dearborn River valley and the captured flood waters moved in a north-northeast direction to the actively eroding Smith Creek and Sun River valleys. Headward erosion of the deep southeast-oriented Dearborn River valley and its Welcome Creek tributary valley in time beheaded this north-northeast flood flow.

Figures 2, 3, and 4 provide a small sample of the topographic map evidence illustrated in more detailed essays describing specific drainage divide areas in the western Sun River drainage basin, or that portion of the Sun River drainage basin located in the high mountain regions near and/or adjacent to the east-west continental divide. The more detailed essays also illustrate and describe only a small sample of the western Sun River drainage basin topographic map evidence available for investigation. Figure 5 and 6 below illustrate a small sample of the topographic map evidence found in the eastern Sun River drainage basin, where the Sun River is flowing on the plains east of the mountains. Again the topographic map evidence demonstrates immense south and southeast-oriented floods flowed across the plains east of the mountains and deeply eroded drainage divide areas between the Sun River valley and the Dearborn River-Missouri River valleys to the south and the Teton River valley to the north. In other words, the topographic evidence demonstrates the Sun River valley and its tributary valleys eroded headward to capture massive south and southeast-oriented flood flow and beheaded flood flow to what were then newly eroded valleys to the south.

Sun River-Little Muddy Creek drainage divide area

Figure 5: Sun River-Little Muddy Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Proceeding north and east from figure 4 onto the Montana plains the figure 5 map area illustrates the Sun River-Little Muddy Creek drainage divide near Crown, Shaw, and Square Buttes. Simms is the small town in the figure 5 northwest quadrant and Fort Shaw is the small town just south of the figure 5 north center edge. The Sun River flows in an east direction along the north margin of the figure 5 northwest quadrant and Simms and Fort Shaw are located in the Sun River valley south of the river. Little Muddy Creek flows in an east-northeast direction from the figure 5 west edge (south half) toward Crown Butte and then flows south of Crown Butte in an east, southeast, and east direction to the figure 5 southeast corner. South and east of figure 5 Little Muddy Creek flows in a southeast direction to join the northeast-oriented Missouri River. Crown Butte Creek flows from the figure 5 west edge (just south of center) in an east direction toward Crown Butte and then turns to flow in a northeast, south-southeast, and southeast direction around the Crown Butte north end to join Little Muddy Creek. Note how south of Simms there is a through valley linking the Sun River valley with the south-southeast oriented Crown Butte Creek valley. But even more impressive are the much deeper through valleys eroded between Crown Butte and Shaw Butte and between Shaw Butte and Square Butte. The figure 5 map contour interval is again 50 meters and the tops of Crown Butte, Shaw Butte, and Square Butte have similar elevations in the 1350 to 1450 meter range. The through valleys between the buttes have floors in the 1100 to 1150 meter range meaning the buttes rise approximately 200 meters above the valley floors, or the valley floors are approximately 200 meters lower than their adjacent valley walls. The buttes are erosional residuals and provide evidence of a one time erosion surface at least as high as the tops of the buttes today. Prior to headward erosion of the deep east-oriented Sun River valley that high level erosion surface was eroded by south- and southeast-oriented flood flow moving to what was then the actively eroding northeast-oriented Missouri River valley (south and east of figure 5). The deep Little Muddy Creek valley eroded headward from that actively eroding Missouri River valley to capture the south and southeast oriented flood flow. Headward erosion of the deep east-oriented Sun River valley then captured the south-oriented flood flow and beheaded the south-oriented flood flow routes to the newly eroded Little Muddy Creek valley. North-oriented Sun River tributary valleys (such as the north-oriented Abode Creek valley segment south of Fort Shaw) were eroded by reversals of flood flow on north ends of the beheaded flood flow routes.

TL Gap between Teton River and Sun River drainage basins

Figure 6: TL Gap between Teton River and Sun River drainage basins. United States Geological Survey map digitally presented using National Geographic Society TOPO software.

Moving north and slightly west of the figure 5 map area and of the Sun River figure 6 will conclude this summary essay by illustrating the Teton River-Sun River drainage divide area at TL Gap. Figure 6 is a reduction of a detailed topographic map and the contour interval is 10 feet. The Teton River flows in a northeast direction across the figure 6 northwest corner and north of the figure 6 turns to flow in more of an east direction. Teton Ridge is labeled and is the high ridge seen in the figure 6 east half (just north of center) and extends eastward for a significant distance. Teton Ridge is the Teton River-Sun River drainage divide with drainage north of the ridge to the Teton River while drainage south of Teton Ridge is to the Sun River. East of the figure 6 map area there are several deep north-south oriented through valleys crossing Teton Ridge, which provide evidence of multiple south oriented flood flow channels. In figure 6 TL Gap is a northwest-southeast oriented through valley linking the northeast-oriented Teton River valley segment seen in the figure 6 northwest quadrant with a southeast-oriented Sun River tributary valley. Bole Bench is the upland seen along the figure 6 south edge (west of center). The southeast-oriented Sun River tributary is a Muddy Creek tributary with Muddy Creek flowing to join the Sun River east of the figure 5 map area. TL Gap is a well-defined through valley which was eroded by southeast-oriented flood flow prior to headward erosion of the much deeper Teton River valley. The TL Gap through valley floor elevation at the drainage divide is between 3930 and 3940 feet. Elevations on Bole Bench to the south rise to 4082 feet in the figure 6 map area and to more than 4100 feet just south of the figure 6 map area. Elevations on Teton Ridge rise to more than 4200 feet to the north of TL Gap. The 4200 foot Teton Ridge surface is probably an erosion surface, which was probably formed as south and southeast-oriented flood flow moved across the region. However there are no higher level markers in figure 6 to indicate how much the region was eroded prior to formation of the Teton Ridge erosion surface. South and southeast-oriented flood flow probably then eroded the Bole Bench surface reducing the level of that surface from the 4200 foot elevation seen on Teton Ridge. The TL Gap through valley was next eroded by southeast-oriented flood flow moving to what was at that time the actively eroding Muddy Creek valley, which had eroded headward from the actively eroding deep Sun River valley. Headward erosion of the much deeper Teton River valley then beheaded the southeast-oriented TL Gap flood flow channel. Flood waters on the northwest end of the beheaded flood flow channel reversed flow direction to erode shallow northwest-oriented Teton River tributary valleys. Links to detailed Essays on which this summary essay is based are provided below. Each of the detailed essays illustrates and discusses two location maps and eight topographic maps for specific Sun River drainage basin drainage divide areas. These detailed essays illustrate and discuss considerable topographic map evidence not illustrated and described in this summary essay, but by no means is all of the available topographic map evidence shown.

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.