Abstract:

Introduction:

• The purpose of this essay is to use topographic map interpretation methods to explore Milk River-Peoples Creek drainage divide area landform origins in Blaine County, Montana. Map interpretation methods can be used to unravel many geomorphic events leading up to formation of present-day drainage routes and development of other landform features. While each detailed topographic map feature provides detailed evidence to be explained, the solution must be consistent with explanations for adjacent area map evidence as well as solutions to big picture map evidence puzzles. I invite readers to improve upon my solutions or to propose alternate solutions that better explain evidence and are also consistent with adjacent map area and big picture evidence. Readers may do so either by making comments here or by writing and publishing their own essays and then by leaving a link to those essays in a comment here.

• This essay is also exploring a paradigm in which erosional landforms are interpreted as evidence left by immense glacial melt water floods. Implied in that interpretation is the immense floods were derived from a thick North American ice sheet that created a deep “hole” in the North American continent and also melted fast. The previously unexplored paradigm being tested in this and similar essays is a thick North American ice sheet, comparable in thickness to the present day Antarctic ice sheet, occupied approximately the North American region usually recognized to have been glaciated and through its weight and erosive actions created a “deep” North American “hole”, through its weight and deep erosion (and perhaps deposition) along major south-oriented melt water flow routes caused significant crustal warping and tectonic change, through its action of melting fast produced immense floods that flowed across the continent, and through its action of melting fast systematically opened up space in the ice sheet created “hole” so headward erosion of newly developed north-oriented drainage systems captured immense south-oriented melt water floods and diverted immense melt water floods north into space the ice sheet had once occupied.

• If this previously unexplored paradigm is correct the geographic region explored by this essay should contain evidence of immense floods that were captured by headward erosion of new valley systems so as to cause the floods to flow in a different direction. Ability of this previously unexplored paradigm to explain Milk River-Peoples Creek drainage divide area landform evidence in Blaine County, Montana will be regarded as evidence supporting the “thick ice sheet that melted fast” paradigm.

Milk River-Peoples Creek drainage divide area location map

Figure 1: Milk River-Peoples Creek drainage divide area location map (select and click on maps to enlarge). National Geographic Society map digitally presented using National Geographic Society TOPO software.

Figure 1 provides a Milk River-Peoples Creek drainage divide area location map. Figure 1 illustrates a region in north central Montana and the US-Canada border is located along the figure 1 north edge. The Missouri River flows east-southeast from Fort Benton near the figure 1 west edge to Fort Peck Lake, which continues to the figure 1 east edge. The lake is a large reservoir impounded behind Fort Peck Dam, which is located east of the figure 1 map area. The Milk River flows southeast from Canada (in the figure 1 northwest corner) to Fresno Reservoir, Havre, Chinook, Harlem, Dodson, Malta, Saco, and Hinsdale. Peoples Creek originates in the Bears Paw Mountains and flows northeast, southeast, northeast, and north to join the Milk River near Dodson. The Milk River-Peoples Creek drainage divide area discussed here is located east of north-oriented Clear Creek, which flows to the Milk River west of Chinook. The south and east boundary of the drainage divide region discussed here is Peoples Creek and the Milk River forms the northern boundary. Based on evidence from the hundreds of Missouri River drainage basin landform origins research project essays published on this website landform evidence illustrated here is interpreted in the context of an immense southeast-oriented flood flowing across the figure 1 map area and which was systematically captured and diverted northeast by headward erosion of deep valleys eroded into a topographic surface at least as high as the figure 1 region highest elevations today. The Milk River valley eroded northwest and west to capture southeast-oriented flood water and to divert flood flow to the newly eroded Missouri River valley east of Nashua, Montana (located east of figure 1. Northeast, east, and north-oriented tributary valleys eroded headward from the newly eroded north-oriented Milk River valley and one such tributary valley was the Peoples Creek valley, which eroded south, southwest, southeast, and southwest from the newly eroded Milk River valley to capture southeast-oriented flood water. Detailed maps below illustrate other valleys theat eroded south and southwest to capture southeast-oriented flood flow and also evidence that flood waters crossed the Bear Paw Mountains. Subsequently, as the deep Milk River valley eroded west it captured  southeast-oriented flood flow moving to the newly eroded Peoples Creek valley. Flood waters on the ends of beheaded southeast-oriented flood flow routes reversed flow direction and eroded the present day northwest and north-oriented Milk River tributary valleys. The Peoples Creek-Beaver Creek drainage divide area essay, the Beaver Creek-Missouri River drainage divide area essay, Milk River-Missouri River drainage divide area between Beaver Creek and Larb Creek essay, the Milk River-Missouri River drainage divide area east of Larb Creek essay, and the Missouri River-Musselshell River drainage divide area essay describe regions located near the Milk River-Missouri River drainage divide area discussed here and can be found under Milk River on the sidebar category list (Musselshell River for the Missouri River-Musselshell River essay). .

Milk River-Peoples Creek drainage divide area detailed location map

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

Figure 2 illustrates a somewhat more detailed map of the Milk River-Peoples Creek drainage divide area discussed in this essay. Blaine County is located in Montana. The red shaded area is the Fort Belknap Indian Reservation. The southeast-oriented Milk River is located in the figure 2 north half and flows through Chinook, Harlem, Savoy, and Coburg before reaching Dodson near figure 2 east edge. Peoples Creek originates in the Bear Paw Mountains southwest of Cleveland (located south of Chinook) and flows northeast, southeast and northeast and finally turns north to join the southeast-oriented Milk River west of Dodson. Clear Creek, which forms the western boundary of the drainage divide area discussed here flows north-northeast near the western Blaine County border and joins the Milk River near Lohman, which is located west of Chinook. Note the large number of southeast-oriented Milk River tributaries coming from the north. Also note the southeast orientation of many valley segments including Milk River and Peoples Creek valley segments. This southeast drainage alignment is evidence major trunk stream valleys eroded headward across and/or along multiple southeast-oriented flood flow channels such as might be found in a large southeast-oriented anastomosing channel complex. As previously mentioned this essay interprets Milk River-Peoples Creek drainage divide evidence in the context of an immense southeast-oriented flood, which crossed the entire figure 2 map area. Headward erosion of the southeast-oriented Milk River valley to the Dodson region occurred first and the Peoples Creek then eroded south, southwest, west-northwest to capture the southeast-oriented flood flow before headward erosion of the Milk River valley beheaded the southeast-oriented flood flow routes. Topographic maps below illustrate how the Peoples Creek valley eroded west into the Bear Paw Mountain region to capture southeast-oriented flood flow, which was subsequently beheaded by headward erosion of the Milk River valley. Evidence presented here requires flood waters to have originally flowed across a topographic surface at least as high as the highest figure 2 elevations today (including Bear Paw Mountains elevations), although it is possible the Bear Paw Mountains region was uplifted while flood erosion was taking place. An alternate possibility is the Bear Paw Mountains region and the Milk River-Peoples Creek drainage divide area were covered by sedimentary rocks and/or ice, which were removed by flood water erosion. Detailed maps tell the story.

White Bear Creek-Peoples Creek drainage divide

Figure 3: White Bear Creek-Peoples Creek drainage divide. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 3 illustrates the White Bear Creek-Peoples Creek drainage divide area at the east end of the Milk River-Peoples Creek drainage divide area. The southeast-oriented Milk River is located in the figure 3 north center area and northeast quadrant. Peoples Creek flows northeast from the figure 3 south edge (east half) to the figure 3 east center area where it turns to flow north-northwest to the southeast-oriented Milk River valley. Note northwest-oriented Mud Creek flowing to the Peoples Creek elbow of capture. Also note southeast-oriented tributaries from the northwest such as Willow Coulee and Kuhr Coulee. White Bear Creek flows southeast from the figure 3 west edge and then east, northwest, and northeast to reach the Milk River. Fifteenmile Creek (the “Creek” located south of White Bear Creek ) flows east, northeast, and northwest to reach White Bear Creek as a tributary. Spring Creek in the figure 3 northwest quadrant flow northwest to reach the Milk River as a barbed tributary. The southeast-oriented valley segments and tributaries provide evidence valleys eroded headward along southeast-oriented flood flow and/or captured multiple southeast-oriented flood flow routes such as might be found in a southeast-oriented anastomosing channel complex. The northwest-oriented valley segments provide evidence of flow reversals on the northwest ends of southeast-oriented flood flow channels. In the case of Peoples Creek the north-northwest valley segment (and the associated northwest-oriented Mud Creek valley) was probably eroded by reversed flood flow on a southeast-oriented flood flow route beheaded by headward erosion of the deep southeast-oriented Milk River valley. The northeast-oriented Peoples Creek valley segment (upstream from the north-northwest oriented valley segment) eroded southwest to capture southeast-oriented flood waters that had not yet been beheaded by headward erosion of the deep Milk River valley. The Milk River valley eroded headward from the southeast to the northwest, which means the White Bear Creek and Fifteenmile Creek valleys were formed slightly later than the Peoples Creek valley. The northwest-oriented White Bear Creek valley segment and northwest-oriented Fifteenmile Creek valley segment were also formed by reversals of flood flow on a southeast-oriented flood flow route beheaded by headward erosion of the Milk River valley. The Fifteenmile Creek valley eroded southwest and west to capture southeast-oriented moving to what was then the newly eroded Peoples Creek valley. Soon thereafter the White Bear Creek valley eroded southwest and west to capture southeast-oriented flood flow to the newly eroded Fifteenmile Creek valley. Headward erosion of the Milk River valley then beheaded southeast-oriented flood flow routes moving water to the newly eroded White Bear Creek valley, including a route along the present day Spring Creek alignment. Flood waters on the northwest-end of that Spring Creek alignment flood flow route reversed low flow direction to erode the northwest-oriented Spring Creek valley.

Milk River-Snake Creek drainage divide area

Figure 4: Milk River-Snake Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 4 illustrates the Milk River-Snake Creek drainage divide area northwest of the figure 3 map area and includes overlap areas with figure 3. The Milk River flows southeast from the figure 4 northwest corner to the figure 4 east center edge. Northwest-oriented Spring Creek flows to Threemile Reservoir just south of the Milk River near the figure 4 east edge. Threemile Creek flows northeast from the figure 4 south edge (west of Snake Butte, which is located along the figure 4 south center edge) to join an unnamed southeast-oriented tributary and then flow southeast to Threemile Reservoir and the Milk River. Box Elder Creek flows east-northeast to a southeast-oriented through valley used by the unnamed southeast-oriented Threemile Creek tributary, however Box Elder Creek turns to flow northwest and join northwest-oriented Snake Creek, which turns to flow north-northeast to the Milk River. Snake Creek flows east from the figure 4 west center edge and then turns north and southeast to join Box Elder Creek and to flow northwest and north-northeast to the Milk River. Note the northwest-southeast oriented through valley now used for short distances by Snake Creek, Box Elder Creek, and Threemile Creek. The valley is parallel to the larger southeast-oriented Milk River valley and provides evidence of a southeast-oriented flood flow channel that was dismembered by headward erosion of north- and northeast-oriented valleys from the deeper and parallel Milk River valley. The southeast-oriented through valley probably was eroded headward to capture southeast-oriented flood waters and for some reason eroded slower than the adjacent and parallel Milk River valley. As the Milk River valley eroded headward it eventually beheaded all southeast-oriented flood flow routes to the through valley from the northwest. However the Threemile Creek, Box Elder Creek, and Snake Creek valleys eroded southwest to capture flood waters not yet beheaded by headward erosion of the Milk River valley (and its parallel valley). Headwaters area of White Bear Creek, Box Elder Creek and Snake Creek are illustrated in figures below.

Little Box Elder Creek-White Bear Creek drainage divide area

Figure 5 illustrates the Little Box Elder Creek-White Bear Creek drainage divide area southwest of the figure 4 map area. Little Box Elder Creek flows northeast in the figure 5 northwest corner. The North Branch of White Bear Creek flows east in the figure 5 southeast quadrant. The South Branch of Box Elder Creek flows east in the figure 5 east center area. Note how a well-defined through valley links headwaters of the east-oriented South Branch of White Bear Creek with headwaters of a northwest-oriented Little Box Elder Creek tributary. A similar through valley links headwaters of the east-oriented North Branch White Bear Creek with a north-northwest oriented Little Box Elder Creek tributary valley west near the figure 5 west edge and also continues west where further linkages are made (see figure 6 below). These through valleys provide evidence that multiple channels of water once flowed across the present Little Box Elder Creek-White Bear Creek drainage divide such as might be found in an anastomosing channel complex. The through valley linking the northwest-oriented Little Box Elder Creek tributary valley with the South Branch White Bear Creek is on the side of hill, which in the context of present day topography makes a southeast-oriented flood seem unlikely. However, the figure 5 topography must be viewed as it existed when southeast-oriented flood waters first flowed across the figure 5 map region. At that time there was no Milk River valley to the north and the north-oriented slope did not exist. The origin of the small lake basins or depressions in the figure 5 southeast quadrant is difficult to determine, but it is possible they provide evidence of the presence of decaying ice sheet remnants that remained after flood flow across the region ceased. If so, the figure 5 map area may have been covered with decaying ice at the time flood waters flowed across the region. With or without the presence of decaying ice sheet remnants the South Branch White Bear Creek eroded a deep valley into the figure 5 map area to capture southeast-oriented flood flow. That valley extended west across the entire figure 5 map area to the west of figure 5. Next the deep North Branch White Bear Creek valley eroded headward into the figure 5 map area to capture southeast-oriented flood flow that was moving to the newly eroded South Branch White Bear Creek valley. As that South Branch valley was eroding the northeast-oriented Little Box Elder Creek was eroding southwest to capture southeast-oriented flood flow moving to the newly eroded east-oriented White Bear Creek valley system. Flood waters on the northwest ends of beheaded flood flow routes reversed flow direction to erode the northwest and north-northwest oriented Little Box Elder Creek tributary valleys (and also the north-oriented slope). Reversed flow on newly beheaded flood flow routes captured significant yet to be beheaded flood flow from flood flow routes further to the southwest, which greatly aided in the erosion process.

Box Elder Creek-Peoples Creek drainage divide area

Figure 6 illustrates a Box Elder Creek-Peoples Creek drainage divide area south and west of the figure 4 map area and includes overlap areas with figure 5. Peoples Creek flows northeast from the figure 6 southwest corner area to north of McCann Butte and then southeast to the figure 6 east edge. McCann Butte is composed of erosion resistant rock and is located south of Peoples Creek. Box Elder Creek is located in the figure 6 northwest quadrant and flows northeast and north from the figure 6 west edge to the figure 6 north edge. Little Box Elder Creek flows northeast in the figure 6 north center area. Note the well-defined northwest-southeast oriented through valley linking the northeast oriented Little Box Elder Creek valley with the southeast oriented Peoples Creek valley. That through valley provides evidence headward erosion of the northeast oriented Little Box Elder Creek valley beheaded a major southeast oriented flood flow route that had been captured by headward erosion of the Peoples Creek valley. Also note the maze of east-oriented through valleys extending east from the northeast oriented Little Box Elder Creek headwaters area. Those through valleys provide evidence of an anastomosing channel complex of which the southeast oriented through valley to the Peoples Creek valley was a significant component. Further, note the through valley on the southwest side of McCann Butte (northwest of the small lake). That through valley links a north-oriented Peoples Creek tributary valley with the northeast- and east-southeast oriented Maggie Creek valley, which flows to Peoples Creek just east of the figure 6 map area. In addition note two well-defined west to east oriented through valleys linking north-oriented Box Elder Creek tributaries with the northeast-oriented Peoples Creek valley. Those through valleys provide evidence headward erosion of the north-northeast oriented Box Elder Creek valley captured southeast and east-oriented flood flow moving to the Peoples Creek valley. In total figure 6 evidence describes a southeast and east oriented anastomosing complex of flood flow channels that was captured by headward erosion of the Peoples Creek-Maggie Creek valley, the Peoples Creek valley, the Little Box Elder Creek valley, and the Box Elder Creek in that order.

Snake Creek-Peoples Creek drainage divide area

Figure 7: Snake Creek-Peoples Creek drainage divide area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 7 illustrates the Snake Creek-Peoples Creek drainage divide area west of the figure 6 map area and includes overlap areas with figure 6. East and northeast-oriented Snake Creek is located in the figure 7 northwest corner. Northeast-oriented Peoples Creek is located in the figure 7 southeast corner. North-northeast oriented Box Elder Creek headwaters are located in the figure 7 north center area. East of the Box Elder Creek headwaters are north-oriented tributaries to Box Elder Creek and the west end of the through valleys linking those tributary valleys with the southeast-oriented Peoples Creek valley seen in figure 6. Figure 7 illustrates some interesting meandering through valleys linking the Snake Creek valley with the Peoples Creek valley and also with the north-oriented Box Elder Creek tributary valley. The road from the ranch along the figure 7 west edge to the Peoples Creek valley makes use of the through valleys as it weaves its way east and south. However, the road avoids a valley segment with the lake and levees in it. Also east of the valley with the lake and the next valley used by the road is a north-oriented through valley that leads to the valley of a north-oriented Box Elder Creek tributary. The through valleys in figure 7 were eroded by anastomosing flood flow channels at the time headward erosion of the deep Peoples Creek valley was capturing the southeast-oriented flood flow and the north-northeast oriented Box Elder Creek valley was capturing southeast-oriented flood flow to the what was then the actively eroding and deep Peoples Creek valley. Headward erosion of the northeast-oriented Snake Creek valley next captured the southeast-oriented flood flow beheading flood flow routes that were supplying water to the anastomosing channel complex. Bedrock composition cannot be determined from topographic map evidence alone, although the name Sand Rocks suggests the presence of resistant sandstone and buttes such as Crown Butte in the figure 7 southwest corner may be composed of even more erosion resistant material. Whatever the bedrock, the anastomosing valleys were eroded into a topographic surface at least 400 meters higher than where Snake Creek joins the Milk River valley to the north and east of figure 7. Crown Butte and McCann Butte stand even higher, however because no valleys have eroded into them their relationship to the flood erosion cannot be determined from topographic map evidence alone.

Timber Butte through valley

Figure 8: Timber Butte through valley. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 8 illustrates the Timber Butte area located immediately south of Crown Butte, which was located in the figure 7 southwest corner area. Peoples Creek flows northeast, north, and northeast from the figure 8 south edge to the figure 8 east edge. Rieve Creek flows west from the figure 8 east edge to join Peoples Creek in figure 8 east center area. Note northwest-oriented Peoples Creek tributaries in the figure 8 southeast corner area. Timber Creek tributaries from the northwest are southeast-oriented. Timber Butte stands tall above the surrounding region probably because it is composed of erosion resistant rock. The northwest-southeast oriented through valley eroded across Timber Butte provides evidence southeast-oriented flood flow once moved on a topographic surface at least as high as the highest Timber Butte elevations today. Otherwise the flood waters would have flowed around Timber Butte, which they eventually did when headward erosion of the Peoples Creek tributary valley on the southwest side of Timber Butte eroded northwest and north and beheaded southeast-oriented flood flow that had been moving across Timber Butte. The east-northeast oriented valley north of Timber Butte drains to a southeast-oriented Peoples Creek tributary east of the figure 8 map area. Timber Butte stands more than 900 meters higher than the Milk River valley floor elevation to the north. When southeast-oriented flood waters flowed across Timber Butte the deep southeast-oriented Milk River valley did not exist. The source of the southeast-oriented flood waters cannot be determined from evidence presented here. However, the hundreds of Missouri River drainage basin landform origins research project essays published on this website when taken as a group can be used to trace flood waters both up flood to source areas and down flood to see where flood waters were going. A logical flood water source would be rapid melting of a thick North American ice sheet located in a deep “hole” occupying approximately the North American location usually recognized to have been glaciated. The deep “hole” would have been created by deep glacial erosion and by crustal warping caused by the ice sheet weight. Such a flood water source would not only explain the immense southeast-oriented floods this essay series describes, but would also explain why deep valleys were eroding headward to capture the southeast-oriented flood waters and diverting the flood waters further and further to the northeast and north into space in the deep “hole” the rapidly melting thick ice sheet had once occupied.

Clear Creek-Milk River confluence area

Figure 9: Clear Creek-Milk River confluence area. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 9 illustrates the Milk River valley north of Timber Butte and also north of the Peoples Creek headwaters area, which is located west of Timber Butte. There is no overlap with previous figures. The Milk River flows east from the figure 9 west edge and turns to flow northeast to Lohman, where it is joined by north-oriented Clear Creek. From Lohman the Milk River flows east-southeast past Chinook to the figure 9 east edge. Note southeast-oriented tributary valleys flowing from the northwest to join the Milk River. Lodge Creek flows southeast and south to join the Milk River at Chinook and shares a valley with south-southeast and southeast oriented Battle Creek, which joins the Milk River near the figure 9 east edge. Southeast-oriented Redrock Coulee flows from the figure 9 northwest corner area into the Milk River valley north of Lohman. These southeast-oriented tributaries provide evidence headward erosion of the deep Milk River valley captured multiple channels of southeast-oriented flood flow such as might be found in a southeast-oriented anastomosing channel complex. Note also the east-oriented through valley linking the north-oriented Clear Creek valley with the northeast, east, and northeast-oriented Threemile Creek valley. The through valley provides evidence east-oriented flood flow once moved from the present day Clear Creek valley area to the present day Threemile Creek valley and was beheaded by headward erosion of the north-oriented Clear Creek valley. The Milk River valley floor north of Lohman, where Clear Creek joins the Milk River, is between 720 and 740 meters in elevation. Remember that elevation when looking at figure 10 below, which illustrates how flood waters moving south in the Clear Creek valley once flowed east to the Peoples Creek valley.

Clear Creek-Peoples Creek through valley

Figure 10: Clear Creek-Peoples Creek through valley. United States Geological Survey map digitally presented using National Geographic Society TOPO software. 

Figure 10 illustrates a through valley linking the north-northwest oriented Clear Creek valley with southeast-oriented Peoples Creek headwaters. Figure 10 is located south of the figure 9 map area and west of figure 8 and there are no overlap areas with previous figures. North-northwest oriented Clear Creek flows from the figure 10 southwest corner area to the figure 10 west center edge. Greenough Coulee is located in the figure 10 northwest quadrant and flows southwest and northwest to join Clear Creek north and west of the figure 10 map area. Peoples Creek flows southeast from the figure 10 north center area into the deep west to east oriented through valley and then flows east-southeast to the figure 10 east edge. West-oriented Battle Creek drains the through valley west end. The drainage divide in the through valley between west-oriented Battle Creek and east-oriented Peoples Creek is more than 700 meters higher than the Milk River valley floor where Clear Creek joins the Milk River north of Lohman. The highest Bear Paw Mountains peaks in the region immediately surrounding the through valley are more than 1100 meters higher than the Milk River valley. Yet, the through valley was eroded by southeast-oriented flood flow that crossed what is today the deep east- and southeast-oriented Milk River valley to the north. The deep Peoples Creek valley eroded west before headward erosion of the deep Milk River valley captured the southeast-oriented flood flow and the north-northwest oriented Clear Creek valley was initiated at least in part as a south-southeast oriented extension of the east-oriented Peoples Creek valley. Headward erosion of the deep Milk River valley then beheaded the south-southeast oriented flood flow route and flood waters on the north end of the beheaded flood flow route reversed flow direction to flow north to the newly eroded and much deeper Milk River valley. Flood waters on the west end of the west to east oriented through valley also reversed flow direction to flow west and to erode the west-oriented Battle Creek valley. It is also possible the Bear Paw Mountains area was being uplifted during the flood as these drainage changes took place. If so the uplift could have been related to crustal warping caused by the presence of a thick North American ice sheet to the northeast and/or could also have been related to crustal warping caused by the removal of significance amounts of overlying bedrock material.

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.