COOK COUNTY ST. LOUIS COUNTY LA KE SU PE RIO R ST. LOUIS COUNTY LA KE SU PE RIO R COOK COUNTY CANADA CANADA CANADA LA KE SU PE RIO R ON TA RIO ONT ARIO 91° 45' W. 91° 30' W. 47° N. 48° 7' 30" N. 47° 7' 30" N. 47° 30' N. 47° 52' 30" N. 47° 15' N. 47° 37' 30" N. 48° N. 48° 7' 30" N. 47° 22' 30" N. 47° 45' N. 91° 22' 30" W. 91° 7' 30" W. 91° 37' 30 W. 91° 15' W. )61 R. 6 W. R. 11 W. R. 10 W. R. 7 W. R. 8 W. R. 9 W. 91° 45' W. 91° 30' W. 91° 22' 30" W. 91° 7' 30" W. 91° 37' 30 W. 91° 15' W. R. 6 W. R. 11 W. R. 10 W. R. 7 W. R. 8 W. R. 9 W. T. 66 N. T. 65 N. T. 64 N. T. 63 N. T. 62 N. T. 61 N. T. 60 N. T. 59 N. T. 58 N. T. 57 N. T. 56 N. T. 53 N. T. 55 N. T. 54 N. T. 52 N. 47° N. 47° 7' 30" N. 47° 30' N. 47° 52' 30" N. 47° 15' N. 47° 37' 30" N. 48° N. 48° 7' 30" N. 47° 22' 30" N. 47° 45' N. T. 66 N. T. 65 N. T. 64 N. T. 63 N. T. 62 N. T. 61 N. T. 60 N. T. 59 N. T. 58 N. T. 57 N. T. 56 N. T. 53 N. T. 55 N. T. 54 N. T. 52 N. )1 )1 )169 )61 )61 )61 )1 Beaver Bay Silver Bay Two Harbors St on y Baptism River Coffee Egge Little Manitou River Ba ptis m R ive r Ing a C re ek Caribou Ho g C ree k Cr eek River Island River Cabin Creek Rock Cut Creek Manitou Balsam Creek Lindstrom Kawishiwi Crown Creek Creek Sn ak e Spur End Creek Isabella Au gu st Cr ee k Osier Creek Cr os s R ive r Sto ny Keel ey C reek Sa wm ill C ree k River Mitawan Creek Isab ella Islan d Co yo te C re ek San d R iver Kn ife R ive r Kn ife R ive r Stewart Silver Encampment River Go os eb err y River Kit Creek Beaver Bea ver R ive r Creek Thirtynine Big East Branch Beaver West River Ea st Lit tle Ri ve r River Creek Creek Branch Branch Baptism Wenho River Gooseberry Ri ve r River Creek River Riv er River Isabella River River Ri ve r Ri ve r Fo rk Sou th Rive r Farm LakeWhite Iron Lobo Lake Stewart Lake Balsam Lake Lax Lake Marble Lake Greenwood Lake Sand Lake Lake Three Gabbro Lake Pietro Lake Turtle Lake Gull Lake Quadga Lake Ferne Lake Rice Lake Isabella Lake Bog Lake Silver Island Lake Wilson Lake Lake Polly Windy Lake Insula Horse Lake Muskeg Cedar Lake Basswood Lake Fall Ensign Lake Manomin Lake Indiana Lake Wood Lake Ella Hall Mud Ojibway Lake Vera Lake Ima Lake Adams Lake Alice Lake Parent Lake Pan Lake Holt Lake White Iron Lake Lake Culkin Denley Lake Bonga Lake Cougar Lake HighlandLakeLong Lake Pine Lake Hjalmer Lake Murphy Lake Woodcock Lake ThomasLake Lake Twentythree Sonju Lake Doyle Lake Egge Lake Fry Lake Johnson Lake Nipisiquit Lake Micmac LakeTetagouche Lake Bean Lake Bear Lake Christianson Lake Upland Lake Katherine LakeCloquet Lake Lillian Lake Sullivan Lake Legler Lake Kane Lake Rota Lake Spruce Lake Phantom Lake ShamrockLake Dunnigan Lake IngaLakeDragon Lake Cat LakeSlateLake Kitigan Lake Grouse LakeGanderLake Two Deer Lake Pike Lake West Chub Lake Flat Horn Lake CampersLake Lake GegokaWampus Lake Stony Lake McDougal Lakes Pagami Lake RockIslandLake Horseshoe Lake Rock ofAges Lake Path Lake Omaday Lake Bogberry Lake HeartLake TonicLake August Lake BairdLakeBeaver Hut Lake HudsonLake Hope Lake Jupiter Lake ManiwakiLake Baskatong LakePose Lake Diana Lake TomahawkLakePelt Lake Pompous Lake Comfort Lake Bunny Lake LupusLakeHoist Lake Delay Lake Round Island Lake DivideLake Bluebill Lake Cabin Lake Helen Lake Jack Lake Plum Lake Wanless Lake Eighteen Lake Katydid Lake Dumbbell Lake Osier Lake WhitefishLake Cross River Lake Dam Five Lake ArlipLakeCrooked Lake HareLakeBonanza Lake ThunderbirdLake NinemileLake Moose Lake Harriet Lake Sister Lake Lost Lake Hazel Lake Watonwan Lake Cook Lake Coffee Lake Cramer Lake ChippewaLake Fourtown Lake Jackfish Lake Section Sixteen Lake Tin Can Mike Lake Sandpit Lake Range Lake Camp Lake Browns Lake Stub Lake Found Lake Wind Lake Good Lake Flash Lake Tofte Lake Triangle LakeGreenstoneLake KemptonsLake Pickerel Lake Rifle Lake SemaLake SpoonLake Pickle LakeDixLake Trident Lake WisiniLake Lake Gerund LakeBootLake Gibson Lake JordanLake Muzzle Lake Kiana Lake Whiz Lake River LakeAmber Lake Starlight Lake FishdanceLake KivanivaLake Trail Lake Koma Lake Gabimichigami Lake Van Lake Raven Lake Elton Lake Makwa Lake Rabbit Lake Ogishkemuncie Lake JasperLake Ester KnifeLake Kekekabic Lake Lake Gijikiki Lake Lake Lake Lake Back Bay Hoist Bay Basswood Lake Jackfish Bay Pipestone Bay Moose Lake Birch Lake Ashigan Lake Skoota Lake Fraser Lake Thomas Lake Snowbank Lake Disappointment Lake Boulder Lake Garden Lake Bald Eagle Lake Knife Lake Crooked Lake Moosecamp Lake Carp Lake Amoeber Lake SuckerLake Splash Lake Alworth Lake Beaver Lake Malberg Lake Lake Two Lake One Lake Four Clear Lake Little Gabbro Lake Clearwater Lake South Wilder Lake Kawishiwi Lake Square Lake Kawasachong Lake Perent Lake Section Twenty-Nine Lake Birch Lake Shallow Lake Clark Lake Stony C reek Skunk Creek Split Rock West East Split Rock River River Split Rock River Lake Ka wis hiw i T Lake Elbow Lake Lake COOK COUNTY ST. LOUIS COUNTY LA KE SU PE RIO R ST. LOUIS COUNTY LA KE SU PE RIO R COOK COUNTY CANADA CANADA CANADA LA KE SU PE RIO R ON TA RIO ONT ARIO 91° 45' W. 91° 30' W. 47° N. 48° 7' 30" N. 47° 7' 30" N. 47° 30' N. 47° 52' 30" N. 47° 15' N. 47° 37' 30" N. 48° N. 48° 7' 30" N. 47° 22' 30" N. 47° 45' N. 91° 22' 30" W. 91° 7' 30" W. 91° 37' 30 W. 91° 15' W. )61 R. 6 W. R. 11 W. R. 10 W. R. 7 W. R. 8 W. R. 9 W. 91° 45' W. 91° 30' W. 91° 22' 30" W. 91° 7' 30" W. 91° 37' 30 W. 91° 15' W. R. 6 W. R. 11 W. R. 10 W. R. 7 W. R. 8 W. R. 9 W. T. 66 N. T. 65 N. T. 64 N. T. 63 N. T. 62 N. T. 61 N. T. 60 N. T. 59 N. T. 58 N. T. 57 N. T. 56 N. T. 53 N. T. 55 N. T. 54 N. T. 52 N. 47° N. 47° 7' 30" N. 47° 30' N. 47° 52' 30" N. 47° 15' N. 47° 37' 30" N. 48° N. 48° 7' 30" N. 47° 22' 30" N. 47° 45' N. T. 66 N. T. 65 N. T. 64 N. T. 63 N. T. 62 N. T. 61 N. T. 60 N. T. 59 N. T. 58 N. T. 57 N. T. 56 N. T. 53 N. T. 55 N. T. 54 N. T. 52 N. )1 )1 )169 )61 )61 )61 )1 Beaver Bay Silver Bay Two Harbors St on y Baptism River Coffee Egge Little Manitou River Ba ptis m R ive r Ing a C re ek Caribou Ho g C ree k Cr eek River Island River Cabin Creek Rock Cut Creek Manitou Balsam Creek Lindstrom Kawishiwi Crown Creek Creek Sn ak e Spur End Creek Isabella Au gu st Cr ee k Osier Creek Cr os s R ive r Sto ny Keel ey C reek Sa wm ill C ree k River Mitawan Creek Isab ella Islan d Co yo te C re ek San d R iver Kn ife R ive r Kn ife R ive r Stewart Silver Encampment River Go os eb err y River Kit Creek Beaver Bea ver R ive r Creek Thirtynine Big East Branch Beaver West River Ea st Lit tle Ri ve r River Creek Creek Branch Branch Baptism Wenho River Gooseberry Ri ve r River Creek River Riv er River Isabella River River Ri ve r Ri ve r Fo rk Sou th Rive r Farm LakeWhite Iron Lobo Lake Stewart Lake Balsam Lake Lax Lake Marble Lake Greenwood Lake Sand Lake Lake Three Gabbro Lake Pietro Lake Turtle Lake Gull Lake Quadga Lake Ferne Lake Rice Lake Isabella Lake Bog Lake Silver Island Lake Wilson Lake Lake Polly Windy Lake Insula Horse Lake Muskeg Cedar Lake Basswood Lake Fall Ensign Lake Manomin Lake Indiana Lake Wood Lake Ella Hall Mud Ojibway Lake Vera Lake Ima Lake Adams Lake Alice Lake Parent Lake Pan Lake Holt Lake White Iron Lake Lake Culkin Denley Lake Bonga Lake Cougar Lake HighlandLakeLong Lake Pine Lake Hjalmer Lake Murphy Lake Woodcock Lake ThomasLake Lake Twentythree Sonju Lake Doyle Lake Egge Lake Fry Lake Johnson Lake Nipisiquit Lake Micmac LakeTetagouche Lake Bean Lake Bear Lake Christianson Lake Upland Lake Katherine LakeCloquet Lake Lillian Lake Sullivan Lake Legler Lake Kane Lake Rota Lake Spruce Lake Phantom Lake ShamrockLake Dunnigan Lake IngaLakeDragon Lake Cat LakeSlateLake Kitigan Lake Grouse LakeGanderLake Two Deer Lake Pike Lake West Chub Lake Flat Horn Lake CampersLake Lake GegokaWampus Lake Stony Lake McDougal Lakes Pagami Lake RockIslandLake Horseshoe Lake Rock ofAges Lake Path Lake Omaday Lake Bogberry Lake HeartLake TonicLake August Lake BairdLakeBeaver Hut Lake HudsonLake Hope Lake Jupiter Lake ManiwakiLake Baskatong LakePose Lake Diana Lake TomahawkLakePelt Lake Pompous Lake Comfort Lake Bunny Lake LupusLakeHoist Lake Delay Lake Round Island Lake DivideLake Bluebill Lake Cabin Lake Helen Lake Jack Lake Plum Lake Wanless Lake Eighteen Lake Katydid Lake Dumbbell Lake Osier Lake WhitefishLake Cross River Lake Dam Five Lake ArlipLakeCrooked Lake HareLakeBonanza Lake ThunderbirdLake NinemileLake Moose Lake Harriet Lake Sister Lake Lost Lake Hazel Lake Watonwan Lake Cook Lake Coffee Lake Cramer Lake ChippewaLake Fourtown Lake Jackfish Lake Section Sixteen Lake Tin Can Mike Lake Sandpit Lake Range Lake Camp Lake Browns Lake Stub Lake Found Lake Wind Lake Good Lake Flash Lake Tofte Lake Triangle LakeGreenstoneLake KemptonsLake Pickerel Lake Rifle Lake SemaLake SpoonLake Pickle LakeDixLake Trident Lake WisiniLake Lake Gerund LakeBootLake Gibson Lake JordanLake Muzzle Lake Kiana Lake Whiz Lake River LakeAmber Lake Starlight Lake FishdanceLake KivanivaLake Trail Lake Koma Lake Gabimichigami Lake Van Lake Raven Lake Elton Lake Makwa Lake Rabbit Lake Ogishkemuncie Lake JasperLake Ester KnifeLake Kekekabic Lake Lake Gijikiki Lake Lake Lake Lake Back Bay Hoist Bay Basswood Lake Jackfish Bay Pipestone Bay Moose Lake Birch Lake Ashigan Lake Skoota Lake Fraser Lake Thomas Lake Snowbank Lake Disappointment Lake Boulder Lake Garden Lake Bald Eagle Lake Knife Lake Crooked Lake Moosecamp Lake Carp Lake Amoeber Lake SuckerLake Splash Lake Alworth Lake Beaver Lake Malberg Lake Lake Two Lake One Lake Four Clear Lake Little Gabbro Lake Clearwater Lake South Wilder Lake Kawishiwi Lake Square Lake Kawasachong Lake Perent Lake Section Twenty-Nine Lake Birch Lake Shallow Lake Clark Lake Stony C reek Skunk Creek Split Rock West East Split Rock River River Split Rock River Lake Ka wis hiw i T Lake Elbow Lake Lake MINNESOTA GEOLOGICAL SURVEY Anthony C. Runkel, Interim Director Prepared and Published with the Support of THE LAKE COUNTY BOARD OF COMMISSIONERS, THE MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND AS RECOMMENDED BY THE LEGISLATIVE-CITIZEN COMMISSION ON MINNESOTA RESOURCES, THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, AND THE MINNESOTA LEGACY AMENDMENT'S CLEAN WATER FUND Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification. LOCATION DIAGRAM GEOLOGIC ATLAS OF LAKE COUNTY, MINNESOTA©2022 by the Regents of the University of MinnesotaThe University of Minnesota is an equal opportunity educator and employer GIS compilation by R.S. Lively Edited by Lori Robinson Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by Corey J. Betchwars. Universal Transverse Mercator Projection, grid zone 15 1983 North American Datum ENVIRONMENT AND NATURAL RESOURCES TRUST FUND SCALE 1:200 000 0 10 MILES55 0 10 15 KILOMETERS55 DEPTH TO BEDROCK By Amy Radakovich Block 2022 COUNTY ATLAS SERIES ATLAS C-54, PART A Lake County Plate 6—Bedrock Topography and Depth to Bedrock SCALE 1:200 000 0 10 MILES55 0 10 15 KILOMETERS55 BEDROCK TOPOGRAPHY By Amy Radakovich Block 2022 551-600 451-500 501-550 851-900 901-950 951-1,000 1,001-1,050 1,051-1,100 1,101-1,150 1,151-1,200 1,201-1,250 1,251-1,300 1,301-1,350 1,351-1,400 1,401-1,450 1,451-1,500 1,501-1,550 1,551-1,600 1,601-1,650 1,651-1,700 1,701-1,750 1,751-1,800 1,801-1,850 1,851-1,900 1,901-1,950 1,951-2,000 2,001-2,050 601-650 651-700 701-750 751-800 801-850 Depth in feet from the land surface to the bedrock surface 0–50 51–100 101–150 151–200 201–250 251–300 301–350 351–400 401–450 EXPLANATION The Bedrock Topography map of Lake County depicts the elevation and relief of the bedrock surface in feet above mean sea level. Each colored unit on the map represents a 50-foot (15-meter) elevation interval (example: 1,601 to 1,650 feet [488 to 503 meters] above mean sea level). For any location within a representative color, the elevation of the bedrock surface is expected to be between the end values depicted on the legend. Elevation ranges are separated by contours, which represent lines of equal elevation. Because bedrock commonly crops out at the land surface in Lake County, locations of mapped outcrop provided primary information in construction of the bedrock topography. Scientific drilling and water-well construction records from Minnesota's County Well Index (CWI), and boring records from the Quaternary Data Index, were also used to provide information about bedrock elevation. Several dozen seismic refraction surveys provided additional constraints on bedrock elevation near the shore of Lake Superior. In areas that lack outcrop, drilling records, or seismic refraction surveys, passive seismic measurements provided additional data to estimate the elevation of the bedrock surface. Reliability of the bedrock topographic surface is directly related to the density and type of available data (see Plate 1, Database Map). For example, bedrock elevation in areas with abundant outcrop or closely spaced drilling records is more reliable than that in areas with only passive seismic data. A 98-foot (30-meter) grid of the elevation of the bedrock surface and all contour lines are available in the supplementary digital and GIS files (https://cse.umn.edu/mgs). The bedrock elevation map presented here was compiled and modified from three maps covering parts of St. Louis and Lake Counties. These three maps were created between 2015 and 2019 and were published as part of the Minnesota Geological Survey's Open-File Report 16-4 (Jirsa, 2016). The majority of the bedrock topographic depiction included within the Open-File Report was new work produced for that mapping project. However, in the area of the Mesabi Iron Range, which extends just slightly into west- central Lake County, new bedrock topography contours were modified from a more detailed bedrock topography map published in Jirsa and others (2005). This plate supersedes the topography for Lake County presented in Jirsa (2016). Block (2022) contains the final Bedrock Topography map for St. Louis County. Some of the highest bedrock elevations occur along a broad, northeast–southwest- trending bedrock high in the south-central part of Lake County, extending roughly northeast from Scott Junction to just northeast of the Wanless Lookout Tower (Fig. 1). There is also a smaller bedrock high in far northeastern Lake County just west of Gabimichigami Lake in the Boundary Waters Canoe Area Wilderness (BWCAW). In both areas, bedrock commonly exceeds 1,900 feet (579 meters) above sea level, reaching a maximum elevation of 2,036 feet (620 meters) at the Wanless Lookout Tower. With the exception of the isolated bedrock high in northeast Lake County, the bedrock surface generally decreases in elevation to both the northwest and to the southeast of the northeast–southwest-trending bedrock high mentioned above. To the north of the bedrock high, bedrock elevation decreases gradually to the northwest, and reaches its lowest point of approximately 1,244 feet (379 meters) above mean sea level in Friday Bay along the St. Louis/Lake County border. This represents a decrease in elevation of 750 feet (229 meters) over approximately 45 miles (72 kilometers). In contrast, the bedrock elevation decreases abruptly to the south of the northeast–southwest-trending bedrock high. From the highest elevation in the county at Wanless Lookout Tower, the bedrock elevation drops to below 550 feet (168 meters) at the Lake Superior shoreline, a decrease of nearly 1,500 feet (457 meters) in elevation over a distance of approximately 15 miles (24 kilometers). The lowest bedrock elevation on land in Lake County is approximately 495 feet (151 meters) above sea level along the Lake Superior shoreline at Thompson Beach, just northeast of Gooseberry Falls. The bedrock surface along the northeast–southwest-trending high appears to be very smooth, while the bedrock surface to the north and south of it appears to be a more intricate and complex series of knobs, depressions, and drainages. This contrasting depiction does not reflect the true nature of the bedrock surface along the bedrock high, but rather the distribution of data. In areas away from the bedrock high, bedrock is commonly exposed at the surface, and 1-meter (3-foot) resolution lidar data allow for detailed contouring of the elevation of the bedrock surface. In contrast, along the bedrock high, bedrock is commonly buried by glacial sediment, and bedrock elevation data come primarily from subsurface drilling records and seismic surveys and measurements. As a result, the bedrock elevation contours are less detailed and the bedrock surface appears smoother in this area. Despite a smoother appearance, the bedrock surface along the northeast–southwest-trending bedrock high in the south-central part of the county is likely equally as intricate as in areas of abundant outcrop. Topography of the bedrock surface in Lake County strongly correlates with the composition of the underlying rocks in several areas (see Fig. 1). Diabase sills are commonly more resistant to erosion than their host rocks. For example, along the North Shore, the Silver Creek diabase (A) and Beaver River diabase (B) are expressed as prominent bedrock highs in comparison to the surrounding bedrock. Similarly, one particular andesitic flow in the North Shore Volcanic Group (C) also corresponds to a local bedrock high. Another example of the link between bedrock composition and bedrock topography is the Bald Eagle intrusion. Where exposed at the surface in the northern part of the intrusion (D), the Bald Eagle is a zoned, funnel-shaped intrusion (see Plate 2, Bedrock Geology). The outer, troctolitic zone of the funnel corresponds precisely with an oval-shaped low in the bedrock topography. The central, gabbroic core of the funnel, by contrast, holds up a topographic high. There are more than 150 feet (46 meters) of relief between the two units in this intrusion, strongly mimicking the gabbro/troctolite contact. Troctolite is more susceptible to weathering than gabbro, explaining the differing elevations. A slightly more tenuous correlation can be made between bedrock composition and bedrock topography in the linear bedrock high that crosscuts the south-central part of the county. Felsic rocks, such as the Mt. Weber granophyre (E), the western part of the Whitefish Lake granophyre (F), and several unnamed rhyolite units of the North Shore Volcanic Group (G), are some of the highest bedrock peaks in this regional bedrock high. Felsic rocks are known to withstand erosion better than mafic rocks, which may explain these highs. However, there are also areas of topographic highs along this ridge that are associated with rocks of mafic composition, so the correlation is not fully understood. Bedrock structures also influence bedrock topography in Lake County, particularly in areas with little to no sedimentary cover (see Fig. 1). It is likely that bedrock topographic lows developed along preexisting faults or fractures in the bedrock surface, following the path of least resistance. For example, the straight trajectory of the South Kawishiwi River (H) just northeast of its intersection with Birch Lake directly coincides with the contact between the Archean Giants Range rocks and the Mesoproterozoic Duluth Complex rocks over a strike length of 4.5 miles (7.2 kilometers). The Archean supracrustal rocks north and west of the Mesoproterozoic Duluth Complex rocks have a distinct northeast–southwest- trending fabric, which formed as a result of complex tectonic activity billions of years ago. There are numerous, notable northeast–southwest lineaments that correlate with bedrock lows throughout the Archean bedrock of the county. These linear bedrock lows of 50 to 100 feet (15 to 30 meters) correspond directly with major and minor regional faults, including the Burntside Lake fault (I), the Shagwa Lake fault (J), the Kawishiwi fault (K), and several unnamed faults in northwestern Lake County (L), as well as countless other linear unit contacts. Because the bedrock topography coincides or strongly correlates with surface topography in many areas of the county where bedrock is near the land surface, surface- water features locally occupy bedrock lows (Fig. 2). For example, in the BWCAW along the Canadian border, bedrock lows correspond almost precisely with interconnected lakes and rivers, such as Basswood Lake (1), Moose Lake (2), Snowbank Lake (3), Knife Lake (4), and more. Farther south, bedrock lows also coincide with Fall Lake (5), Birch Lake (6), and Bald Eagle Lake (7). Wilson Lake (8), though sitting at a much higher overall elevation than the other lakes discussed, still occupies a depression in the local bedrock surface. Additionally, deeply incised bedrock valleys that deepen toward Lake Superior are commonly occupied by modern-day rivers, such as Crow Creek (9), Split Rock River (10), Baptism River (11), and Manitou River (12). REFERENCES Block, A.R., 2022, Bedrock topography, pl. 6 of Jirsa, M.A., project manager, Geologic atlas of St. Louis County, Minnesota: Minnesota Geological Survey County Atlas C-51, pt. A, 6 pls., scale 1:200,000. Jirsa, M.A., project manager, 2016, Preliminary geologic maps of Lake and St. Louis Counties, northeastern Minnesota: Minnesota Geological Survey Open-File Report 16-4, scale 1:100,000. Jirsa, M.A., Setterholm, D.R., Bloomgren, B.A., Bauer, E.J., and Lively, R.S., 2005, Bedrock geology database, bedrock topography, and depth to bedrock maps of the eastern half of the Mesabi Iron Range, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-158, scale 1:100,000. EXPLANATION The Depth to Bedrock map depicts the thickness of unconsolidated sediment above the bedrock surface. Depth to the bedrock surface is a calculated value, determined by subtracting a grid of bedrock-surface elevations from a corresponding grid of land-surface elevations. The land-surface elevation grid was resampled from a 1-meter (3-foot) digital elevation model (DEM) derived from lidar data provided by the Minnesota Department of Natural Resources, making this grid highly detailed. In contrast, the bedrock-surface elevation grid is much more generalized because it is derived from the 50-foot (15-meter) elevation contours shown on the Bedrock Topography map. As a result, the intricate contours shown on the Depth to Bedrock map in parts of the county represent an artificial level of detail that results from subtracting the more generalized bedrock-surface grid from the detailed land-surface grid. The accuracy of depth to bedrock at any location is a direct reflection of the abundance and quality of data available, as depicted on Plate 1, Database Map, and described in the Bedrock Topography explanation. Additionally, because the thickness of unconsolidated sediment can vary greatly over short distances, the 1:200,000-scale Depth to Bedrock map presented here is unable to accurately resolve depths for site-specific situations. The Depth to Bedrock map is colored in 50-foot (15-meter) intervals. The depth to bedrock at any location on the colored map is interpreted to be within the 50-foot range shown on the legend with the corresponding color. Bedrock crops out at the land surface across much of Lake County, (see Plate 1); in these areas, the thickness of unconsolidated sediment (drift) atop the bedrock is zero. In areas where bedrock does not crop out at the land surface, the bedrock is covered by a variable thickness of unconsolidated materials that consist of Quaternary glacial sediment overlain by a thin soil horizon. For more information about the composition of the unconsolidated sediment, consult Plate 4, Quaternary Stratigraphy. Thickness of the unconsolidated sediment is highly variable, ranging from 0 to approximately 400 feet (0 to 122 meters). In over two-thirds of the county, unconsolidated sediment is very thin, and the bedrock is at or within 50 feet (15 meters) of the land surface (Fig. 1). Drift thicknesses of greater than 50 feet (15 meters) occur only in central, west-central, and southwest Lake County (Fig. 1, gray units). In these locations, some of the highest land-surface elevations in the county overlie bedrock-surface lows, and a thick stack of glacial sediment is present, trending in a northeast–southwest orientation. In an extremely localized area in central Lake County, where an approximately 12-mile x 12-mile (19-kilometer x 19-kilometer) depression has been identified in the bedrock surface, the glacial sedimentary sequence reaches thicknesses ranging from 250 to over 400 feet (76 to 122 meters; Fig. 1, dark gray units). This bowl-shaped depression cannot be attributed to differences in bedrock composition, nor to any known bedrock structure. The present-day Baptism River has incised the bedrock from the shore of Lake Superior northwest to this depression, forming a drainage from central Lake County to Lake Superior. The area of thick glacial sediment exists at the former junction of the Rainy, Brainerd, and Superior lobes (Fig. 1) of the Laurentide Ice Sheet, which covered parts or all of Lake County at various times throughout the last glacial cycle. In the region overlying the pronounced bedrock low, the sediment stack largely comprises sorted basin-fill deposits and is fringed by the continental (Laurentian) drainage divide. To the south and west, the thick sediment stack extends onto and forms part of the Highland moraine complex, a positive-relief band of ice-stagnation topography that demarcates a long-sustained ice-marginal position of the Superior lobe (Fig. 1). Repeated interaction of the Superior and Brainerd ice lobes along strike with the Highland moraine may have contributed to glacial overdeepening—a process in which deep, closed depressions are eroded into the bedrock surface beneath a glacier (Cook and Swift, 2012). Such overdeepenings are common in interlobate glacial settings such as Lake County, where multiple ice lobes make contact with one another. In such areas, ice flux, subglacial meltwater discharge, and associated subglacial erosion rates are orders of magnitude greater than over areas that were subject to persistent areal scour by a single ice lobe. Following bedrock incision, the receding Brainerd and Superior lobes contributed abundant sand and gravel to the eroded valley in the form of proglacial meltwater deposits. Though there are few wells in this thick Quaternary sequence, those present record sedimentary sequences that are consistent with the above interpretation. See Plate 3, Surficial Geology, for more information on the glacial sediment in this area. ACKNOWLEDGEMENTS Thanks are extended to Kaleb Wagner, formerly of the Minnesota Geological Survey, for insight on the nature of the anomalously thick section of Quaternary sediment described above. REFERENCE Cook, S.J., and Swift, D.A., 2012, Subglacial basins: Their origin and importance in glacial systems and landscapes: Earth-Science Reviews, v. 115, no. 4, p. 332-372, . GIS compilation by R.S. Lively Edited by Lori Robinson Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by Corey J. Betchwars. Universal Transverse Mercator Projection, grid zone 15 1983 North American Datum Elevation of the bedrock surface in feet above mean sea level Figure 1. Depth to the bedrock surface in Lake County highlighting areas where the depth to bedrock is 50 feet (15 meters) or less (pink). Areas with a greater depth to bedrock are shown in 50-foot intervals in successively darker shades of gray. Blue line indicates the Baptism River, a drainage that connects the anomalous bedrock depression in central Lake County with Lake Superior. Black line indicates the Laurentian Divide, a continental drainage divide. Dashed purple and magenta lines indicate the Isabella moraine and the Highland moraine, respectively, which separate areas of the map that have been glaciated by the Rainy, Brainerd, and Superior lobes during the last glacial cycle. Scale 1:600,000. Rainy lobe 0 4 8 12 162 Miles Baptism River 0-50 51-100 101-150 151-200 201-251 251-300 301-350 351-400 401-450 Isabella moraine Hig hla nd m ora ine Brainerd lobe D I V I D E L A U R E N T I A N DEPTH TO BEDROCK (feet above sea level) Superior lobe Figure 1. Bedrock elevation map of Lake County colored to emphasize the highest elevations on the bedrock surface. All elevations above 1,901 feet (579 meters) are white, and other colors are the same as on the Bedrock Topography map. The Boundary Waters Canoe Area Wilderness (BWCAW) is outlined in green. Gabimichigami Lake and the South Kawishiwi River (H) are light blue with black outlines. Thin black lines are generalized boundaries of bedrock units (see Plate 2, Bedrock Geology) that correlate with bedrock topography as described in the text. Dashed black lines indicate areas where unit contacts have been truncated, modified, or simplified for the purposes of this illustration. Boxed letters A through G delineate bedrock geologic units as follows: (A) Silver Creek diabase; (B) Beaver River diabase; (C) andesitic flow of the North Shore Volcanic Group; (D) Bald Eagle intrusion; (E) Mt. Weber granophyre; (F) Whitefish Lake granophyre; (G) unnamed rhyolite of the North Shore Volcanic Group. Thick black lines and boxed letters I through L indicate faults (see Plate 2). Scale 1:600,000. 0 4 8 12 162 Miles Gabimichigami Lake D Bald Eagle intrusion A Silver Creek diabase B Beaver River diabase C North Shore Volcanic Group andesite flow E F Mt. Weber granophyre Whitefish Lake granophyre North Shore Volcanic Group rhyolite G I J K H South Kawishiwi River SHAG WA L AKE F AULTBU RN TS IDE LA KE FA ULT KAWISHIWI FAULT Neoa rchea n Meso prote rozoic Thompson Beach Scott Junction Wanless Lookout Tower Friday Bay L A K E S U P E R I O R L Figure 2. Bedrock elevation map of Lake County. Colors are the same as on the Bedrock Topography map. The Boundary Waters Canoe Area Wilderness (BWCAW) is outlined in green. Lakes are outlined in blue. Numbers 1 through 8 highlight the following lakes that occupy lows in the bedrock surface: (1) Basswood Lake; (2) Moose Lake; (3) Snowbank Lake; (4) Knife Lake; and more. Further south, bedrock lows also coincide with (5) Fall Lake; (6) Birch Lake; (7) Bald Eagle Lake; and (8) Wilson Lake. Numbers 9 through 12 highlight the following rivers that occupy lows in the bedrock surface: (9) Crow Creek; (10) Split Rock River; (11) Baptism River; and (12) Manitou River. Scale 1:600,000. 0 4 8 12 162 Miles 8 6 5 1 2 3 4 7 12 11 9 10