BEDROCK TOPOGRAPHY DEPTH TO BEDROCK

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BEDROCK TOPOGRAPHY By Andrew J. Retzler and Julia R. Steenberg 2016 MINNESOTA GEOLOGICAL SURVEY Harvey Thorleifson, Director Prepared and Published with the Support of THE WASHINGTON COUNTY BOARD OF COMMISSIONERS, THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF ECOLOGICAL AND WATER 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. COUNTY ATLAS SERIES ATLAS C-39, PART A Washington County Plate 6—Bedrock Topography and Depth to Bedrock LOCATION DIAGRAM GEOLOGIC ATLAS OF WASHINGTON COUNTY, MINNESOTA ©2016 by the Regents of the University of Minnesota The University of Minnesota is an equal opportunity educator and employer DEPTH TO BEDROCK By Andrew J. Retzler and Julia R. Steenberg 2016 CONTOUR INTeRvAL 25 MeTeRS 1 0 1 2 3 4 5 MILES 8 KILOMETERS SCALE 1:100 000 1 0 1 2 3 4 5 6 7 Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by the Minnesota Geological Survey. Elevation contours were derived from the U.S. Geological Survey 30-meter Digital el evation Model (DeM) by the Minnesota Geological Survey. Universal Transverse Mercator Projection, grid zone 15 1983 North American Datum GIS compilation by R.S. Lively edited by Lori Robinson EXPLANATION The bedrock elevation surface of Washington County is represented by colors assigned to 50-foot (15- meter) contour intervals (example: 751 to 800 feet [229 to 244 meters] above sea level) on the Bedrock Topography map. The position of contour intervals was determined from water well construction records from the County Well Index, engineering test borings, and bedrock outcrop mapping. Passive seismic soundings were used to further constrain the bedrock surface in some areas, most notably across bedrock valleys buried by Quaternary sediments. Geomorphic features visible on lidar imagery and indicative of near-surface bedrock conditions were also taken into consideration and include prominent steep rocky bluffs, rock-cored mesas, and areas of karst terrain (marked predominantly by field-located sinkholes and springs; Cleland and others, 1990). These features generally indicate shallowly buried bedrock, or less than 50 feet (15 meters) of overburden. The distribution of data can be seen on Plate 1, Data-Base Map, and should be considered when assessing the reliability of the map at specific locations. Fewer wells reach the bedrock surface in the northern part of Washington County because the overlying glacial deposits yield sufficient water. The bedrock elevation surface of Washington County ranges from more than 1,000 feet (305 meters) above sea level in the southern half, to less than 400 feet (122 meters) above sea level along the southern border beneath the Mississippi River. The average bedrock elevation throughout the county is about 750 feet (229 meters) above sea level. Several bedrock valleys exist at or near the land surface along the eastern edge of the county, and coincide with the modern St. Croix River and its tributaries. The most prominent bedrock topography feature of Washington County is a nearly 20-mile-long (32-kilometer-long), south trending, buried bedrock valley in the south-central part of the county. Several lakes lie atop this valley, most notably Lake Elmo and Long Lake. Modern stream drainages appear to follow the bedrock valley to the north along Brown's Creek in Stillwater, and to the south along intermittent streams in east Cottage Grove. Many of the less prominent buried bedrock valleys in the western part of the county also coincide with modern lakes, such as Clear Lake, Forest Lake, White Bear Lake, and others. The relationship between modern lakes and streams and buried bedrock valleys may be due to the melting of remnant, buried ice blocks, settling and compaction of the fill within the bedrock valleys, or a combination of both. Where bedrock valleys are filled with permeable sediment, they can potentially focus groundwater discharge towards major rivers. Groundwater within bedrock valleys of northwestern Washington County drains through the bedrock valleys of Anoka and Ramsey Counties to the west and into the modern Mississippi River valley. Groundwater in bedrock valleys of southwestern, central, and eastern Washington County drains into the ancestral and modern Mississippi River valley in northern Dakota County to the south and into the St. Croix River to the east. Bedrock valleys also provide pathways for recharge to deeper aquifers where they incise regional aquitards. The resistance of the underlying bedrock to weathering and erosion, as well as the presence of fractures and joints, can greatly affect the shape of the bedrock topography surface. As a result, the bedrock topography exhibits some correlation with rock units. Weakly cemented sandstone and shale are easily eroded by flowing water. Limestone and dolostone are more resistant to physical erosion, but prone to chemical weathering in slightly acidic surface water and groundwater conditions. Once resistant layers are breached by drainage, erosion of underlying sandy units creates narrow, steep-sided valleys. The flat uplands in southern Washington County are mostly underlain by resistant limestone and dolostone of the Prairie du Chien Group. This area is also more susceptible to dissolution and karst development, as indicated by the increased presence of sinkholes and springs. REFERENCE Cleland, J.M., Tipping, R.G., Bloomgren, B.A., and Meints, J., 1990, Data-base maps, pl. 1 of Swanson, L., and Meyer, G.N., eds., Geologic atlas of Washington County, Minnesota: Minnesota Geological Survey County Atlas C-5, scale 1:100,000, 7 pls. 351-400 401-450 451-500 501-550 551-600 601-650 651-700 701-750 751-800 801-850 851-900 901-950 951-1,000 1,001-1,050 Elevation of the bedrock surface in feet above mean sea level T. 32 N. T. 32 N. T. 31 N. T. 31 N. T. 30 N. T. 30 N. T. 29 N. T. 29 N. T. 28 N. T. 28 N. T. 27 N. T. 27 N. T. 26 N. R. 22 W. R. 22 W. R. 21 W. R. 21 W. R. 20 W. R. 20 W. R. 19 W. 92° 52' 30" W. 93° W. 44° 52' 30" N. 44° 45' N. 44° 45' N. 45° N. 45° 7' 30" N. 92° 45' W. 45° 15' N. 92° 45' W. 45° 15' N. 45° 7' 30" N. 45° N. 44° 52' 30" N. 93° W. 93° W. ANOKA COUNTY CHIS AGO COU NTY CHISAGO CO UNTY DAKOTA COUNTY ANOKA COUNTY WISCONSIN WISCONSIN DAKOTA COUNTY PIERCE COUNTY ST. CROIX COUNTY POLK COUNTY PIERCE COUNTY ST. CROIX COUNTY POLK COUNTY RAMSEY COUNTY RAMSEY COUNTY Forest Clear Lake Mud Lake Sylvan Lake Lake Nielsen Lake German Lake Lake Lake Sea Goose Fish Big Marine Lake Long Lake Lake Lake Sand Hay Square Lake Lake Terrapin Turtle Lake Lake Mud Lake Lake Oneka Lake Rice Egg Lake School Section Sunset Lake Lake Long Lake Round Lake Horseshoe White Rock Lake Lake Bald Eagle Lake Pine Tree Lake Mann Lake Fish Lake Long White Bear Lake Lake Long Lake Lake Olson Demontreville Clear Lake McDonald Lake Cloverdale Lake Lake Long Lily Lake Lake McKusick Lake Loon Silver Lake Louise Lake Lake Carnelian Little Lake Masterman Benz Lake Sunfish Lake Lake Point Eagle Lake Goose Lake Lake Tanners Lake Creek Battle Wilmes Margrafs Powers Lake Lake Colby Lake Lake Carver Lake La Baldwin Lake Conley Lake Lake Edith Lake Mooers Lake Big Carnelian Lake Lakes Twin Lake Jane Lake Elmo Bone Lake Hardwood Creek Creek Creek Hardwood Creek Creek Creek Creek River River St. Croix Mississippi Mississippi River River St. Croix River St. Croix Old Mill Stream Clearwater Silver Brown's Valley Valley Branch Trout Brook Brown's 6 31 1 1 31 36 6 6 6 6 6 6 6 36 36 36 1 1 1 1 31 31 31 31 36 35 36 36 31 31 31 31 6 6 6 6 1 1 2 2 35 36 36 36 6 6 6 6 1 1 1 25 31 31 31 31 4 2 ¤ 61 ¤ 61 ¤ 61 ) 36 ) 36 ¤ 10 ¤ 10 ¤ 10 ¤ 61 ¤ 8 § ¦ 35 § ¦ 494 § ¦ 694 § ¦ 94 § ¦ 494 ) 95 ) 95 ) 95 ) 95 ) 95 ) 97 ¤ 61 ) 97 ) 244 ) 5 ) 5 ) 95 ) 96 § ¦ 94 § ¦ 94 SCANDIA F OREST LAKE HUGO M AY GRANT S TILLWATER LAKE E LMO BAY T OWN WE ST LAKELAND L A K E L A N D AFTON WOODBUR Y DENMARK COTTAGE GROVE GREY CLOUD ISL AND St Paul Park Newport Cottage Grove Woodbury Lakeland Shores Beach St Croix Lake Point St Mary's Bayport Oak Park Heights Stillwater Marine St Croix On Forest Lake Hugo Dellwood Oakdale Pine Springs Mahtomedi 225 250 275 300 325 275 275 275 275 275 275 275 300 300 300 275 275 275 275 300 275 275 225 300 300 250 275 300 275 300 275 300 275 325 300 325 300 300 275 300 300 250 300 275 300 300 300 225 300 275 275 300 275 275 275 300 300 225 300 275 300 300 300 300 300 300 300 275 300 225 275 300 250 300 300 275 300 300 275 300 275 300 275 300 250 275 300 300 250 300 250 300 300 300 275 275 300 275 325 250 300 275 250 225 300 300 275 300 325 275 275 300 300 275 300 275 275 300 275 275 275 275 250 300 300 300 275 300 300 300 300 275 300 275 325 275 225 300 275 275 275 275 225 225 225 92° 52' 30" W. CONTOUR INTeRvAL 25 MeTeRS 1 0 1 2 3 4 5 MILES 8 KILOMETERS SCALE 1:100 000 1 0 1 2 3 4 5 6 7 EXPLANATION The depth to bedrock is equal to the depth from the land surface to the underlying bedrock surface, and represents the thickness of Quaternary sediment cover over bedrock. To calculate this thickness, a digital grid of bedrock-surface elevations was subtracted from a corresponding digital grid of land-surface elevations. The surface elevation grid was resampled from a 1-meter digital elevation model derived from lidar by the Minnesota Department of Natural Resources, whereas the bedrock elevation grid was derived from the Bedrock Topography map, which was inferred from outcrop mapping and interpretation of water well records, engineering test borings, passive seismic data, and geomorphic features. The residual grid was then classified at 50-foot (15-meter) intervals to produce the color-coded Depth to Bedrock map. Thickness of the Quaternary sediments can vary greatly over short distances, and mapping at this scale (1:100,000) may not properly resolve prominent variations. For this reason, it is best to consult site-specific data (such as water well records, engineering test borings, and passive seismic soundings) wherever available. The detailed appearance of the Depth to Bedrock map is related to surficial landforms because the land surface topography model is based on higher resolution data than the interpreted bedrock surface model. The thickest Quaternary sediments in Washington County occur within the Mississippi and St. Croix River valleys and their tributaries, where the depth to bedrock is as much as 400 feet (122 meters). Elsewhere, more than 300 feet (91 meters) of overlying sediment occurs within a buried bedrock valley in the northwest corner of the county near Forest Lake, and in the 20-mile-long (32-kilometer-long) valley passing through the south-central part of the county near Cottage Grove. Areas where bedrock is at or within 50 feet (30 meters) of the land surface occur within the southern one-third of Washington County, and along the eastern part of the county following the St. Croix River bluff. Depth in feet from the land surface to the bedrock surface 1–50 51–100 101–150 151–200 201–250 251–300 301–350 351–400 401–450 T. 32 N. T. 32 N. T. 31 N. T. 31 N. T. 30 N. T. 30 N. T. 29 N. T. 29 N. T. 28 N. T. 28 N. T. 27 N. T. 27 N. T. 26 N. R. 22 W. R. 22 W. R. 21 W. R. 21 W. R. 20 W. R. 20 W. R. 19 W. 92° 52' 30" W. 93° W. 44° 52' 30" N. 44° 45' N. 44° 45' N. 45° N. 45° 7' 30" N. 92° 45' W. 45° 15' N. 92° 45' W. 45° 15' N. 45° 7' 30" N. 45° N. 44° 52' 30" N. 93° W. 93° W. ANOKA COUNTY CHIS AGO COU NTY CHISAGO CO UNTY DAKOTA COUNTY ANOKA COUNTY WISCONSIN WISCONSIN DAKOTA COUNTY PIERCE COUNTY ST. CROIX COUNTY POLK COUNTY PIERCE COUNTY ST. CROIX COUNTY POLK COUNTY RAMSEY COUNTY RAMSEY COUNTY Forest Clear Lake Mud Lake Sylvan Lake Lake Nielsen Lake German Lake Lake Lake Sea Goose Fish Big Marine Lake Long Lake Lake Lake Sand Hay Square Lake Lake Terrapin Turtle Lake Lake Mud Lake Lake Oneka Lake Rice Egg Lake School Section Sunset Lake Lake Long Lake Round Lake Horseshoe White Rock Lake Lake Bald Eagle Lake Pine Tree Lake Mann Lake Fish Lake Long White Bear Lake Lake Long Lake Lake Olson Demontreville Clear Lake McDonald Lake Cloverdale Lake Lake Long Lily Lake Lake McKusick Lake Loon Silver Lake Louise Lake Lake Carnelian Little Lake Masterman Benz Lake Sunfish Lake Lake Point Eagle Lake Goose Lake Lake Tanners Lake Creek Battle Wilmes Margrafs Powers Lake Lake Colby Lake Lake Carver Lake La Baldwin Lake Conley Lake Lake Edith Lake Mooers Lake Big Carnelian Lake Lakes Twin Lake Jane Lake Elmo Bone Lake Hardwood Creek Creek Creek Hardwood Creek Creek Creek Creek River River St. Croix Mississippi Mississippi River River St. Croix River St. Croix Old Mill Stream Clearwater Silver Brown's Valley Valley Branch Trout Brook Brown's 6 31 1 1 31 36 6 6 6 6 6 6 6 36 36 36 1 1 1 1 31 31 31 31 36 35 36 36 31 31 31 31 6 6 6 6 1 1 2 2 35 36 36 36 6 6 6 6 1 1 1 25 31 31 31 31 4 2 ¤ 61 ¤ 61 ¤ 61 ) 36 ) 36 ¤ 10 ¤ 10 ¤ 10 ¤ 61 ¤ 8 § ¦ 35 § ¦ 494 § ¦ 694 § ¦ 94 § ¦ 494 ) 95 ) 95 ) 95 ) 95 ) 95 ) 97 ¤ 61 ) 97 ) 244 ) 5 ) 5 ) 95 ) 96 § ¦ 94 § ¦ 94 SCANDIA F OREST LAKE HUGO M AY GRANT S TILLWATER LAKE E LMO BAY T OWN WE ST LAKELAND L A K E L A N D AFTON WOODBUR Y DENMARK COTTAGE GROVE GREY CLOUD ISL AND St Paul Park Newport Cottage Grove Woodbury Lakeland Shores Beach St Croix Lake Point St Mary's Bayport Oak Park Heights Stillwater Marine St Croix On Forest Lake Hugo Dellwood Oakdale Pine Springs Mahtomedi 225 250 275 300 325 275 275 275 275 275 275 275 300 300 300 275 275 275 275 300 275 275 225 300 300 250 275 300 275 300 275 300 275 325 300 325 300 300 275 300 300 250 300 275 300 300 300 225 300 275 275 300 275 275 275 300 300 225 300 275 300 300 300 300 300 300 300 275 300 225 275 300 250 300 300 275 300 300 275 300 275 300 275 300 250 275 300 300 250 300 250 300 300 300 275 275 300 275 325 250 300 275 250 225 300 300 275 300 325 275 275 300 300 275 300 275 275 300 275 275 275 275 250 300 300 300 275 300 300 300 300 275 300 275 325 275 225 300 275 275 275 275 225 225 225 92° 52' 30" W. Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by the Minnesota Geological Survey. Elevation contours were derived from the U.S. Geological Survey 30-meter Digital el evation Model (DeM) by the Minnesota Geological Survey. Universal Transverse Mercator Projection, grid zone 15 1983 North American Datum GIS compilation by R.S. Lively edited by Lori Robinson

Transcript of BEDROCK TOPOGRAPHY DEPTH TO BEDROCK

Page 1: BEDROCK TOPOGRAPHY DEPTH TO BEDROCK

BEDROCK TOPOGRAPHY

By

Andrew J. Retzler and Julia R. Steenberg

2016

MINNESOTA GEOLOGICAL SURVEYHarvey Thorleifson, Director

Prepared and Published with the Support of THE WASHINGTON COUNTY BOARD OF COMMISSIONERS,

THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF ECOLOGICAL AND WATER 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.

COUNTY ATLAS SERIESATLAS C-39, PART A

Washington County Plate 6—Bedrock Topography

and Depth to Bedrock

LOCATION DIAGRAM

GEOLOGIC ATLAS OF WASHINGTON COUNTY, MINNESOTA©2016 by the Regents of the University of Minnesota

The University of Minnesota is an equal opportunity educator and employer

DEPTH TO BEDROCK

By

Andrew J. Retzler and Julia R. Steenberg

2016

CONTOUR INTeRvAL 25 MeTeRS

1 0 1 2 3 4 5 MILES

8 KILOMETERS

SCALE 1:100 000

1 0 1 2 3 4 5 6 7

Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by the Minnesota Geological Survey.

Elevation contours were derived from the U.S. Geological Survey 30-meter Digital elevation Model (DeM) by the Minnesota Geological Survey.

Universal Transverse Mercator Projection, grid zone 151983 North American Datum

GIS compilation by R.S. Livelyedited by Lori Robinson

EXPLANATION

The bedrock elevation surface of Washington County is represented by colors assigned to 50-foot (15-meter) contour intervals (example: 751 to 800 feet [229 to 244 meters] above sea level) on the Bedrock Topography map. The position of contour intervals was determined from water well construction records from the County Well Index, engineering test borings, and bedrock outcrop mapping. Passive seismic soundings were used to further constrain the bedrock surface in some areas, most notably across bedrock valleys buried by Quaternary sediments. Geomorphic features visible on lidar imagery and indicative of near-surface bedrock conditions were also taken into consideration and include prominent steep rocky bluffs, rock-cored mesas, and areas of karst terrain (marked predominantly by field-located sinkholes and springs; Cleland and others, 1990). These features generally indicate shallowly buried bedrock, or less than 50 feet (15 meters) of overburden. The distribution of data can be seen on Plate 1, Data-Base Map, and should be considered when assessing the reliability of the map at specific locations. Fewer wells reach the bedrock surface in the northern part of Washington County because the overlying glacial deposits yield sufficient water.

The bedrock elevation surface of Washington County ranges from more than 1,000 feet (305 meters) above sea level in the southern half, to less than 400 feet (122 meters) above sea level along the southern border beneath the Mississippi River. The average bedrock elevation throughout the county is about 750 feet (229 meters) above sea level. Several bedrock valleys exist at or near the land surface along the eastern edge of the county, and coincide with the modern St. Croix River and its tributaries. The most prominent bedrock topography feature of Washington County is a nearly 20-mile-long (32-kilometer-long), south trending, buried bedrock valley in the south-central part of the county. Several lakes lie atop this valley, most notably Lake Elmo and Long Lake. Modern stream drainages appear to follow the bedrock valley to the north along Brown's Creek in Stillwater, and to the south along intermittent streams in east Cottage Grove. Many of the less prominent buried bedrock valleys in the western part of the county also coincide with modern lakes, such as Clear Lake, Forest Lake, White Bear Lake, and others. The relationship between modern lakes and streams and buried bedrock valleys may be due to the melting of remnant, buried ice blocks, settling and compaction of the fill within the bedrock valleys, or a combination of both.

Where bedrock valleys are filled with permeable sediment, they can potentially focus groundwater discharge towards major rivers. Groundwater within bedrock valleys of northwestern Washington County drains through the bedrock valleys of Anoka and Ramsey Counties to the west and into the modern Mississippi River valley. Groundwater in bedrock valleys of southwestern, central, and eastern Washington County drains into the ancestral and modern Mississippi River valley in northern Dakota County to the south and into the St. Croix River to the east. Bedrock valleys also provide pathways for recharge to deeper aquifers where they incise regional aquitards.

The resistance of the underlying bedrock to weathering and erosion, as well as the presence of fractures and joints, can greatly affect the shape of the bedrock topography surface. As a result, the bedrock topography exhibits some correlation with rock units. Weakly cemented sandstone and shale are easily eroded by flowing water. Limestone and dolostone are more resistant to physical erosion, but prone to chemical weathering in slightly acidic surface water and groundwater conditions. Once resistant layers are breached by drainage, erosion of underlying sandy units creates narrow, steep-sided valleys. The flat uplands in southern Washington County are mostly underlain by resistant limestone and dolostone of the Prairie du Chien Group. This area is also more susceptible to dissolution and karst development, as indicated by the increased presence of sinkholes and springs.

REFERENCE

Cleland, J.M., Tipping, R.G., Bloomgren, B.A., and Meints, J., 1990, Data-base maps, pl. 1 of Swanson, L., and Meyer, G.N., eds., Geologic atlas of Washington County, Minnesota: Minnesota Geological Survey County Atlas C-5, scale 1:100,000, 7 pls.

351-400

401-450

451-500

501-550

551-600

601-650

651-700

701-750

751-800

801-850

851-900

901-950

951-1,000

1,001-1,050

Elevation of the bedrock surface in feet above mean sea level

T. 32 N.T. 32 N.

T. 31 N.T. 31 N.

T. 30 N. T. 30 N.

T. 29 N.T. 29 N.

T. 28 N. T. 28 N.

T. 27 N.T. 27 N.

T. 26 N.

R. 22 W.

R. 22 W.R. 21 W.

R. 21 W. R. 20 W.

R. 20 W.

R. 19 W.

92° 52' 30" W. 93° W.

44° 52' 30" N.

44° 45' N. 44° 45' N.

45° N.

45° 7' 30" N.

92° 45' W.

45° 15' N.

92° 45' W.

45° 15' N.

45° 7' 30" N.

45° N.

44° 52' 30" N.

93° W.

93° W.

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CHISAGO COUNTY CHISAGO COUNTY

DAKOTA COUNTY

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ST. CROIXCOUNTY

POLKCOUNTY

RAMSEYCOUNTY

RAMSEY COUNTY

Forest

Clear Lake

MudLake

SylvanLake

LakeNielsen

LakeGerman

Lake

Lake

Lake

Sea

Goose

Fish

Big Marine Lake

Long Lake

Lake

LakeSand

Hay

SquareLake

LakeTerrapin

TurtleLakeLake

Mud

Lake

Lake

OnekaLake

RiceEgg

Lake

SchoolSectionSunset

LakeLake

Long

LakeRound

LakeHorseshoe

WhiteRockLake

Lake

Bald Eagle

Lake

PineTree

LakeMann

LakeFish

LakeLong

White BearLake

LakeLong

Lake

LakeOlson

Demontreville

ClearLake

McDonaldLake

CloverdaleLake

LakeLong Lily

Lake

LakeMcKusick

LakeLoon

Silver

Lake

Louise Lake

LakeCarnelianLittle

LakeMasterman

BenzLake

SunfishLake

LakePoint

Eagle

LakeGoose

Lake

Lake

TannersLake

CreekBattle

Wilmes

Margrafs

Powers

Lake

Lake

ColbyLakeLake

Carver

LakeLa

BaldwinLake

ConleyLake

LakeEdith

LakeMooers

Lake

BigCarnelian

Lake

LakesTwin

LakeJane

LakeElmo

Bone

Lake

Hard

wood

Cree

k

Creek

Creek

Hardwood

Creek Creek

Creek

Creek

River

River

St. Croix

Mississippi

Mississippi

River

RiverSt. Croix

RiverSt. Croix

Old M

ill Stream

Clearwater

Silver

Brown's

Valley

Valley

Branch

TroutBrook

Brown's

6

31

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31 36

66

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66

6

36

3636

11

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31

31

3131

36

3536

3631

31

3131

66

6 6

1

1 2

2

3536

3636

6

6

6

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1

1

1

25

31

31 31

31

4

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¤61

¤61

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¤10

¤10

¤10

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§¦694

§¦94

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§¦94

§¦94

SCANDIAFOREST LAKE

HUGO MAY

GRANTSTILLWATER

LAKE ELMO

BAYTOWN

WESTLAKELAND

LAKE

LAN

D

AFTONWOODBURY

DENMARK

COTTAGE GROVE

GREYCLOUDISLAND

St PaulPark

Newport

Cottage Grove

Woodbury

LakelandShores

BeachSt Croix

Lake

PointSt Mary's

Bayport

Oak Park Heights

Stillwater

MarineSt Croix

On

Forest Lake

Hugo

Dellwood

Oakdale

Pine Springs

Mahtomedi

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225

92° 52' 30" W.

CONTOUR INTeRvAL 25 MeTeRS

1 0 1 2 3 4 5 MILES

8 KILOMETERS

SCALE 1:100 000

1 0 1 2 3 4 5 6 7

EXPLANATION

The depth to bedrock is equal to the depth from the land surface to the underlying bedrock surface, and represents the thickness of Quaternary sediment cover over bedrock. To calculate this thickness, a digital grid of bedrock-surface elevations was subtracted from a corresponding digital grid of land-surface elevations. The surface elevation grid was resampled from a 1-meter digital elevation model derived from lidar by the Minnesota Department of Natural Resources, whereas the bedrock elevation grid was derived from the Bedrock Topography map, which was inferred from outcrop mapping and interpretation of water well records, engineering test borings, passive seismic data, and geomorphic features. The residual grid was then classified at 50-foot (15-meter) intervals to produce the color-coded Depth to Bedrock map. Thickness of the Quaternary sediments can vary greatly over short distances, and mapping at this scale (1:100,000) may not properly resolve prominent variations. For this reason, it is best to consult site-specific data (such as water well records, engineering test borings, and passive seismic soundings) wherever available. The detailed appearance of the Depth to Bedrock map is related to surficial landforms because the land surface topography model is based on higher resolution data than the interpreted bedrock surface model.

The thickest Quaternary sediments in Washington County occur within the Mississippi and St. Croix River valleys and their tributaries, where the depth to bedrock is as much as 400 feet (122 meters). Elsewhere, more than 300 feet (91 meters) of overlying sediment occurs within a buried bedrock valley in the northwest corner of the county near Forest Lake, and in the 20-mile-long (32-kilometer-long) valley passing through the south-central part of the county near Cottage Grove. Areas where bedrock is at or within 50 feet (30 meters) of the land surface occur within the southern one-third of Washington County, and along the eastern part of the county following the St. Croix River bluff.

Depth in feet from the land surface to the bedrock surface

1–50

51–100

101–150

151–200

201–250

251–300

301–350

351–400

401–450

T. 32 N.T. 32 N.

T. 31 N.T. 31 N.

T. 30 N. T. 30 N.

T. 29 N.T. 29 N.

T. 28 N. T. 28 N.

T. 27 N.T. 27 N.

T. 26 N.

R. 22 W.

R. 22 W.R. 21 W.

R. 21 W. R. 20 W.

R. 20 W.

R. 19 W.

92° 52' 30" W. 93° W.

44° 52' 30" N.

44° 45' N. 44° 45' N.

45° N.

45° 7' 30" N.

92° 45' W.

45° 15' N.

92° 45' W.

45° 15' N.

45° 7' 30" N.

45° N.

44° 52' 30" N.

93° W.

93° W.

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DAKOTA COUNTY

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ST. CROIXCOUNTY

POLKCOUNTY

PIERCECOUNTY

ST. CROIXCOUNTY

POLKCOUNTY

RAMSEYCOUNTY

RAMSEY COUNTY

Forest

Clear Lake

MudLake

SylvanLake

LakeNielsen

LakeGerman

Lake

Lake

Lake

Sea

Goose

Fish

Big Marine Lake

Long Lake

Lake

LakeSand

Hay

SquareLake

LakeTerrapin

TurtleLakeLake

Mud

Lake

Lake

OnekaLake

RiceEgg

Lake

SchoolSectionSunset

LakeLake

Long

LakeRound

LakeHorseshoe

WhiteRockLake

Lake

Bald Eagle

Lake

PineTree

LakeMann

LakeFish

LakeLong

White BearLake

LakeLong

Lake

LakeOlson

Demontreville

ClearLake

McDonaldLake

CloverdaleLake

LakeLong Lily

Lake

LakeMcKusick

LakeLoon

Silver

Lake

Louise Lake

LakeCarnelianLittle

LakeMasterman

BenzLake

SunfishLake

LakePoint

Eagle

LakeGoose

Lake

Lake

TannersLake

CreekBattle

Wilmes

Margrafs

Powers

Lake

Lake

ColbyLakeLake

Carver

LakeLa

BaldwinLake

ConleyLake

LakeEdith

LakeMooers

Lake

BigCarnelian

Lake

LakesTwin

LakeJane

LakeElmo

Bone

Lake

Hard

wood

Cree

k

Creek

Creek

Hardwood

Creek Creek

Creek

Creek

River

River

St. Croix

Mississippi

Mississippi

River

RiverSt. Croix

RiverSt. Croix

Old M

ill Stream

Clearwater

Silver

Brown's

Valley

Valley

Branch

TroutBrook

Brown's

6

31

11

31 36

66

66

66

6

36

3636

11

11

31

31

3131

36

3536

3631

31

3131

66

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1

1 2

2

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3636

6

6

6

6

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1

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31

31 31

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SCANDIAFOREST LAKE

HUGO MAY

GRANTSTILLWATER

LAKE ELMO

BAYTOWN

WESTLAKELAND

LAKE

LAN

D

AFTONWOODBURY

DENMARK

COTTAGE GROVE

GREYCLOUDISLAND

St PaulPark

Newport

Cottage Grove

Woodbury

LakelandShores

BeachSt Croix

Lake

PointSt Mary's

Bayport

Oak Park Heights

Stillwater

MarineSt Croix

On

Forest Lake

Hugo

Dellwood

Oakdale

Pine Springs

Mahtomedi

225

250

275

300

325

275

275

275

275

275

275

275

300

300

300

275

275

275

275

300

275

275

225

300

300

250

275

300

275

300

275

300

275

325

300

325

300

300275

300

300

250

300

275

300

300

300

225

300

275

275

300

275

275

275

300

300

225

300

275

300

300

300

300

300

300

300

275

300

225

275

300

250

300

300

275

300

300

275

300

275

300

275

300

250

275

300

300

250

300

250

300

300

300

275

275

300

275

325

250

300

275

250

225

300

300

275

300

325

275

275

300

300

275

300

275

275

300

275

275

275

275

250

300

300

300

275

300

300

300

300

275

300

275

325

275

225

300

275

275

275275

225

225

225

92° 52' 30" W.

Digital base modified from the Minnesota Department of Transportation BaseMap data; digital base annotation by the Minnesota Geological Survey.

Elevation contours were derived from the U.S. Geological Survey 30-meter Digital elevation Model (DeM) by the Minnesota Geological Survey.

Universal Transverse Mercator Projection, grid zone 151983 North American Datum

GIS compilation by R.S. Livelyedited by Lori Robinson