BEDROCK TOPOGRAPHY DEPTH TO BEDROCK
Transcript of 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.
ANO
KA C
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CHISAGO COUNTY CHISAGO COUNTY
DAKOTA COUNTY
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WIS
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DAKOTACOUNTY
PIERCECOUNTY
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
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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
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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.
ANO
KA C
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CHISAGO COUNTY CHISAGO COUNTY
DAKOTA COUNTY
ANO
KA C
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WIS
CO
NSI
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WIS
CONS
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DAKOTACOUNTY
PIERCECOUNTY
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
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31 36
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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
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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