PRELIMINARY BEDROCK TOPOGRAPHY AND DEPTH TO BEDROCK MAPS
OF THE CENTRAL ARROWHEAD AREA
Amy L. Radakovich, Val W. Chandler, R. S. Lively, and Dale Setterholm
February 2017
Prepared and published with the support of
the St. Louis and Lake County Boards of Commissioners, 
and the Minnesota Environment and Natural Resources Trust Fund
as recommended by the Legislative-Citizen Commission on Minnesota Resources

BedrockTopography_DriftThickness Folder Contents:
1.	README—this file.
2.	PDFs of Bedrock topography; Drift thickness (depth to bedrock); and a PDF of the Bedrock topography Database (BT_Database.pdf).
3.	Map Package containing an mxd and the following associated data sets:
•	USGS 7.5-minute quadrangle map outlines and names.
•	Outcrop—includes 88,880 polygons representing bedrock at or near the land surface, including both those visited in the field and those inferred from LiDAR and air photo.
•	Passive Seismic—includes 308 points representing stations from which data on depth to bedrock were acquired.
•	QDI-Quaternary data locations—static view of the Quaternary Data Index; an internal working database of the MGS. Some of the locations were used in the construction of the bedrock topography, mostly drill holes such as soil borings, engineering borings, and Giddings sites (listed in attribute table as “SA,” “GD,” “SR,” and “SR” types)
•	CWI—static version of County Well index as of January 2017 containing the records of 9652 water wells and other drill holes, subdivided into those that intersected bedrock (bedrock drill holes) and those that did not intersect bedrock (Quaternary drill holes).
•	Drift Thickness raster—represents depth to bedrock at 25’ intervals.
•	Bedrock Topography—contour line-file at 50’ contour intervals.
•	Bedrock Topography polys—polygon file at 50’ intervals.
•	DNR 30m DEM—land surface elevation at 30m, derived from 1m LiDAR data. Contains lake bathymetry where available.
•	Bedrock Topography raster—image represents bedrock elevation, feet above sea level.
 
The coordinate system for all data in the folder is UTM, NAD83, Zone 15N.
The BedrockTopography_DriftThickness.zip contains files associated with part two of MGS Open-File Report OFR2016_04. Part one of the Open-File Report, released in July 2016, included bedrock geology and surficial geology maps and data.
 This 2017 addition to the OFR contains the GIS files in an ESRI map package (bt_dt_openfile.mpk) such as bedrock topography in ESRI raster, line, and polygon format, and the depth to bedrock in ESRI raster format, and PDF files of selected maps. The map package also includes the data sets used to construct the maps including bedrock outcrops, Quaternary drill holes, the County Well Index (CWI) of water well and other drill hole locations, and passive seismic stations. The map package, when double clicked, will open an ArcMap session (bt_dt_openfile.mxd) with the data as saved. The maps and data presented here are preliminary and should be considered “regional scale.” More site-specific investigations within this map area will require acquisition of additional data. 
The study area known as the Central Arrowhead covers ~1600 mi2 in both St. Louis and Lake Counties. It is the first of several map areas that when completed and combined will be refashioned into County Geologic Atlases for the two counties. The Central Arrowhead area is bounded by the 7.5 minute quadrangles of Vermillion Dam in the northwest, Ojibway Lake in the northeast, Greenwood Lake East in the southeast and McKinley in the southwest. Part of the area includes the eastern end of the Mesabi Iron Range, where the bedrock topography was mapped and published as Miscellaneous Map series M-163. The M-163 bedrock topography represents the pre-mining bedrock surface circa 1899 contoured in 20 foot intervals. For the current project, pre-existing bedrock topographic lines in the M-163 area were locally minimally modified to reflect the current bedrock surface using new data, and were then used to create the bedrock topography raster. The bedrock topography lines for the Central Arrowhead were drawn based on water well, scientific, and exploratory drill hole data, the pre-existing bedrock topographic lines from M-163, outcrop locations, 1 m LiDAR imagery and topographic lines derived from the LiDAR, and depth to bedrock determined at passive seismic stations Note that the CWI and Quaternary data included with this package are static and may change slightly over time. Check with the County Well Index (CWI) online for updated information. Also note that the most generalized areas of the bedrock topography reflect a lesser amount of available data, whereas more intricate contouring was derived from areas with more abundant data. Additionally, the 50 foot intervals contoured from outcrop and subsurface data are necessarily much more generalized and less detailed than either the one-meter, or the 30 meter LiDAR-based land surface topography. It is important to understand that because of the large differences in data density, the bedrock raster will never exactly match the land surface, even where bedrock is very close to or at the land surface.
A set of bedrock topography contour lines at a 50 foot interval were drawn within the Central Arrowhead study area. Contour lines within the mine land areas were redrawn from the pre-mining contours available in M-163 and relied on new data from LiDAR, passive seismic stations, water wells and Quaternary drill holes not available when M-163 was developed. From these 50 foot contours, a bedrock topography raster was created and adjusted to agree with the land surface topography and outcrop elevations. The Depth to Bedrock (Drift thickness) raster was then calculated by subtracting the bedrock topography raster from the land surface 30m DEM and color-classified for display at 25 foot intervals. Note that some lakes and mine pits on the DEM contain bathymetric data; in these areas the calculated drift thickness is necessarily thinner than in areas without bathymetry, where the water surface may become the top of bedrock. 
The bedrock topography lines were edited to be topologically correct and used to create bedrock topography polygons representing the 50-foot intervals of bedrock elevation. Some minor discrepancies may exist between the bedrock topography contours that were drawn in late 2016 and the enclosed CWI data set sampled in early 2017. Data acquisition is on-going. Modifications of the maps to accommodate these and other new data will occur as the St. Louis and Lake County atlas products are completed.