Update 12/08/2010:  The bedrock topography PDF and the Depth to Bedrock PDF have been updated in 2010 
as part of the new bedrock geology map of Minnesota.  These are listed under the Open File 
report OFR10_02, but have been added to the PDF folder here as well.




Minnesota Geological Survey Open File Report OFR2006_02

Maps of Gridded Bedrock Elevation and Depth to Bedrock in Minnesota

R.S. Lively, E.J. Bauer, and V.M. Chandler
Minnesota Geological Survey
January, 2006

This open-file of bedrock elevation (be) and depth to bedrock (dt) grids and data points 
for Minnesota represents the first phase of an ongoing process to create new statewide 
maps from these data.  The grids and data can be viewed using either ArcView 3.x with 
Spatial Analyst or ArcMap 8, 9.x. The grids and point data use the UTM, NAD 83, Zone 
15 projection and datum.  ArcMap .lyr files are provided to color each map in ArcMap at 
a presset interval.  No legend files are provided if using ArcView to view the grids.  The 
Mn state outline and County outlines are also provided.

As a first step we created two grids, "be_loc" and "dt_loc," from the most accurate 
primary data available—wells in the County Well Index (CWI) database with verified 
locations that intersect the bedrock surface.  These grids are the best algorithm-based 
representations of bedrock topography and bedrock depth available at the present time.  
The CWIpt_loc data set should be reviewed so that the user can see where accurately 
located data are plentiful and where they are sparse or absent.  The likelihood that the 
grids accurately represent real conditions is less in areas of sparse data.  As a second step 
we created two additional gridded surfaces, "be_loc_unloc" and "dt_loc_unloc" that 
added data from wells without verified locations from the CWI data set.  The positions of 
wells in the unlocated set were derived from township, range, and section data that 
appeared on the original records.  Additional uncertainty is attached to elevations that 
were extracted from a 30 meter DEM grid, and interpretations of what constitutes the 
bedrock surface may be in error through lack of interpretation.  Nevertheless, the increase 
in detail gained by using the unlocated wells may be significant, so the maps are provided 
with the caveat that errors may be found and changes will occur.  Both sets of maps 
represent point-in-time data models that can be verified or altered with acquisition of 
additional data. 

The utility of these maps varies with the density of data in any given area—some areas 
might appropriately be used for some purposes at scales as large as 1:24,000, but in other 
areas, where data are widely distributed or nonexistent, usage may not be appropriate at 
any scale.  The greatest value of the current grids is in the statewide perspective they 
provide.  Locally, state bedrock elevation and depth to bedrock maps have been created 
by more reliable and comprehensive methods.