U.S. Department of the Interior Scientific Investigations 3392 U.S. Geological Survey Sheet 2 of 2

112°W 108°W 104°W 100°W 96°W " Pembina Warroad " A NORTHWEST SOUTHEAST B NORTHWEST SOUTHEAST FEET FEET Havre " 88°W " Shelby 1,600 1,600 Malta Rugby " " 92°W Big Falls 1,400 1,400 " Thief River Falls " Glasgow " Williston " 48°N Land Surface Land Surface 1,200 1,200 48°N " Grand Forks

112°W 1,000 1,000

Bemidji " 104°W " 800 800 Hibbing 68°W Houghton " 600 600 108°W Quaternary Sediment Quaternary Sediment

" Moorhead Jamestown " 400 "Bismarck Duluth 400 " Unsmoothed Surface Smoothed Bedrock Surface 200 " MAP OF BEDROCK Marquette " 200 Ashland " " Brainerd To create the bedrock , the simplest approach would be to subtract Quaternary sediment thickness grid values from land-surface " Ironwood Wakefield topography. That method would produce adequate results if the level of detail in both source were similar. This, however, is not the case for those Figure 2. Diagrammatic comparison of computed bedrock topographic surface. This example is from a profile partway across the Lower Peninsula of two datasets. In general, surface topography is directly observable, hence actual measurements are abundant and the level of detail for this surface is 84°W Michigan. A, Bedrock topography derived by simple subtraction of sediment thickness from land surface. Note the translation of fine topographic detail downward to the bedrock surface. B, Bedrock topography derived by subtraction of sediment thickness from land surface that was smoothed according relatively fine. In contrast, sediment thickness generally is inferred for large areas based on sparse, unevenly distributed observations. If we performed a 46°N 46°N 46°N 46°N simple subtraction, the fine detail of the surface topography would translate downward to the bedrock surface, as shown in figure 2A. In areas where the to focal-mean process explained in the text. glacial sediment is thin (for example, in New England), this approach produces a realistic result because the bedrock and land topographic surfaces are " St. Cloud Escanaba very nearly the same in many places. In that region, upland areas largely are comprised of patchy glacial deposits and exposed bedrock, whereas thick " Aberdeen glacial deposits are confined to -defined lowland areas. However in other regions where glacial deposits are thicker and cover a larger area, such as " Rogers City " the Midwest, there is little or no relation between land-surface and bedrock topography. There, bedrock drainage systems that developed prior to and Petoskey " between glaciations are buried and have no surface expression. In such areas, a simple subtraction produces artifacts; for example, the finely detailed St. Paul topography of a drainage network is translated downward to a bedrock surface buried by several hundred feet of glacial sediment (see fig. 2A). " 72°W Montevideo Minneapolis " " " Newport" To prevent translation of land-surface topographic features onto deeply buried bedrock surfaces, the following approach was taken. In areas of Eau Claire Wausau " Bangor thick Quaternary sediment, the surface topography was pre-processed in order to remove the high-frequency (fine) topographic detail, thereby minimiz- " 80°W " ing the likelihood that those features would appear as artifacts in the bedrock topographic surface. Our method consisted of the application of a Traverse City Green Bay focal-mean smoothing technique to the surface topography/ grid. The ArcGIS focal-mean technique recalculates a value for each cell as the " Burlington " " " Pierre Owen Sound 76°W mean value among a specified number of neighboring cells that surround each cell. By increasing the number of cells in the average, the grid can " Brookings " " Montpelier " effectively be smoothed. The smoothing was applied most strongly to the surface topography in areas of thickest sediments, and less strongly in areas of " Barrie " Augusta 68°W Manistee thinner sediments. Specifically, a mean statistic with a rectangular neighborhood of 3 cell units was applied to the land-surface grid in areas where " " Mankato " Manitowoc Kincardine Rochester " " 44°N 44°N Watertown sediment thickness ranges from 26–50 ft, a mean statistic with a rectangular neighborhood of 5 cell units was applied in areas where sediment thickness 100°W " 44°N 44°N ranges from 51–100 ft, and a mean statistic with a rectangular neighborhood of 10 cell units was applied in areas where sediment thickness is greater Port Austin La Crosse " Worthington Austin Portland than 100 ft. No focal functions were applied to areas where sediment was less than 26 ft thick. " Mitchell " Rutland " Sioux Falls " " The resultant smoothed land-surface regions (areas where sediment is 0–25, 26–50, 51–100, and >100 ft thick) were clipped based on original " regional extents, in order to remove edge artifacts introduced by the focal functions. The clipped grids then were merged to generate a single, smoothed " Saginaw land-surface grid. Rochester " " The bedrock topography grid then was generated by subtracting sediment thickness from the smoothed land-surface grid. In areas where the Glens Falls Madision Syracuse Utica " " Grand Rapids Flint " computed bedrock topography elevation exceeded land-surface elevation, bedrock topography cell values were replaced with land-surface values. Milwaukee " " London " " " Manchester Yankton Buffalo " Keene This approach yields an improvement over a simple subtraction, because it selectively filters the fine surface topographic detail, and it also tends to " " more accurately reflect the state of knowledge of the bedrock surface configuration (fig. 2B). In areas of thick sediment, subsurface information is " Lansing " Albany relatively sparse, and the generalized, somewhat “out of focus” appearance of the map directly conveys the relative uncertainty of fine bedrock Waterloo topographic features in those areas. " Sioux City " " Dubuque " Fort Dodge " Boston" Rockford Detroit " " Worcester DISCUSSION OF MAP FEATURES " Erie " " " Binghamton 42°N Cedar Rapids 42°N By comparing the regional configuration of the bedrock surface beneath Quaternary sediments (in other words, the bedrock topography) with the EXPLANATION Norfolk " Springfield Chicago inset map of land-surface topography (fig. 3), the reader may note many similarities and differences. These can be grouped as follows: (A) topographic Elevation, in feet (approximate) " " Providence " accentuation by of glacial sediments, (B) topographic masking by glacial sediments, (C) regional lithologic contrasts and deep-seated 5000 Des Moines " " " Toledo Poughkeepsie structural features, and (D) artifacts. " South Bend Cleveland Davenport " " 4000 Williamsport Scranton " A. TOPOGRAPHIC ACCENTUATION Omaha " " New Haven

3000 " " Large masses of glacial ice, such as the Laurentide Ice Sheet that covered the map area, do not flow in a uniform manner; rather, near the ice Galesburg " Fort Wayne Youngstown margin they adapt to the underlying bedrock topography, flowing more rapidly in the lowlands than over the uplands, thereby assuming a lobate form. Lincoln 80°W 72°W 2000 " Lima Newark The pre-glacial uplands (or topographic highs) were underlain by resistant bedrock that diverted the glacial ice (see discussion in Soller, 1992). With ice Peoria " " " 76°W New York flow thereby focused in adjacent lowlands, erosion in those areas was more pronounced than on the bedrock topographic highs. In contrast, along the " 1000 margins between two adjacent ice lobes, relatively thick sediment was deposited—especially where a bedrock high strictly constrained the ice lobes—thereby accentuating the preexisting bedrock topography. For example, note the Lower Peninsula of Michigan (fig. 3, locations A1 and A2) and 0 Muncie the Prairie Coteau region of eastern South Dakota (fig. 3, location A3). Stratigraphically, these thick accumulations of glacial sediment tend to be " Champaign " Falls City " " Kirksville relatively complex, and contain abundant coarse-grained sediment that serve as . 40°N " 40°N -1000 Columbus Indianapolis St. Joseph " Jacksonville " " B. TOPOGRAPHIC MASKING Hannibal " " Springfield " -2000 Dayton " Where old river courses were aligned at right angles to the ice flow, the glaciers tended to flow over them without significantly altering their form, Concordia Terre Haute " and filled them with glacial sediment. These old topographic features are now essentially masked, as there is little or no evidence of them at the land surface. One of the most notable of such preserved buried valleys is the network of drainageways known as the “Teays River System” (fig. 3, location Manhattan " Bloomington B1, shows one segment of the Teays). Extending through Ohio, Indiana, and Illinois, this feature actually is a composite of various ancient river " " Cincinnati " Columbia drainages that have no modern descendant, for example the Mahomet Bedrock Valley in Illinois (Kempton and others, 1991). The Mahomet Bedrock Topeka " Scioto" Valley (fig. 3, location B2) is buried, with no visible trace at land surface; this Valley is filled with a thick deposit of and that serves as an 96°W D3 important source of ground water for the region. 84°W A3 Huntington 1 Mount Vernon " A C. REGIONAL LITHOLOGIC CONTRASTS AND STRUCTURAL FEATURES 92°W " " The bedrock topographic surface was formed by water and glacial ice eroding the exposed geologic units. Each unit, and local areas within each Louisville Base modified from U.S. Geological Survey digital data, Evansville GIS database and digital by David R. Soller unit, withstood erosion to a different degree. The topography reflects this differential response. For example, granites and well-lithified sandstones 38°N " 38°N 1:1,000,000-scale without significant fractures or other planes of weakness tended to resist erosion and formed topographic highs, whereas more poorly lithified or and Christopher P. Garrity A2 fractured rocks tended to erode and are today found beneath topographic lows. By referring to regional bedrock geologic maps, correlations between Web Mercator projection, World Geodetic System of Digital cartographic production by Christopher P. Garrity 1984 D2 bedrock lithology, structural features, and the pre- and post-glacial topography can readily be identified. Edited by Regan Austin B2 Manuscript approved for publication December 1, 2017 88°W D. ARTIFACTS B1 In some cases, prominent features shown on the map are merely artifacts of the data. For example, (1) In most places along the margin of the glacier, sediment thickness decreases to zero. However, in two areas the glacial sediments extend far beyond the limit of glacial ice, and the map area cuts arbitrarily across these deposits; these are the thick glacial outwash deposits in the Mississippi River Valley (fig. 3, location D1), where deposits of glacial origin extend southward into the Gulf of Mexico, and eastern Nebraska (fig. 3, location D2) where glacial sediments are interlayered with western-source and . Because of the arbitrary limit of mapping, these areas on the bedrock topographic map appear to have abrupt edges. D1 (2) In eastern South Dakota (fig. 3, location D3), a large wedge-shaped feature known as the Prairie Coteau is visible at land surface. This feature is underlain in places by resistant bedrock, but the configuration of the bedrock surface is not precisely known. The bedrock topographic map shows Figure 3. Shaded relief map of land surface topography, derived from USGS National Elevation Dataset (NED). By examining the land surface and the bedrock sharp, well-defined eastern and western edges to this feature, similar in nature to its surface expression. Although it is possible that the land-surface and topographic maps, along with the drift thickness map, areas underlain by relatively thick and thin deposits of unconsolidated, Quaternary sediment can be compared bedrock topography are quite similar, it is more plausibly an artifact produced by slightly inaccurate placement of sediment thickness contour lines and contrasted. The selected locations (for example, A1, D2) highlight areas of particular interest (see “Discussion of Map Features”). (which are highly interpretive) on the source map, relative to abrupt changes in elevation (which is directly measurable, and not interpretive). Because this map was derived strictly by computation, and did not benefit from geologic interpretation, artifacts such as this are unavoidable, especially in areas of thick Quaternary sediment. It also is likely that the pitted nature of the bedrock surface on Michigan’s Lower Peninsula (fig. 3, location A1) is an artifact caused by incomplete smoothing of the land-surface elevation data.

Map of Bedrock Topography

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not Quaternary Sediment Thickness and Bedrock Topography of the Glaciated United States East of the Rocky imply endorsement by the U.S. Government This map or plate is offered as an online-only, digital publication. Users should be aware that, because of differences in rendering processes and pixel resolution, some slight distortion of scale or color may occur when viewing it on a computer screen or when printing it on an electronic plotter, even when it is By viewed or printed at its intended publication scale. Digital files available at https://doi.org/10.3133/sim3392 Suggested citation: Soller, D.R., and Garrity, C.P., 2018, Map of bedrock topography, sheet 2 in Quaternary David R. Soller and Christopher P. Garrity sediment thickness and bedrock topography of the glaciated United States east of the Rocky Mountains: ISSN 2329-132X (online) U.S. Geological Survey Scientific Investigations Map 3392, 2 sheets, scale 1:5,000,000. 2018 https://doi.org/10.3133/sim3392 https://doi.org/10.3133/sim3392.