We mapped the glacial geomorphology preserved on the MnIS bed using the best available digital bathymetry data (fig. S1). This work supersedes all previous compilations (16, 17, 43, 49), as much of the bathymetric data used in this study was not available at that time.

To perform geomorphological analyses, we produced the following derived datasets in ArcGIS 10.4: (i) grayscale shaded relief maps (1× and 3× vertical exaggeration) with NW and SW illuminations, (ii) bathymetric surface contour maps (at 1-, 2-, and 5-m vertical intervals), (iii) slope (gradient) maps (0° to 90°), (iv) slope aspect maps (000° to 359°), and (v) profile curvature maps (in degrees m−1). To avoid mapping errors caused by azimuth bias (50, 51) and to ensure faithful landform representation, 2D cross-sectional profiling techniques were used (in Fledermaus and ArcGIS), alongside the first- and second-order derivatives of the bathymetric grid. Color and grayscale shaded relief maps (geotiffs) of digital elevation models were primarily used for 3D visualization purposes and cartographic presentation. All maps and datasets (onshore and offshore) were projected in Geographic Coordinate System GCS-WGS-1984 to minimize geolocation errors. Submarine glacial landforms within the former flow path of the MnIS were digitized in ArcGIS 10.4. Because of the large size of the study area (~10,000 km2) and differing resolution bathymetric datasets (see fig. S1), it was not possible to map all of the study area at the same level of detail, although mapping was typically conducted at 1:10,000 scale. In this study, we paid particular attention to the following: large transverse ridges within the ice-stream trough, other ice-marginal features (moraines, etc.), other evidence of ice-stream/ice-shelf grounding, and areas of hard substrate (bedrock) at seabed.

For computational convenience, large transverse ridges (mapped as GZWs) are referred to by their distance along the ice-stream trough relative to the trough centerline, starting at the continental shelf break datum (0 km; see Fig. 2 for examples). For the purposes of this work, the MnIS trough is defined by the 100-m present-day water-depth contour (except for in the outermost shelf west of 6°30′W where the 120- and 130-m contours are used, as the 100-m bathymetric contour does not come close enough to the shelf break to generate a closed polygon). The trough is defined at the shelf break by the 200-m water-depth contour. A generalized “trough outline” polygon was generated on the basis of a 1-km regridded surface (contour) model of best-available bathymetry. From this polygon, a centerline (equidistant from both trough margins) was generated using a script in ArcGIS.

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