The imprint of glacial isostatic adjustment has long been recognized in shoreline elevations of oceans and proglacial lakes, but to date, its signature has not been identified in river long profiles. Here, we reveal that the buried bedrock valley floor of the upper Mississippi River exhibits a 110-m-deep, 300-km-long overdeepening that we interpret to be a partial cast of the Laurentide Ice Sheet forebulge, the ring of flexurally raised lithosphere surrounding the ice sheet. Incision through this forebulge occurred during a single glacial cycle at some time between 2.5 and 0.8 million years before present, when ice-sheet advance forced former St. Lawrence River tributaries in Minnesota and Wisconsin to flow southward. This integrated for the first time the modern Mississippi River, permanently changing continental-scale hydrology and carving a bedrock valley through the migrating forebulge with sediment-poor water. The shape of the inferred forebulge is consistent with an ice sheet ~1 km thick near its margins, similar to the Laurentide Ice Sheet at the Last Glacial Maximum, and provides evidence of the impact of geodynamic processes on geomorphology even in the midst of a stable craton.
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