Glacial landform assemblages associated with the retreat of the Rainy and Superior lobes of the Laurentide Ice Sheet in Minnesota provide a record of spatial and temporal variations in glaciological processes. The two lobes advanced contemporaneously to their maximum at the St. Croix moraine. The portion of the moraine formed by the Rainy lobe is characterized by large frontal outwash plains, broad ice-stagnation topography, glacial thrust systems, and an absence of streamlined landforms and features produced by subglacial drainage. Recessional moraines are similar to the terminal moraine, but largescale glacial thrust systems are even more common. In contrast, the part of the moraine formed by the Superior lobe is composed of closely spaced recessional ridges with little ice-stagnation topography. Recessional ice margins are marked by the termination of eskers and tunnel valleys into subaerially deposited outwash fans. Drumlins are abundant, and sugblacial drainage features are well developed, as indicated by the numerous tunnel-valley and esker systems. The thermal regime of the Rainy lobe changed little during retreat. Landforms indicate that a frozen toe existed at the terminal and recessional positions, and that conditions were too cold to allow surface meltwater to penetrate to the glacier bed. The Superior lobe, on the other hand, apparently underwent an evolution from a subpolar glacier at its maximum to a temperate glacier during ice recession. Englacial and subglacial drainage systems became better developed during retreat, and drumlin formation was favored during recessional phases.
|Number of pages
|Bulletin of the Geological Society of America
|Published - Jan 1 1990