Glacial landforms, sedimentology, and depositional environments of the Des Moines Lobe, northern Iowa

Geologic investigation of seven counties in north-central Iowa reveals that the Wisconsinan Des Moines Lobe deposited an intricate assemblage of glacial landforms much more complex than the simple end moraine and ground moraine areas of previous studies.;Upland landforms were mapped using a descriptive classification consisting of ridges, escarpments, plateaus, ramp-like slopes, discontinuous ridges and hummocks, and plains. These types were further subdivided based on the presence or absence of hummocky topography, patterns on aerial photographs, and local relief. These landforms are composed of complex sedimentary successions that differ between types.;Three types of drainage and depressional features were mapped: present drainage lines (major stream valleys); abandoned channels and linked-depression systems; and large semi-closed and closed depressions. Linked-depression systems have not been previously identified. They are valley-like landforms with semi-closed depressions along their courses. They connect present drainage lines, giving the drainage system a complex, interconnected pattern.;Broad tracts of a new type of hummocky topography occur at or near margins of former glacial advances. In these areas, the "hummocks" are not knobs rising above adjacent uplands. Instead, linked-depression systems are inset into the landscape, and the "hummocks" are rounded areas remaining next to the linked-depressions. The internal stratigraphy and geometry of the hummocks and the occurrence of the linked-depression systems can be explained by the development and collapse of a glacier-karst drainage system. After the glacier advances and stagnates, melt-out till is deposited and a karst-like drainage system develops in the ice, removing debris from the tunnel areas. With further ice wastage, the tunnels collapse and material is resedimented off the hummock cores toward the former tunnel areas; the collapsed tunnel areas form linked-depression systems. Any supraglacial deposits are eventually let down, draping the underlying deposits.;The distribution of landforms and the associated sedimentary successions suggest deposition by a surging glacier, one that makes very rapid advances and then stagnates. Surging behavior also explains why the Des Moines Lobe advanced later and has different landforms than lobes further east.