1 May 2011

Iceberg Drag Marks on the Bottom of Glacial Lake Agassiz

Posted by John Freeland

Remnant iceberg drag marks near Pembina, ND. (Source: NRCS Web Soil Survey)

The linear streaks shown on the aerial photograph to the right are believed to have been gouged by icebergs dragging on the bottom of wind swept Glacial Lake Agassiz. The streaks (Lat. 48 50′ 45″ N; Lon. 97 16′ 30″ W) are up to several miles long, hundreds of feet wide and several feet deep.

Glacial Lake Agassiz, named after the 19th Century naturalist Louis Agassiz, was the largest of the proglacial lakes formed at the end of the Pliestocene. Between 10,000 and 7,000 years ago, as glacial ice melted northward, the shoreline of Lake Agassiz shifted as new outlets were opened and the earth’s crust readjusted to the release of the glacier’s enormous weight in a process known as isostatic rebound.

The history of Lake Agassiz corresponded to major global climate change and the migration of the Clovis people into a regional ecosystem that transitioned from ice cover, to periglacial tundra, to spruce forest, to prairie in a few thousand years.

The Sherack Formation, a lacustrine silty clay deposit, is the parent material for a suite of soils found on the landscape that once was the bottom of Lake Agassiz. Driving north toward Winnipeg on Interstate 29, it’s difficult to imagine a flatter landscape in the world, but the drag marks create enough microtopography to cause contrasting soil conditions in the grooves versus the “ridges” between the grooves.

Soil map units related to scour marks (NRCS Web Soil Survey)

The figure (right) is a soil map from the Natural Resources Conservation Service (NRCS) Web Soil Survey. Two dominant soil types are shown on the map:
(BrA) Bearden silty clay loam, somewhat poorly drained, saline.
(Pu) Perella silty clay loam, poorly drained.

The Bearden soil is found on the “ridges” that appear as lighter streaks in the aerial photo. The Perella soils, often found in wetlands, are wetter and appear darker in the aerial photo compared to the Bearden. More information is available regarding these soils at NRCS Official Soil Series Descriptions

The pattern of mapped soil units illustrates the strong influence of parent material, i.e., the glacio-lacustrine sediments and the processes that deposited them – all made possible by Earth’s dynamic climate.