FACTORS AFFECTING THE SPATIAL PATTERNING OF LOESS IN WESTERN WISCONSIN

Many loess deposits cover large swaths of the land surface as more-or-less continuous blankets, becoming progressively thinner and finer-textured downwind. However, in areas like the upper Great Lakes region, where loess deposition was relatively rapid, and at sites farther from prodigious source areas, loess deposits are thinner and spatially discontinuous, with inlier areas that lack loess entirely. I discuss generally the reasons for such spatial variation, with examples from the Peoria-aged loess landscapes of western Wisconsin, where our research group has amassed loess data from >1000 sites.

The study area, centered on the Chippewa River valley, is a mix of bedrock-controlled uplands in the west, and lower-relief, till-covered plains in the east. The river valley carried Wisconsin-aged outwash, and was a loess source. The thickest (> 5m) deposits are on the lee (SE sides) of large bedrock uplands, possibly due to wind-shadow effects. These deposits can extend downwind for 1-3 kms, and follow a clear NW-SE trajectory, indicative of transport on strong NW winds. The NW sides of these ridges have either thin loess, or none at all. Many ridges are eroded down to bedrock, presumably through a combination of periglacial stripping and eolian erosion. Published luminescence dates indicate that loess deposition may have continued here into the early Holocene, suggesting that many ridges, rendered unstable by Late Pleistocene periglacial activity, later stabilized and, in preferred locations, were later covered with loess. Most of the smaller ridges were apparently too low to develop a notable wind shadow, and thus lack measurable loess on all sides.

Loess sources and topography in the region also variously impacted loess textures. Loess is coarsest in the immediate lee of large ridges and near broad valley trains. A belt of fine-textured loess, however, exists in the lee of the broad, Late Wisconsin moraine; hummocky topography here inhibited sand transport out of the moraine. These fine silts were primarily sourced from ice-walled lake plain deposits, which emerged as lakes periodically drained, or fresh exposures of till that formed as ice blocks melted. In summary, loess patterns in this region can be mainly attributed to silt transport pathway-topography interactions, mainly driven by northwesterly winds.