Yes. If you had an extraterrestrial event, and impactors struck the ice sheet and broke it up, you would expect interruption of normal flow of the ice, increased subglacial flow, and perhaps formation of large lakes peripheral to the ice sheet at time of disablement. Ice ejecta beyond the periphery of the ice sheet, perhaps covered with dust, would form kettle lakes. This mechanism could possibly explain the tunnel channels as well. There are eskers in Muncie, Indiana which may predate this single event. Kettle lakes are common in WI, MI, OH, MN. There are relatively young impact craters (rel. small) peripheral to the LIS at the Younger Dryas. These are the Corossol Crater (Higgins 2011) and the Bloody Creek STructure (Spooner et al 2009). Charity Shoals may be another in Lake Ontario (Holcombe 2001). There are many pockmarks on the floor of Hudson Bay, St Lawrence Seaway, and the Scotia shelf, that may be impact scars, or could be methane seeps. Lajeunesse and Duchesne are studying these.

I think the Finger Lakes and perhaps Great Slave Lake, Lake Athabasca, and Lake Winnepeg may mark the extent of the LIS at the time of this event, particularly if they are deep lakes and contain a lot of sediment.

Thank-you for your reply. The timing is probably the critical concern here. For the drumlins and other bedforms to form in large fields with a coherent pattern there must have been large-scale, coherent flows at least as extensive as the drumlins themselves. There is debate on how drumlins form, but in this discussion, genesis of individual forms is not important—it becomes important when we use evidence to interpret formation. There are four or five large fields extending from the District of Keewatin in the NWT, and I think they formed at different times, though the evidence for this is not strong. All of this marks very well organized flow systems, and a significant part of my question is the pristine preservation of the glacier bed. If an impact occurred before the drumlins and flutings, it doesn’t seem to have influenced their formation. If it occurred after the drumlins (bedforms) formed, and, just as you say, the ice sheet was broken up, then the pristine, subglacial bed would not have been preserved.

In some explanations, tunnel channels are closely related to drumlin formation, and with eskers, are also part of the pristine bed.

I think the above provides evidence that there wasn’t a major impact in the area of the major drumlin fields of the Laurentide Ice Sheet, though I am aware of the idea of a Younger Dryas impact on the Laurentide Ice Sheet, by Firestone et al., but I don’t think any impact structures have been found. This idea has come under a lot of evidential criticism, and I am not sure of its status. There was also the idea of drumlin formation by meteorite impact on the Laurentide ice sheet (circa 1987) by Chester Davis; unfortunately I can’t find this by Google.

For the ice-block missiles explanation for kettles, if the ice contained debris, you might find “exotic” material in the kettles, and there should be deformed ground, and may be a rim around each kettle.

Your point on the lakes and their sediment suggests that iridium may be found.

I am sorry that this is a rather negative response. You have a lot of new ideas about the ways things could have happened. I am not any different; I am also suggesting how things could have happened, not how they did happen. If you don’t follow my reasons or have another point of view, please write.

Hi John

Thanks for your feedback.

I think your response is quite reasonable. This impact to the LIS is just an idea I have been thinking about but have not studied the subglacial evidence and geomorphology systematically.

The impact could have struck part of the LIS and not affect subglacial landforms in another region. Was any of the LIS cold-based? that might provide partitioning of effects of this extraterrestrial impact idea. You could possibly vaporize some of the ice sheet and not disturb the subglacial landforms.

Also, an impact resulting in surges of meltwater from the top of the LIS delivered to

the bottom, could bring about drumlin fields at the time of the event. As the remaining ice decays around these structures, they would be preserved. Just thinking about alternative mechanisms. I think I saw a study on drumlins forming today, in Iceland so you might not need a catastrophe, but on a large scale it might work, particularly since the LIS is no longer with us.

I listed the impact structures that have been found in my first post, and I listed ongoing research into Hudson Bay, St. Lawrence seaway et al pockmark structures.

Wu et al 2013 found microspherules in Pennsylvania that they sourced to Quebecia terrain, and interpet these as ejecta. Many others have published papers in the last few years so the hypothesis is getting support.

Iridium for the KT impact was in amounts of PPB. Ir is not routinely analyzed for in lake sediments, which I what I study.

Petaev et al 2013 found Pt in the Greenland Ice Sheet for YDB. Perhaps we should look for Pt in lake sediments.

If the ice chunks impacted soft sediment during possible meltwater overland runoff, you might not see a rim.

With an ET impact, you could potentially transport ice to the North Atlantic, depending on the angle the impactors struck (speculation of course), without having an outlet from glacial Lake Agassiz, for instance.

Joanne