Rather than concluding that the object that hit Canada 12,900 years ago was a comet, I wonder whether there might not be an alternate reason that geologists haven’t discovered a large hole. If a meteor hit a kilometer-thick glacier, would it have left a crater in the rock underneath the ice?

Peter Shor
Wellesley, Mass.

Scientists “haven’t discovered a large, smoking hole” left by the event. Have they considered that James Bay and Hudson Bay look remarkably like what you’d expect an impact crater to look like?

Wm. Carter Elliott
Reseda, Calif.

Now I am inclined to believe that this comet explosion had more to do with the demise of the Pleistocene mammoths, bears, camels, and other animals than superefficient hunting by Clovis hunters did. I have always had doubts that the Clovis hunters wiped out the large animals.

Michael F. Crowe
Aurora, Colo.

Previous expeditions to the area around Hudson and James Bays haven’t found incontrovertible signs of an extraterrestrial impact, such as shocked grains of quartz or cone-shaped zones of shattered bedrock, says Allen West of Geoscience Consulting in Dewey, Ariz. However, he notes, the thick ice sheet overlying the area at the time might have softened the blow from space somewhat, eliminating or minimizing such evidence. On the topic of extinctions, the impact had only a regional impact and didn’t wipe out mammoths living in what are now Mexico and Siberia. —S. Perkins