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There should be many small craters made by those comets. However,even if they said each has the power of megaton bomb, its depth cannot be as deep as the one we tend to associate with the power of such magnitude. It does not help that earth was still covered with thick ice and when glacier finally melt, the part of crater itself could have been washed out. Then, there is a chance that comet broke up in the atmosphere.

I remember somebody tried to find out exactly how many craters Earth has, upon seeing so many on the Moon. He figured that Earth must have as many as the Moon. He wondered where they were. Or Earth is really lucky to escape barrage from space? I think he used some earth-penetrating radar(?) to sweep the earth and found out there are a lot more crater impressions than we see on the surface. Most have been covered up.

I am sure many comets landed on earth and made some marks. We have to track down all those crater impressions under the surface and ascertain their age. That is a lot of work but I hope they are able to do it someday.

It does not help that earth was still covered with thick ice and when glacier finally melt, the part of crater itself could have been washed out.

Actually, I am near the southern edge of the last advance, marked in North Dakota by the Missouri River. About half of North America was not covered by the ice at that point, although just north of the river the thickness estimate for the ice sheet is over a kilometer.

While glacial outwash, loess (windblown sediment), and normal processes since then have doubtless obscured many of the features, satellite imaging in the infrared spectrum and radar mapping have disclosed many features both natural and artificial which are otherwise not readily apparent in high altitude imagery. I would hope the writers would pursue attempting to identify such features, and if they find any possibles in NW North Dakota or NE Montana I would be willing to put in some spare time doing field work and sampling to help verify or refute individual sites. (I have a strange concept fo 'fun', at least to most folks.)

My interest is in finding what may have driven any climate change at that point.

I do not find the evidence supportive of Anthropogenic Global Warming/Climate Change (not beyond a local scale), but I do believe natural processes have been at work, including impacts of material from space.