No. 97: Jan-Feb 1995
Back in 1883, I. Donnelly wrote his seminal Ragnarok, to which he attached as a subtitle: The Age of Fire and Gravel. He hypothesized that all those sheets of unconsolidated rocky debris strewn across the planet--called the "drift" -- were the consequence of impacts of comets. But in Donnelly's day, all geologists were uniformitarians and wedded to glacial theory. Donnelly's "age of fire and gravel" was really a succession of Ice Ages. Quite a difference in mechanism!
Glacial theory, however, has difficulty in explaining apparent glaciations during periods when the earth was supposed to be very warm. Nor does it account easily for glacial-like debris in equatorial regions.
With the current ascendancy of "impact geology," some brave geologists are reinterpreting supposed glacial deposits in terms of sheets of ballistic ejecta from the impacts of comets and/or asteroids. Modern estimates of terrestrial cratering suggest that 10% of our planet's surface could be covered by 10+ meters of ejecta, and 2% by 200+ meters. Now that's a lot of ejecta!
(Rampino, Michael R.; "Tillites, Diamictites, and Ballistic Ejecta of Large Impacts," Journal of Geology, 102:439, 1994.)
Comment. The Ice Ages won't be melted completely away by such reinterpretations. Nor will I. Donnelly ever get any credit for his research and vision. But we are making progress.