Snowball earth? (1835–40)
Snowball earth? (1835–40)
The explanatory problems raised by erratics and scratched bedrock remained at the heart of debates about geohistory as a whole, because they impinged crucially on the problems of reconstructing the part of geohistory closest to the present world. This chapter describes how these problems came much closer to being resolved. The three rival explanations of erratic blocks and scratched bedrock surfaces—that they were traces of a “diluvial” mega-tsunami, or of submergence in a sea with drifting and melting icebergs, or of vast glaciers extending from a newly elevated mountain range—were joined in the later 1830s by a fourth and even more sensational possibility. Louis Jean Rodolphe Agassiz proposed that the whole earth—or, at least, the northern hemisphere as far south as north Africa—had been covered by a static sheet of ice during an “Ice Age,” before the sudden and geologically recent upheaval of the Alps; the latter had merely produced a tilted surface of ice, down which the erratics could slide from Alps to Jura. Only after this had a warming climate melted the static ice-sheet and turned its remnants into slowly retreating valley glaciers. Agassiz integrated this “glacial theory” into his larger picture of geohistory, by suggesting that the Ice Age had been just the most recent in a succession of catastrophic climatic crises in the history of life.
Keywords: erratics, bedrock, geohistory, Ice Age, Alps, Louis Agassiz
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