Background: Important geological concepts were established as naturalists began studying the rock formations of the Alps in the 18th century. In the mid-19th century the now defunct theory of geosynclines was used to explain the presence of "folded" mountain chains but by the mid-20th century the theory of plate tectonics became widely accepted.
Question: When did the studying of rock formations begin in the Alps?
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<extra_id_0> the 18th century<extra_id_1>