This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
U.S.G.S. engineer Bailey Willis(† February 19, 1949) was known for his unorthodox approach to geological questions. Puzzled by the geological structures he discovered in mountain ranges, long before computer-models were available, he constructed a machine to simulate the mountain-forming process.
In a box with a moveable piston he folded and crushed layers of beeswax and compared the structures with the large tectonic folds and thrusts he had mapped in the Appalachian Mountains. He realized that folds and nappes could form also by horizontal movements and compressive forces - not, as still many geologists argued, only by vertical movements (easier to explain at the time).
Fig.1.Willis´"Compression Machine for Experiments" from "The Mechanics of Appalachian Structure" (1891), all images in public domain.
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Fig.2. Miniature mountains made by the "compression machine" - the strata first form regular folds, however as the shortening continues, shear zones develop and single "tectonic nappes" start to pile up, as seen in real mountains.
Fig.3. Folded strata in the central Appalachian Mountains. In later years Willis proposed a first version of plate tectonics to explain mountain formation processes - the Atlantic Ocean was formed when a "bubble" of magma pushed apart the American and European continents, along the borders the layers of rocks were compressed and folded up - the Appalachian Mountains formed. Unfortunately this mountain range is significantly older than the Atlantic…
Fig.4. Willis subdivided mountain ranges in a central zone, characterized by folds, and an outer zone, characterized by shear zones (geological map of Cleveland in Tennessee). Today we know that the conformation can be much more complicated than that.