Charles G. Spencer

Picked up at the 2013 GSA convention, along with a few hundred dollars worth of other books. For high-speed geology Missouri suffers in that most of the intriguing stuff is off the main highways; I-70 runs through relatively boring flat sedimentary rock (author Charles Spencer notes that the limestone formations are difficult to tell apart in outcrop even under close examination, much less out the window at 75mph). However, there are some interesting geofeatures on the back roads.


I’ve posted a couple times before about my annoyance with people who imagine that the geography of the United States has somehow remained unchanged since time immemorial; I’ve countered this by commenting that when humans arrived in North America there were no Great Lakes, no Cape Cod, and no Mississippi River (well, nothing like the Mississippi we see today at least). It turns out the same holds true for the Missouri River; almost its entire course was under ice during the Pleistocene and the ancestral Kansas River more or less took its place. Alas, glacial geology and ancient river channels are interesting but subtle, not the kind of thing that you can see from the highway. Missouri has the distinction that almost all of its glacial geology is pre-Wisconsinan; the last glacial advance, for one reason or another, didn’t extend a lobe into the state.


The main sightseeing geology in the State is in the Ozarks and St. Francois Mountains; the St. Francois, in particular, are a set of overlapping calderas similar to Yellowstone but a billion and a half years earlier. The same general area produced what was for a while the world’s largest lead mining district; again the main sites are not on major highways. Spencer provides an explanation of lead ore formation through hydrothermal processes.


One of the most interesting features – again, alas, with absolutely nothing that can be seen from the roadside – is the 38th parallel lineament. This is a series of “cryptoexplosion” structures – the Weableau-Osceola structure, the Decaturville Structure, the Hazelgreen Structure, the Crooked Creek Structure, the Furnace Creek Structure, and the Avon Diatreme, more or less lined up along the 38th Parallel. (Spencer doesn’t mention it but there are other structures in Kansas (Rose Dome); Illinois (Hicks Dome); and Kentucky (Jeptha Knob) that are also more or less aligned. The 38th parallel structures have an interesting relation to geological philosophy. They were originally called “cryptovolcanic” rather than “cryptoexplosion”; it was assumed that brecciation, uplift and general circular structure were somehow volcanic. However, further examination didn’t find any other signs of volcanism – no lava or volcanoclastics. Various geologists speculated on maar explosions or some other sort of volcanism that didn’t involve melting any rock; this led eventually to the structures being called “cryptoexplosion” structures rather than “cryptovolcanic” structures. In the late 1950s and early 1960s, some geologists began suggesting these things (and others around the world) were the remnants of meteorite impacts. This met with considerable resistance from the geological establishment; one prestigious geologist compared belief in meteorite impacts with belief in UFOs and suggested impact proponents get psychiatric help. The 38th parallel structures were particularly cited by impact opponents, who ridiculed the idea that multiple meteorites would strike along a line. The worm gradually turned as more and more evidence accumulated for the reality of impacts; multiple impacts on Jupiter by comet Shoemaker-Levy in 1994 finally gave the impact geologists an example they could point to. Except then the worm turned back again; further geological examination of the 38th parallel structures showed that the ages are different; for example the Weaubleau-Osceola structure seems to be Middle Mississippian while the Furnace Creek structure is preCambrian. At the moment it seems like the Decaturville and Crooked Creeks structures are most likely impacts; the Weaubleau-Osceola structure is probably impact; the Avon Diatreme is volcanic, the Furnace Creek structure is probably volcanic, and the others are still up for grabs. The mixed origin seems to confound both parties, as both had explanations for the lineament – some sort of fault or rift line for the volcanologists and multiple impacts for the meteorists.


The Missouri bootheel is, of course, famous for the 1811-1812 New Madrid earthquakes; once again, unfortunately, there’s not much that can be seen from the highways. The quakes caused numerous “sand blows” that geysered sand to the surface; these are still there, as roughly circular infertile patches in the middle of cornfields. Seismologically fascinating but not very interesting as seen from a car window.


Very well illustrated, with geological maps for each part of the state plus photographs of all the interesting outcrops – and, alas, a lot of not very interesting ones. The State of Missouri would have been better suited for the Geology Underfoot… series where some geological feature is a destination in itself rather than a roadside attraction.… (altro)