25 December 2023

The Greywacke, by Nick Davidson

Posted by Callan Bentley

Just finished a geologically focused book that the readers of this blog might be interested in. It is a history of work on delineating the early part of the Phanerozoic timescale in Wales and Scotland. The majority of the book is about Adam Sedgewick and Roderick Murchison and their close collaboration and increasing divergence and eventual total estrangement as they sought to sort out the boundaries of different periods of geologic time. Sedgewick was focused on the Cambrian, and Murchison on the Silurian, and they bickered over a the boundary in between. A certain section of strata was seen by both men as integral to “their” chosen period, and they were not able to resolve their differences prior to their deaths. Resolution only came with the meticulous work of Charles Lapworth, who sorted through the chaotically folded and faulted strata at Dobb’s Linn, near Moffat, using careful tracking of the graptolite fossils to distinguish five packages of shale which contained three graptolite faunas – the middle of which he assigned to a whole new period, the Ordovician, which dwelt in the disputed zone of overlap. Author Nick Davidson also points out that a side theater was the North West Highlands of Scotland, where Murchison and James Nicol geologized and also came to disagreement, and once again Lapworth swooped in and was able to resolve the mess, this time invoking the brand new concept of mylonite as a marker of fault surfaces (“gliding planes”).

Davidson is an enthusiastic writer, and this volume is a slimmer, trimmer read than David Oldroyd’s or Martin Rudwick’s volumes on similar topics. The downside is that because Davidson is not a professional geologist, some errors creep in. Because I’m personally more familiar with Scotland’s geology than the Welsh strata, I noticed these in particular during Davidson’s discussion of the Highlands. For instance, he uses “quartz” instead of “quartzite” for the basal strata in the Ardvreck Group, treating the mineral name as if it were a rock, unselfconsciously pairing it with limestone. He gets the direction of transport on the Moine Thrust backward, and illustrates the Moine showing the fault clearly cutting across bedding at a steep angle instead of being parallel to bedding, which is the circumstance that made it so tricky for Victorian geologists to sort out. Furthermore, every time “Mylonite” appears in the text, it’s capitalized. I feel like these are mistakes a professional geologist familiar with the area wouldn’t have made, although that sort of thing can also get caught during fact-checking by an assiduous, geologically-conversant editor. When goofs in basic stuff like that appears in a text, it makes me distrust the wider body of information presented.

Those critiques aside, I feel like I learned a lot more about the origin of some of the prominent divisions of the geologic timescale, and gained insight into some of the insidious issues that bedeviled the pioneers of our science. For instance, there is apparently an unconformity within one unit, the Caradoc Sandstone, that everyone missed for a long time since the strata above and below the unconformity were lithologically identical. One technique that Davidson uses in his narrative is constantly redrawing the geological timescale as a graphic, as the geologists refined their understanding and/or arrived at new interpretations. One issue that I feel never really got the full focus of the book’s attention is the discrepancy between the way we use the term “greywacke” (graywacke) today and the way Davidson indicates it was apparently used in Victorian times, as a catch-all for early Paleozoic strata, but including shales and limestones as well as sandstones. A final chapter jumps to the Heezen/Ewing insight about submarine landslides being a mechanism for depositing graywacke in the deep ocean (after the snapping of telegraph cables due to the 1929 Grand Banks earthquake turbidity current), but how this relates to the main focus of the book is never really tied off in any sort of satisfying way.

All told, a mixed bag.