May 16 2006. Returning to the topic of the history of geology raised by the visit to the Prestwich 1853 section at Southend, there are books out on his English contemporary “Charles Darwin, Geologist” (by Sandra Herbert, 2005 in Cornell University Press, Ithaca N.Y.) and “The map that changed the World” made by William Smith in 1801 (by Simon Winchester 2001 Viking Press, 2002 Penguin Books). Sandra Herbert suggests that Darwin was a true geologist but has tended not to be regarded as such because he did not do English stratigraphical work like Prestwich and of course is better known as a biologist and geological thinker. Prestwich is much closer to the concept of a Victorian field geologist inspired by the work of William Smith and making major corrections to his stratigraphy and mapping of south-east England. Smith appears to have had the new idea of correlating rocks by their animal fossil content, particularly ammonites, in the Bath to Yorkshire tract of south-east tilted layers, which he regarded as being deposited on this slope and overlain by loose gravels which he interpreted as Diluvium formed by the Biblical Flood. During the period 1800 to 1807 Smith worked partly on the Norfolk coast and produced a lost work, evidently seen by the first proper geological author on that area Samuel Woodward (see Yorkshire Geological Society Proceedings v.15 p.23 by Dr. L.R. COX). He also set up house in London during this later period and must have encounted the common knowledge that the London Clay rested upon sands and then the chalk in artesian wells then starting to be sunk there. He therefore presumed that the fifty-foot cliffs of stratified glacial deposits of clay at Happisburgh in Norfolk were also London Clay and not the generally thinner and unstratified Diluvium. Glaciation as an origin of these deposits was not fully accepted until after 1840 and taxonomy of ammonites was not exact or confident enough to determine that his subsequently illustrated London Clay ammonite from Happisburgh was the same as those in the Jurassic clay concretions of Yorkshire and had actually been reworked into the later deposit. Meanwhile in the London area, his ideas were taken up by James Sowerby and other gentleman who promptly found additional ammonites from the Yorkshire Jurassic in the London Clay sites of Highgate Hill and Minster Cliffs (opposite Southend) which they more correctly correlated using the nautiloids from the London Clay itself. It is still unclear, at least to me, where these ammonites came from in these southeast England sites lying beyond the subsequently mapped area of Boulder Clay till deposited by the most extensive ice sheets. The Sowerby specimen of Decipia decipiens (originally from the Jurassic Corallian strata) is described as drift Highgate Hill, and his middle has Amaltheus margaritatus (figured as London clay Ammonites acutus) as drift at Minster in the catalogue of Natural History Museum ammonites by D. Phillips (published 1977). Neither locality shows very much in the way of post-Eocene gravels to confuse early investigators with extensive London Clay to look at there. I wonder whether the Minster specimen was not actually ballast in a ship from Yorkshire, which had called at the port of Sheemess near Minster and then sold to tourists. The Highgate specimen could also have been mixed-up by dealers in fossils, or introduced as building stone etc. Certainly there are specimens labelled Highgate in museums, which came from elsewhere. Reading Museum has a very large Yorkshire Jurassic nautiloid concretion dug up recently in the road near Windsor Castle, and on the mapped London Clay, rather than Boulder Clay or gravels. On the other hand, it is evident from even larger stones incorporated into old churches (probably from pagan sites) and smaller ones commonly seen in fields that some Yorkshire or other northern rocks do occur south-east of the officially mapped area of glacial till in Essex. But as noted last week it is far more common to find loose stones that have been moved north by rivers during the ice age from the Cretaceous rocks of Dent where neither of the Sowerby Jurassic ammonites occur insitu. What interested me last week was that even a later skilled investigator like Prestwich had made rather vague notes compared to what I could do in a few minutes on a less clean cliff, with a modern builders tape measure.