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"Therefore on my return to Shropshire I examined sections and coloured a map of parts round Shrewsbury."
Darwin in his autobiography (1876)
"A map is always a decisive criterion of they who aspire to the rank of geologists [E]very one who has not compiled a map, wants the necessary talent of combination . The spirited Darwin, with all his remarkable vivacity of mind, is for me no Geologist, only an able history maker of what nature as he believes, has done, and what never she did... This man could never make a tolerable geological map."
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Geologist Leopold von Buch* in a private letter to colleague Roderick Impey Murchison (1846)
In 1831 Charles R. Darwin attended a life changing expedition - not considering the voyage on board of the "H.M.S. Beagle". The botanist John Stevens Henslow introduced the 22-year old Darwin to 46-year old Adam Sedgwick, self-educated naturalist and professor for geology and botany at Cambridge University. Even if Darwin was a student at Cambridge, he seems not to have attended Sedgwick´s lectures on geology, as he regrets in an autobiographic note that
"Had I done so I should probably have become a geologist earlier than I did."
At the time Sedgwick was studying the geology of Wales and invited Darwin to join him at a field trip from Shrewsbury, Darwin's hometown. Sedgwick was especially interested in the stratigraphic succession exposed in North Wales (Sedgwick will later use his observations to define the geologic epoch of the "Cambrian"), Darwin was interested to acquire the basics of geological field work. Darwin wrote in July to a friend
"I am now mad about Geology & daresay I shall put a plan which I am now hatching, into execution sometime in August, …[]"
Darwin was well equipped for his geological field investigation. He purchased a new clinometer with an incorporated compass for structural analysis and a geological hammer for the collection of rocks.
He visited Llanymynech (located west of Shrewsbury) alone and started to colour a map, mapping outcrops of sandstone and coal measures.
Sedgwick arrived to Shrewsbury on the 2nd August, visiting in the next days some outcrops located south-west of the city, where he recognized limestone and volcanic rocks. It's not clear if he met Darwin already, for sure both geologist left Shrewsbury on 5th August venturing north. They spend a week trying to find Old Red Sandstone. Sedgwick was interested in the geological formations underlying the Old Red Sandstone (Silurian to Carboniferous in age), as the age of these rocks was still unknown and according to the large-scale geological map published by George Greenough in 1819 such rocks should be found in the area. However despite their combined efforts and a meeting in Llangollen with another great geologist, Robert Dawson, no Old Red Sandstone was found.
In his autobiography Darwin affirms that he left Sedgwick at Capel Curig, however it seems reasonable to assume that he visited with Sedgwick the island of Anglesey and even made a short trip to Dublin (as Sedgwick did, on Anglesey he found also the Red Sandstone he was after). During his voyage on the Beagle, Darwin will recognize on the Cape Verde Islands Serpentine, this kind of rock he could have only previously seen on Anglesey.
Fig.1. Geology of North Wales, after Reynolds 1860, 1889, Woodward 1904 (click to enlarge), with the route of Darwin and Sedgwick after ROBERTS 2001. The first part of the route, starting from Shrewsbury, follows the contact of the Silurian limestone (pink-coloured) and younger sediments (blue colour; Carboniferous to Permian), as both geologist hoped to find the Old Red Sandstone formation. Sedgwick found it (dark-orange) only on the island of Anglesey (original map in public domain).
Twenty pages of notes made by Darwin during this tour are still today conserved - in his autobiography he will later remember: "This tour was of decided use in teaching me a little how to make out the geology of a country…"
When Darwin returned to Shrewsbury on 29th August, a letter from Captain Robert FitzRoy was offering him a position as gentlemen companion on board of the Beagle. The rest is history...
*It seems as von Buch didn't appreciate Darwin's later publication on the volcanoes he visited during his voyage, where he rejected von Buch's theory on the origin of volcanoes.
Bibliography:
HERBERT, S. (2005): Charles Darwin, Geologist. Cornell University Press: 485
ROBERTS, M. (2001): Just before the Beagle: Charles Darwin's geological fieldwork in Wales, summer 1831. Endeavour Vol. 25(1): 33-37