"Mad about Geology" - Geologizing with Darwin

"A journey of a thousand miles begins with a single step."
Chinese proverb

January 16, 1832 the H.M.S.Beagle, with Charles Darwin on board, arrived to the barren "Quail Island" (today Island of Santa Maria, Cape Verde Islands). It was the first time that Darwin geologized alone in a foreign country, however he was well prepared...

In 1831 Charles R. Darwin went on a life changing field trip – not to mention the voyage on board of the Beagle later in that year. 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 in 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“) and 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 "on my return to Shropshire I examined sections and coloured a map of parts round Shrewsbury", 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 August 5th 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 may be possible 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 - or maybe he used Sedgwick notes.

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.

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 home to Shrewsbury August 29th a letter by botanist John Stevens Henslow, in name of Captain Robert FitzRoy, was offering him a position as gentlemen companion and naturalist on board of the Beagle. 

Read on...

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