BGS thin sections: 150,000th image taken! by Isla Simmons

BGS is currently running a programme to digitise the entire collection of rock thin sections. This consists of 100,000 thin sections in the Scottish Sliced Rock (S) Collection, 11,000 in the later Scotland and Northern England (N) Collection and 80,000 in the England and Wales (E) Collection. A number of minor collections will also be captured.



Sample number: S57865. Coarse-grained Metabasic Rock, Scourie dyke suite, W side of Loch Claidh, Scotland



Sample number: S3216. Quarry N of Stichill, 3.5 miles N by W of Kelso, Scotland



 Since March 2012 a team of volunteers in Edinburgh has been documenting, cleaning and photographing every thin section in the Scottish collection. Each section is photographed twice, in plane-polarised light and under crossed-polars, and last week saw us taking the 150,000th image!

When we’ve finished the Scottish (S) collection we’ll be moving on to the Scotland and Northern England (N) Collection. A team of volunteers is also being established in the BGS headquarters at Keyworth to begin digitising the England and Wales (E) Collection.

Sir Archibald Geikie



The origins of Petrology

Petrology was introduced in the Survey in the mid 1800s with the advance of the petrological microscope allowing detailed analysis of rock thin sections. Sir Archibald Geikie, then Director-General of the Survey created a slicing department in Edinburgh because of the relative abundance of crystalline rocks in Scotland, and it was here that most early thin sections were cut. Specialist petrographers were appointed in Edinburgh and in the Survey headquarters in London. Field geologists could send rock samples to one of the petrographers who would slice a thin section, examine it then send a report back to the geologist. The first thin section, numbered S1, was collected by Geikie himself around the early 1860s – a sample of dolerite taken from the Salisbury Crags in Edinburgh, though later renamed teschenite.

  



Sample number: S57774. Foliated tuff? Girvan-Ballantrae No. 6 bore (NX 18NE/5) at 288 feet, Scotland.

The thin section photography process

The first task in digitising the collection was to create a list, an inventory of every single thin section. Some sections were missing, either lost over the years or borrowed for examination and never returned. These had to be noted. Some had duplicates which had to be numbered individually as every section must have a unique number to allow us to link images to the correct thin section. As thin sections have been in the collections for up to 100-120 years they all have to be cleaned, and any broken slides have to be repaired. This second task is often a great social occasion with the volunteers sitting around a table cleaning sections while discussing recent field trips, coming exams or the latest hot topics in geology!



Photography of the thin sections

Once the thin sections have been cleaned we can start on the real business of the project – photographing them. A digital SLR camera with a 60 mm macro lens is permanently set up on a custom made jig and the thin sections are illuminated by a LED lamp.

To capture the images, all that is required is for the operator to follow the previously compiled inventory of thin sections, insert the section in a holder, take an image in cross polarized light (XPL), remove the analyser and take another image in plane polarised light (PPL). The camera image number must then be recorded against the number of the thin section in the inventory so that the images can be referenced back to the thin section they correspond to.



Sample number: S57855. Ultrabasic body in gneiss; Laxfordian? E side of Loch Claidh. Shieldinish? Scotland



Once the photography is complete, the next stage will be to link the images to Britrocks, the BGS database and web application for the rock and thin section collections: http://www.bgs.ac.uk/data/britrocks.html The overall purpose of the project will thus be achieved – to allow the public to view the images. Britrocks already has data for every single thin section in the collection – rock type, where it is from, who collected it and when it was collected. This project will supply the two images of each section to allow researchers and the public to search, browse and now view representative images of the thin sections.

For now though, we’re still cracking on with the cleaning and photography!

Isla Simmons

BGS Volunteer, Murchison House, Edinburgh.