Category Archives: Science

Rocks rediscovered

Geological maps are often some of the most colourful and striking in the collection, especially the early ones on which the different rock types are coloured by hand. Luckily the Map Room receives all the geological maps published in the UK on Legal Deposit (as discussed a couple of weeks ago in this post on electronic Legal Deposit).

Research has been structured differently over the years, and at one time a large quantity of nineteenth century scientific mapping was transferred from the central Bodleian Library to the Radcliffe Science Library. Most of these maps have now been reunited with the rest of the map collection in the Bodleian Map Room, and a set of 39 large bound volumes of early geological maps has just been fully catalogued to modern standards. They include horizontal and vertical sections, detailed large scale geological mapping of certain counties, studies of areas of particular geological interest as well as standard series mapping of the British Isles at one inch to a mile. They have considerably enhanced the collection of early geological mapping.

Some of the most striking are detailed geological maps at the large scale of six inches to a mile; these are available for a few counties across England and Scotland. Most of the maps were based on Ordnance Survey mapping, made by military surveyors as early as the 1850s; the geological survey might be 20 or more years later.The early sheets were coloured by hand, often in astonishingly bright colours. This map of the area around Eastgate in County Durham was geologically surveyed in 1876-1877, published 1880. It’s at a scale of six inches to a mile. Paler blues represent sandstone and shale and the darker blue limestone, with basalt standing out in a vivid red; coal seams are picked out in gold. The maps were published by the Ordnance Survey.

The names of the geological surveyors for each sheet are generally recorded; this particular sheet was “geologically surveyed in 1876-77 by D. Burns, W. Gunn and C.T. Clough … under the superintendence of H.H. Howell.” The same names often come up repeatedly on many sheets. Fortunately, researching them is easy on the Pioneers of the British Geological Survey pages provided by the BGS Earthwise site. This has biographical information for dozens of early surveyors, sometimes including education, publications and even photographs of them in action.

The underlying geology of an area obviously has a profound effect on its landscape. This detailed geological map of Edinburgh from 1864 shows how the area around the castle, which is on a conspicuous hill above the city, is on basalt; the observatory is on another hill (mainly of felstone, now usually known as felsite, another igneous rock) to the east, while most of the surrounding area, coloured in grey, is sandstone.

These nineteenth century maps continued to be reproduced and updated well into the twentieth century, although the colour has long been printed rather than done by hand. This extract is from a one inch map of the area around Wigtown, first geologically surveyed in 1877 although this map was published in 1925.  The Map Room already held a considerable collection of these maps which has been augmented by the addition of the early volumes of geological maps. Modern geological maps continue to be published by the British Geological Survey at the slightly larger scale of 1:50,000.

Geological Survey of Great Britain : Durham. Southampton: Ordnance Map Office, 1880. C15 a.11/15. 

Geological Survey of Scotland : Edinburghshire. Southampton: Ordnance Map Office, 1864. C15 a.11/15.

Wigtown – Geological Survey of Scotland. Southampton: Ordnance Survey, 1925. C18 (33), sheet 4.

True north

We are used to having north at the top of our maps. This has been the most common orientation for hundreds of years, largely because of the use of the magnetic compass. Compasses do not, however, point exactly north. The northern magnetic pole wanders around the Canadian Arctic, and anyone requiring precise direction for navigational purposes needs to keep this in mind. It is common for maps to have a diagram showing the difference between magnetic and true north, as in this sea chart from 1870 (which also includes a date for the declination and, elsewhere on the chart, the current rate of change).

The discovery that the earth’s magnetic field fluctuates, and does not line up with its geographical axis, is nothing new. European navigators were aware of this issue from the fifteenth century. Edmond Halley had begun charted the magnetic declination across much of  the world at the end of the seventeenth century, and this map by John Senex from 1725, based on his work, shows the “Line of no variation in the year 1700” curving sinuously across the Atlantic. Lines of equal declination – isogonic lines – are marked around it.

This line where magnetic and true north coincide – properly called the agonic – is also in constant motion and we recently heard the exciting news that it is about to reach the Royal Observatory at Greenwich, so compasses there will point to true north for the first time in 360 years. More information can be found here on the website of the British Geological Survey.