This blog will follow my digging (excuse the pun) into Brenton Symons Victorian geological map as I prepare for my talk next year to the Caradon Geology group. This post continues with the subject of the granite boundary by comparing Benton Symon’s 1863 map with a modern one published by the British Geological Survey. In comparing the two maps I hope to be able to verify how much use such a Victorian map is to those studying geology today, and maybe gain an insight into the accuracy of Brenton Symon’s work.
This series of posts are slanted towards those interested in the geology of the Liskeard area, but if your interests are cartography or Cornish mining history there may be some information of interest.
I have chosen to expose to Mr. Symon’s cartography is BGS Sheet 337- Tavistock. A map now available on the excellent BGS website. The two maps are broadly in agreement (dates) on the rough course of the contact, so this post will concentrate on a selection of areas where the two diverge the largest.
The first is the intriguing little kink in the Trecombe. Symons shows the Killas here forming a
estheticaly pleasing wave, its crest toppling to the west at the head of the combe. The BGS
version is far less pleasing to the eye, but displays some far more interesting geology. The modern map portrays the wave displaced slightly to the east, but more importantly has a NNW running fault replacing the western curve of the wave; a fault explaining the kink in the contact.
To the east of this point the two maps disagree on where the granite lies on the southern slope of Caradon Hill. The modern survey places it significantly to the norrt, a surprising discrepancy due to the importance of the South Caradon Mine. On the BGS map the contact is shown running through Holman’s shaft ( or as it is known now, the Man in the mine), partly following the Caunter Lode.
Moving on from there the two maps agree as they cross the East Caradon sett, but soon after rapidly part company. As it skirts the eastern slopes of Caradon Hill the granite boundary is shown running close east of the main road on the modern map, whilst Symons shows it further eastwards passing near to the round at Tokenbury, quite a large difference.
Inside the Marke Valley, just east of Minions Village the Geology gets more complex, this is the area of the overlaying killas tongue that influenced the minerals of Phoenix United mine. In 1863 this Killas was shown as having a curved form, but the BGS display it bordered by two faults. This slab of killas extends westwards across the South Phoenix Sett to a point just north of the Hurlers stone circle, much further then indicated by Symons.
Again, like at South Caradon Mine there is a major difference in the maps at the important mine of Phoenix United. Brenton Symons shows the mine being sunk on granite, with the extension of the rock reaching a point just east of Knowles farm. The BGS show a completely different situation. On their map only the western part of the mine, west of the Clananacombe, is in granite. However, what the modern map does show is a small outlier granited, an isolated outcrop close east of Knowles farm.
The northern part Symon’s map shows a simpler course of the boundary at it passes eastwards out of the coverage. The modern map shows the contact distruptted by faults, and its eastern extremity further west than indicated in 1863.
What are the key differences?
This comparison has revealed the main differences in the depiction of the granite/killas contact as:
In many places the granite contact is shown extending further into the surrounding country by Brenton Symons
The 1863 map shows a simpler course for the contact. Its course is formed of curves with no harsh lines caused by faults.
Within the two most important mines within the map’s coverage there are significant differences in the location of the granite.
The last point has certainly sparked my curiosity- which one is correct? My next little project will be start turning the pages of some of the books on my shelves to look for clues on where the granite really is. Unless of course any one reading this post already has information to answer that question.
For a copy of the complete 1863 map, and information about the mines in area see my book “The Liskeard Mining Area in 1863”.