|
Date: |
Sunday 14 June 2006 |
|
Location: |
Mountsorrel, Leicestershire |
|
Leader: |
John Carney |
|
Author: |
Peter Knuddsen |
In a cold wet year, the forecast this particular Sunday was for it to be very hot. And so it proved to be. The quarry manager, Trevor Warren, welcomed us with the stringent and necessary health and safety requirements to be observed, including the need to take drinking water to the quarry floor.
John Carney, from BGS, led us for the day with his now familiar and much appreciated expert knowledge, professional manner – and handouts! He outlined both the historical and geological context of the quarry. There had been extraction of rock since at least the mid 18th century and probably much earlier, even back to Roman times. The rock itself is a granodiorite of Ordovician age, dated to about 450 Ma both by radiometric analysis as well as in its relationship with the surrounding Charnian calc-alkaline tuffs. It was formed as part of the Eastern Caledonides when subduction was closing the Iapetus Ocean to join Laurentian Scotland to Avalonian England and Wales. These rocks then became the platform for Carboniferous and later deposits. There are also a couple of significant faults that have affected subsequent geology – the Thringstone and Sileby Faults – which were reactivated during the later Variscan orogeny about 350-290 Ma. This re-exposed much earlier strata which were then truncated, weathered and covered with Triassic, Jurassic and Cretaceous deposits, the latter two having been entirely removed in this part of England. In fact the Mountsorrel Complex is but a tiny tip of a small granodiorite batholith that has yet again resurfaced to become the largest hard-rock quarry in Europe.
Being minibussed to the quarry floor, aka ‘the Pit’, had a touch of descending into Dante’s Inferno – especially with the heat. We were at c100m below sea level according to my Garmin. I was quite astonished at the sheer height of the rock faces surrounding the bowl in which we stood. The cloudless blue sky contrasted with the grey, brown and pink of the walls rising vertically towards it. John showed us the relatively solid stone bedding in some of the faces, contrasted with the fractured and sheared zone trending ESE. As much of the quarried stone is not in a condition for creating large usable blocks, it has in the past been made into kerbstones and setts. Now it is crushed and reconstituted as concrete aggregates or roadstone. Much of the production goes off to the south east by rail and recently it was used for construction of the Channel Tunnel rail link.
 |
| Discussion in the pit bottom (Don Cameron) |
A lot of time was spent looking at the various minerals. Biotite, black and shiny, is in evidence although in very small flakes. There is epidote (the lighter green) and chlorite (the darker green). Of great interest are the slickensides, evidence of the grinding and sliding of the rock in fault zones, but more particularly the slickenfibres of epidote that grow along the line of movement. There are also pink and finely grained aplite veins (the final juice of the melt) that criss-cross the granodiorite. These are typically about 5mm diameter although some may be up to 80mm. One vein we saw had a tiny but quite beautiful growth of chalcopyrite in it – stunning when magnified! Yet perhaps the most exciting discovery made was of a piece of molybdenite embedded in the granodiorite. It measured only 5 mm along its longest axis, but John told us that to his knowledge it was the first time it had been discovered in that quarry. Molybdenite, although superficially like galena, has a distinctive bluish-grey appearance and is extremely soft. It is associated with high temperatures in pneumatolytic veins.
The quarry also demonstrated it is more than a geologist’s paradise when we heard the distinctive call of peregrine falcons that slowly circled at the top of the bowl, easily spotted against the blue sky. One landed on a rocky ledge high up.
During the lunch break we were bussed to a higher level although wherever we sat to eat shade was scarce. Here we were shown a dolerite dyke that traversed the quarry. In truth I would have missed it entirely without someone patiently pointing it out, and then it became all too obvious! On examining a fallen block the difference with the granodiorite was clear – dark-grey and fine-grained although various subtle tints of colour could be seen. The dyke has not been dated but it is thought to have been intruded during the Carboniferous. It is of tholeiitic composition containing quartz rather than olivine. Close to the dyke is a text-book quality example of a small shear zone with the shattered and brecciated rock curving upwards.
Our move took us to see the unconformity between the Ordovician granodiorite and the red Triassic Mercia Mudstones. At the end of the Triassic the ancient surface had been eroded and desert sediments, fine silts and mud, filled in the exposed valleys. This is clearly seen with the bowing or catenary dip in the middle of the valleys where compaction and compression was greatest. The redness of this rock, due to oxidization, is broken by grey calcareous and dolomitic beds. This was an arid environment prone to flooding, and the grey beds contain sedimentary structures, and excellent examples of ripple marks were seen. There are tiny gypsum veins traversing both red and grey beds. A short distance away we saw evidence of Triassic weathering when exposed granodiorite had been rounded, fluted and sandblasted by sand grains in high winds – 240 Ma!
There are also Quaternary deposits in the quarry, from the Anglian Glaciation about 440,000 years ago, which reached as far south as the Thames, unlike the more recent Devensian which never got as far as Leicestershire. Two tills have been identified in the area. The Thrussington Till, which came from the north and the Oadby Till which came from the east. It is the latter till that is most in evidence at Mountsorrel. Dark blue-grey in colour and clay rich it incorporates chalk, flint and even Jurassic Gryphaea from Lincolnshire and East Yorkshire.
I had a final look into the Pit from the quarry top and took a couple more photos which rounded off the visit quite nicely. It had been an excellent day. We had been shown so much thanks to John’s excellent leadership. Thanks also to the organizers, it was much appreciated. I wonder just how many more quarries there are in the East Midlands that can be earmarked for future visits.