This is an extract from the Black Country Geological Society (BCGS) Newsletter 210 December 2011.
In many of the articles I have read about local glacial erratics, there arise the problems related to age; which glaciation was responsible for their transport and deposition. I thought that with a few days concentrated research I might throw some light on the problem with regard to the erratics that members of the BCGS have recorded, but I have just confirmed that “the more you know, the more you realise what you don’t know”. Indeed, perhaps the title of this article should be ‘Devensian or Anglian or Wolstonian?’ I must point out at the outset that I am no Quaternary specialist.
The last glaciation was the Devensian which lasted from 110,000 or 120,000 years ago until about 18,000 years ago. During this glacial event there were four warm interstadials as well as small advances and retreats. These ice sheets removed or reworked sediments, including erratics, that may have been deposited by previous ice sheets. The maximum extent of the ice sheet is indicated by the extent of till and erratics. The ice front was from Bridgnorth, through the area south of Bobbington, Highgate Common and Trysull, north of Wombourne to cross the Black Country through Bilston, Wednesbury, south of Walsall before swinging northwards. It did not, therefore, directly affect the main Birmingham area.
Theoretically, any erratics found to the south or east of this ‘Wolverhampton line’ are Anglian or Wolstonian. The base of the Anglian is at 350,000 years ago, followed by the Hoxnian interglacial at 280,000 years and then the Wolstonian glaciation at 250,000 years. There is some doubt about the extent of the Wolstonian and it is assumed that it did not reach our area with any significance if at all. Some books put the Anglian and Wolstonian together as the two main cold stages of the Anglian.
The next step was to consult the maps of the British Geological Survey (BGS). The 1:50,000 Dudley sheet 167, is based on an old survey; it marks Boulder Clay and Sand & Gravel with no differentiation regarding glaciation. The BGS Geology of Britain viewer names Devensian deposits as Devensian, but when it looks at the Birmingham area, it calls the glacial material ‘Mid Pleistocene’ which includes both the Anglian and Wolstonian. The Wolverhampton sheet 153, produced in 2001 has Devensian deposits recorded in great detail, but there are no pre-Devensian glacial deposits in that area.
But to return to our erratics, we have to remember that although there is a definite line for the maximum front, the periglacial area of sand and gravel and river deposits are extensive, and we know that the rivers flowing out from beneath ice sheets can be very powerful at times, capable of moving large boulders. Small erratics have been found in glacial gravels in Kingswinford for example. There is also evidence that in very cold spells the ice sheet may push out small tongues of ice for a short time, and so carry boulders in that way. It was also pointed out to me that the BGS have found patches of till in some higher areas of Dudley but did not record them because they are too thin.
Having spent my youth in Essex, the Black Country impressed me by its altitude, indeed there used to be a Pub Quiz question asking for the highest Football League ground in England, the answer being the Hawthorns in West Bromwich. Sedgley Beacon is 238m, Turners Hill 271m, Clent 315m and this north to south ridge would have been a considerable barrier to any ice sheet. Perhaps the ice never covered it, even in Anglian times, but it does seem to divide the Devensian of the west, from the Anglian to the east, as far as remaining deposits are concerned.
So this article has not given any definitive answers, but I think we can safely assume that the Birmingham erratic’s are Anglian. Reading the works of Harrison has thrown up some interesting information and some very enlightening photographs. Above is his picture of the Cannon Hill Park ‘Arenig’ boulder, taken around 1900. It should be compared with the second photograph (left), which is taken from the almost identical spot in 2011. The similarity is remarkable, particularly with the trees on the far side of the lake; a similar line and both with some leafless trees higher than the rest.
Image by Julie Schroder