BCGS

Monday 16 October (Indoor Meeting): 'Erratic Tales'. Speaker: Ian Fairchild, University of Birmingham and Herefordshire & Worcestershire Earth Heritage Trust (HWEHT).

I will start the lecture by giving some examples of erratic boulders in the UK, Iceland and the Alps which provide us with interest and instruction about geological processes.

Over the past two years, the Black Country Geological Society has partnered with the HWEHT, the Birmingham Open Spaces Forum and the Lapworth Museum of Geology to deliver the Heritage Lottery Funded project “Birmingham’s Glacial Erratics: Heritage of the Ice Age”. The legacy of this project includes much better documentation and visibility of these boulders in SW Birmingham and NE Worcestershire, the creation of eight walking and cycling trails, a programme of public engagement events, and a website erraticsproject.org which includes some marvellous essays on the historical study of the erratics by Julie Schroder. The nature of these stones, through their exotic nature and sometimes impressively large size makes them attractive to the public and a means to foster pride in the local area. There is scope for further projects in other geographic areas to build on this interest.

The Birmingham boulders are distinctive in having a provenance from North Wales and originating in one or more older ice ages, notably including the Anglian around 450,000 years ago. Most boulders are composed of siliceous volcanic ash representing the product of pyroclastic flows following explosive eruptions. The most recent glaciation (the Devensian) led to ice advance as far as Wolverhampton, but Birmingham was untouched. The Devensian erratic suite in the West Midlands, by contrast, is dominated by granites from SW Scotland and the Lake District. These deductions were made in the 19th century, prior to urbanization, during an intensive period of mapping the position of boulders stimulated by the Erratic Blocks Committee of the British Association for the Advancement of Science. Essentially trains of boulders pointed back to the source. The map of Macintosh (1879) covered an enormous area and established that the white Criffel Granite and the pink Eskadale Granite as sources, whilst showing a distinctive train of volcanic boulders from the Arenig massif of North Wales which a model of the Welsh Ice shows was underneath the crest of the ice sheet. At the time the apparent crossing of flowlines on the map was a source of confusion, but it arises from the different ice ages, the Welsh Ice being relatively thinner and hence slower moving in the Devensian. Martin (1890) showed much more detail of the Midlands area whereas the posthumous map of Harmer (1928) covers the area to the north and east of the Midlands. It would not be possible to do such work today, since our urban areas are now swamped with boulders imported for decoration, including glacial erratics, but which have no heritage value.

The original enthusiasm for documenting boulders and mapping glacial flowlines barely survived WWI. The Birmingham geological memoir barely mentions them, yet they are the most widespread evidence for deep time processes visible to the public and remain of considerable scientific significance.

Click here for a Google map of the location.