The Glacial Geology of Rossendale Forest & North Manchester - Field Report 10-06-2006
On June 10th three MGA members joined up with a Leeds Geological Association field excursion led by Dick Crofts of the BGS to investigate the Quaternary deposits on our doorstep. Not only did we see a number of interesting localities but we also got an insight into some current issues which people working in this area are addressing. It shouldn’t come as a surprise that, as with most fields of human scientific endeavour, there are different camps on a range of topics. Particularly in this field trip:
i. whether some deposits were of primarily glacial origin or the result of subsequent landslips
ii. and how many and of what size were the lakes from which much of the sediment originated.
Cliviger Gorge near Cornholme was the location illustrating the first point. What to many looked like a glacial till deposit cut by the Calder and now to the north east side of the river, has recently been reinterpreted as a landslip deposit originating on the southwest side of the valley. The valley itself was scoured and deepened to the end of the last glaciation, the Devensian, c 20,000 years BP. As this last ice retreated the valley acted as a drainage channel for much of the surrounding Pennines and we were shown a schematic of a series of stratagraphic logs which correlated sediments in the valley from Lydgate through 70km to Knottingly.
On Worsthorne Moor over Burnley, we visited Shedden Hushings. This was a centre for a lime extraction industry utilising limestone boulders in the till. Such was the efficiency of the operations we were challenged, and failed, to find any remaining limestone. The remains of kilns littered a small area. These kilns operated from the 15th century until the opening of the Leeds Liverpool canal rendered the industry uncompetitive to imported lime in the early 19th century.
That these ‘imports’ came from Skipton served to illustrate how localised economies once were and the earth-shattering impact canals had on England.
At several stops in the Rawtenstall/Haslingden/Ramsbottom area the varying theories for the extent and number of glacial lakes were put to us. Different workers at different times have presented evidence of complete inundation (including sea ice flows) to the complete absence of lakes. Current thinking favours a number of lakes, this is supported by borehole data showing varved deposits typical of lacustrine environments. These lakes were centred on Rawtenstall, Bolton and Dunscar.
Tandle Hill is a country park in Oldham which provides magnificent views over Manchester to the south and west. From the top, a number of moraines marking key stages in the recession of the Devensian ice sheet were pointed out to us. One point best illustrated here, which is obvious when told, is that the ice did not simply retreat from south to north, but instead retreated from low ground to high ground. So in this area of the Pennines ice was retreating south as well as north.
It was made clear that the stratigraphy and paleogeographical interpretations of all the areas visited is the subject of ongoing work, with differing views among the experts, so hopefully the MGA can revisit this topic again in the future both in the field and the lecture theatre.