Field Trip to Park Bridge, 17 July 2010

Intermittent heavy rain failed to deter 18 hardened geologists from attending a field trip to Park Bridge, near Ashton-under-Lyne, lead by Chantal Johnson. This part of the Medlock valley is now a haven of peace tucked away from the surrounding urban area, but is the site of a once-thriving industrial community. Coal had already been mined here for many years when the Lees family set up an ironworks, using the local coal as fuel and supplying machine parts to the mills of the surrounding towns.

We began by examining an exposed bank showing a coal layer - part of the Foxhole seam - with a grey clay layer below. Chantal treated us to a vivid description of conditions in the equatorial swamp of 300Ma, with 20m horsetails, giant dragonflies and dog-sized millipedes. Indeed, a few minutes fossil hunting revealed a beautiful surface impression of Lepidodendron, a giant clubmoss.

We then walked through part of the village, built for the workers when the ironworks were in their heyday, employing 800 people, and into Rocher Vale. Here we looked at the river Medlock, an example of a 'young' river, with angular blocks of sandstone on its bed as well as areas of silt deposition where the flow is slower. Chantal used this to compare with the conditions existing when the sandstone of the nearby quarry was laid down. Here she demonstrated bedding planes in a medium grained sandstone, deposited in a dynamic river delta - a mature river system. Above the sandstone (exposed in the quarry) was a layer of buff coloured rippled siltstone more obvious a short distance away. We discussed possible reasons for the change in conditions which lead to silt deposition rather than medium sand. Chantal also described to us an area further along the rock face with evidence of marine incursion, sadly inaccessible currently as the severe winter had left the rock face unstable.

Next we returned to the river and walked a little up the valley, looking at evidence of coal mining and talking about the fossils the miners had found, some of which are now kept in Manchester Museum (as part of the George Wild collection). We also looked at the differing profiles of the two valley sides and discussed possible reasons why one side is so much more prone to landslips than the other.

Finally we returned along the valley to the ironworks, Chantal pointing out evidence of the industrial past as we went. We closed back at the Heritage Centre which has interesting displays about the ironworks, the geology and the wildlife which has made the site its home.

Many thanks must go to Chantal for putting on such an enthusiastic and interesting trip following the cancellation of the previously arranged trip, and to Jane as ever for organising the event.

Linda Jones
Exposed bank showing coal and clay layers
Exposed bank showing coal and clay layers
Chantal describes the conditions in which the sandstone was laid down.
Chantal describes the conditions in which the sandstone was laid down.