Leader Fred Owen gathered a group of SciBarians at the Lymm Dam car park for a briefing on the geological history of the area. He explained that Lymm is situated in the centre of the northern part of the Cheshire Basin, which is one of a series of basins extending from the east Irish Sea to the English Channel. The sediments filling the basin are the familiar red sandstones eroded from a mountain range of Himalayan proportions about 230 million years ago, when the ‘English’ tectonic plate was roughly where the Sahara Desert is today. It would have been in an inland, arid desert climate, on a super-continent on the north of the mountain range. The south east of the Cheshire Basin would have been deep enough to bury the Matterhorn.
Later earth movements caused the sea to transgress the desert to form finer inter-tidal sediments, known as the Tarporley Siltstone, which were seen at the southern end of the lake. Today’s picturesque landscape around Lymm Dam was formed during the last Ice Age, by a combination of erosion by glacial ice and glacial melt-water around 20k to 10k years ago. The ice, 200m or more thick, flowing from the northwest, scoured out the gorge now occupied by the lake formed when the dam was built in 1824 on the site of a 3 meter waterfall in the Bradley Brook.
The group examined a remarkable set of seven deep, narrow, sinuous channels cut into the red Helsby Sandstone, which were aligned more or less parallel to the ice flow direction. It is deduced that they were cut by melt-water below the water table, known as ‘phreatic’ water. Less obvious were five horizontal channels at right angles to the deep ones, believed to have been eroded later by melt-water above the water table, known as ‘vadose’ water. The only other example of such channels in England is at Thurstaston on the Wirral. An alternative hypothesis has been proposed that these ‘melt-water’ channels were really formed by children with trainers and mountain bikes!
Further along the path a sandstone outcrop exhibited numerous vertical, elongate ‘scallops’ eroded into the sandstone, believed to have been formed by turbulent, phreatic melt-water flowing between active ice and the rock face.
Fred reflects: “It was interesting that here we were standing on the longest day in real-time on the edge of a temperate thunderstorm, on sandstone formed in an arid dessert, observing channels cut under 200m of ice by melt-water. Climate is always changing! The real issue is the rate of the change to which life can adapt to survive.”
In answer to a specific question about tectonic plate thickness Fred explained that oceanic plates are about 7 km thick while continental plates average 35 km thick. Oceanic crust is much denser than continental crust so is repeatedly ‘subducted’ beneath continents and recycled. It has a maximum ‘lifespan’ of 200 million years. There is an excellent free app ‘Earthviewer’ which shows how the plates have moved over geologic time and gives much information about fossils, mass extinctions etc.
The walk concluded with a convivial meal at the Spread Eagle in Lymm village.
Thanks to Fred for conducting the walk and for writing this blog entry.
For further reading, Fred suggests “Leviston, D. 2001. Subglacial meltwater channels at Lymm Dam, Cheshire. Proceedings of the Geologists’ Association, 112, 147-154.”