Continued… Normally, underlying solid rocks are only visible at seaside cliffs, quarries, in mines, riverbeds, motorway cuttings or bore holes. Here, in Roundhay Park, they are exposed in the Gorge – a V shaped valley cut by glacial melt water from the last ice age, revealing shales and sandstones laid down more than 300 million years ago, when this part of the earth’s crust was close to the equator.
Shales are the result of fine mud particles which are only deposited as sediment in very quiet water conditions. At some sites they contain both fresh water and marine fossils, so these sediments were laid down in lagoons fed mainly by rivers, but with occasional invasions of the sea. Fine grain sandstone horizons within the shale, having larger particles, indicate periods of stronger currents while, at the highest levels in the Gorge, coarse sandstone, containing large (cm) quartz pebbles, required very fast flowing rivers for their transport. These are seen at site 7, (Photo: Scouts’ Quarry) – and also at the same elevation in the golfcourse west of the Gorge – the dune bedding planes defining the direction of flow.
The overall picture is one of a giant river delta, extending across what is now Yorkshire and Lancashire, the sediments being derived from the erosion of a huge mountain range to the north.
There is a good example of a fold in the shales, which were originally laid down horizontally, but were later subject to compressional forces. (Photo: Folds in the shale) Elsewhere there is evidence of further folding and faults within the Gorge.
At a higher level the shales give way to sandstone which is porous in nature. When water percolating through the sandstone reaches the impervious shale it has no option but to move sideways and emerge as a spring. (Photo: Dog’s Mouth Spring.)
Where the gorge stream enters Waterloo Lake, an abrupt change has taken place; the steep sided gorge gives way to the much broader shallow basin underlying the lake. There has been a major fault here, the rocks to the south have slipped down by some 150 metres and so the layers of sediment on this side are about 1 million years younger than the adjacent layers in the Gorge. This fault runs just south of the Mansion and extends westward for many kilometres. It defines the northern boundary of the Yorkshire coalfield. The sandstones to the south of the fault are of good quality and the Mansion was built from blocks derived from a local quarry.
This text is just a taster – do attend a Trail Walk to learn more from a friendly geologist!