For the answer to that question we have to go back 20,000 years.
Before the last ice sheet covered Lancashire the valley bottoms were well below present day levels, 18,000 years ago a stream of ice flowed west from Ribblesdale enlarging the valley, 5,000 years later it melted and dumped masses of tons of sand and gravel which completely buried the old valley, (these can clearly be seen at Pleasington golf course)
The modern River Darwen began to flow again on top of these deposited soft sediments but its twisting course only roughly followed the line of the old river, as it cut down it ran into the rock at many points to the North and South of the now buried valley.
Where it has cut into rock the river today runs into a gorge and there is little or no floodplain.
We can tell however where the old valley lies because the water has cut a clear path through the deep sand and gravel to foirm a wide valley and floodplain.
The result of all this is a deep cavernous gorge here at Hoghton, the towering rock sides that are left behind are formed of GRITSTONE, common to the area, more about which can be read HERE
Gritstone is a sedimentary rock composed of coarse sand grains with inclusions of small stones. It is a coarser version of sandstone. It was laid down in the late (upper) Paleozoic era, in the Carboniferous period, in delta conditions.
The Paleozoic Era (from the Greek palaio, old and zoion, animals, meaning ancient life) is the earliest of three geologic eras of the Phanerozoic eon. ... The Carboniferous is a major division of the geologic timescale that extends from the end of the Devonian period, about 359m years ago.
Gritstone is a sedimentary rock and frequently shows signs of cross-bedding or current bedding. Two types of sedimentary rock: limey shale overlaid by limestone. ... Cross bedding is a geological term referring to the way a sedimentary deposit is affected by water currents, during its formation.
It is quarried for building material. British gritstone was used for millstones to mill flour and grindstones to sharpen blades, giving rise to its other common name of MILLSTONE GRIT.
If you walk along from the given coordinates at the viaduct to the large weir at N53.43.849 W002.33.971 and look down towards where the river flows you will be able to where the river has clashed with the sides of the gritstone and been swirled around forming small whirpools, these can still be seen today at times - especially during heavy river flow, also where the rock has been eroded by these whirlpools there are curious circular shapes left behind by the flowing water.
To claim this Earthcache please answer the following questions BY EMAIL and not in your logs
1. Estimate the height of the gorge from River level to highest point under the viaduct.
2. What are the 2 sub periods of the Carboniferous period ?
3.Finally please take a photo of the gorge including you and your GPSR, and also a photo if you can of the shapes in the rocks below left by the whirlpools ( please be careful!)
I hope you enjoy your visit to the gorge and hope that the information above makes it more enjoyable, learning how it was created and seeing the end result.
Enjoy the area !
PLEASE NOTE: I receive a very high number of Earthcache emails, I can’t reply to them all otherwise I’d be doing nothing else all day, as has always been the case there is no need to await a reply from me regarding your answers…. However due to numerous people thinking they can just log these caches without emailing any answers, and/or completing the required tasks these will be picked up, and the logs will be deleted without further communication. To facilitate this Please email your information either before, or AT THE SAME TIME OF LOGGING THE CACHE, Thanks.