A larger version of Bill Fraser's annotated image is available in pdf format via the following link:
The text beneath the image reads:
At Force Gill the Simonstone Limestone was quarried to provide stone for the viaduct. This is a hard, dark blue-grey limestone approximately 4 metres thick made up of a number of individual beds, up to 1 metre thick, separated by thin layers of mudstone. In the highest workings nearly half of the limestone lies below the level of the river. Individual blocks were prised from the ground using crow bars to exploit the natural lines of weakness. They were then rough cut to size before being transported up the access tramway then along the light railway that ran alongside the main route, to the viaduct site for finishing. The steep hillsides restricted the width of the quarry meaning that rock had to be worked below the water level which caused problems of flooding. Temporary diversions or dams made of timber or spoil were required to prevent the workings filling with water although this was not always successful.
© William Fraser (2017)
The viaduct referred to in the text is Ribblehead Viaduct.