A larger version of Bill Fraser's annotated image is available in pdf format via the following link:
The text beneath the image reads:
In Little Dale the Hardraw Scar Limestone was quarried to provide stone for the viaduct. Here it is around 7 metres thick and made up of individual beds of limestone approximately 40-50 cm thick, separated by thinner layers of mudstone. The thickness of the beds was ideal for the size of stones required in the viaduct. The rock is a dark blue-grey and while hard could be quarried by hand using hammers and crowbars to break up the layers along their natural lines of weakness. Once extracted the rock was rough cut to the required sizes before being taken to the viaduct site for finishing. To get sufficient rock the quarries had to work stone below the level of the river and overall the stream bed was lowered by up to 4 metres. This had the added advantage of enabling it to be bridged and to stop water flooding the cutting. To work below river level the stream was dammed using spoil and timbers and its course altered. There were still problems with flooding though and a steam pump was employed in an attempt to keep the workings dry. The rough cut blocks were moved to the viaduct site along the light railway that ran alongside the main route.
© William Fraser (2017)
The viaduct referred to in the text is Ribblehead Viaduct.