The Askrigg Block
Visitors to the Yorkshire Dales often comment on the distinctive nature of the landscape, and in particular the horizontal white limestone scars and terraces which run along the length of many of the valleys (e.g. Upper Wharfedale and Littondale etc).
This kind of "terracing" is generally due to the predominantly horizontal rock strata of Great Scar Limestone and the overlaying "Yoredale Series" of limestones and shales. Given the age of the rocks (which mainly were laid down during the carboniferous period) this horizontal structure is perhaps somewhat surprising. The reason that the rocks do not appear to have been thrusted or folded by earth movements to any significant degree is that much of the area lies over a huge mass of underlying granite which is deep underground and is believed to have been very stable even over the vast geological timescales involved - including the many millions of years during which the hills and valleys of the Yorkshire Dales were formed and developed. Known to geologists as the "Askrigg Block", the existence of this underlying granite mass was proved only relatively recently by bore samples taken from a deep drilling rig set up near Semerwater.
At the southern and western edge of the Askrigg Block is the Craven Fault (marking the boundary of the overlaying limestone uplands of the higher dales and the gentler scenery to the south and west) while at the north western edge is the Dent Fault which runs along Dentdale (marking the boundary with the Howgill Fells near Dent and Sedbergh).
To the east the overlaying rocks of the Askrigg Block disappear beneath newer sediments towards the Vale of York and the Vale of Mowbray, where the land flattens out as the main drainage route to the sea.