If geology is a layer cake, Wharfedale is a two-slice portion. The upper valley is the Lower Carboniferous area, roughly north-west of Burnsall, where the Great Scar Limestone forms a basement to the overlying Yoredale Beds, a 300-metre sequence of hard limestones, sandstones and soft shales. These strata have been only slightly tilted, down to the east. To the south-east the Millstone Grit, of the Upper Carboniferous, begins, with its heather moorland and hard crags and tors.
Weathering of the Yoredales has produced the classic stepped profile which can be seen in the valley sides, with a shelf of limestone, sometimes grassy but often displaying karst features such as limestone pavement, gorges and sinkholes. Above it a slope of shale slopes up to the next limestone shelf, and hills such as Great Whernside and Pen-y-Ghent are crowned with small caps of Millstone Grit. On the tops of the hills can be found unexpected ponds – Birks Tarn, Fountains Fell Tarn – where the impermeable layers hold up little peaty pools. The limestones below contain corals and brachiopods and also minerals, of which lead once formed a major local industry.
The well-known Craven fault series, North, Mid and South, is seen to best advantage a little further west, south of Malham Moor, and separates the Craven uplands from the lowlands, but runs out to the east near Burnsall where the Millstone Grit takes over. A series of reef knolls runs west-east immediately south of the Mid-Craven fault.
The Great Scar limestone and the Yoredale layers are noted for extensive cave systems. At Birks Fell caves, west of Buckden, there are two separate systems which formed during the ice age. They are roughly parallel to Wharfedale itself but are otherwise quite independent of present drainage lines. Some of the country’s best twisted stalactites in Britain are found here.
Boreham cave has a remarkably extensive system, formed by water under pressure and deepened while the glaciers were carving out Littondale. The upper passages were left fairly dry but have since accumulated glacial deposits and, in one cave, ‘a display of calcite straw stalactites, unsurpassed in Britain.’
At Brants Gill there is a network of potholes feeding underground streams – Hull Pot has the largest entrance in Britain and Pen-y-ghent Pot is the country’s finest underground streamway. The Dow Cave system contains the remarkable Dowbergill Passage, 10-25 metres high but only 1 metre wide and completely straight for 1,500 metres. It was produced by solution along a fault and the stream goes through a spur of Great Whernside, sinking on one side and rising on the other.
Black Keld has two systems, Langcliffe Pot and Mossdale Caverns, which include over 20 kilometres of passages. They are unusual since they are not restricted to the Yoredale strata but break through the shale and sandstones to the Great Scar limestone below. These systems are important examples of maze caves.
And at Strans Gill, near Hubberholme, is ‘probably the most beautifully decorated cave passage anywhere in Britain,’ with magnificent stalagmites and stalactites.
The surface features are no less dramatic. In the last glaciation, a local ice cap at the head of the Dales fed valley-glaciers and produced the classic U-shaped profiles. There was probably a glacial lake, overlooked by Kilnsey Crag, and opposite is the narrow dry meltwater channel of Conistone Dib, with a deeply-incised gorge called the Gurling Trough. To the south, Dib Scar is a dry former waterfall, similar to the better-known Malham Cove in Airedale. The glacier scoured the limestone, producing the pavements which have since weathered into raised ‘clints’ and the cracks, ‘grikes’, between them.
Pen-y-Ghent Gill is another limestone gorge, cut into the Great Scar Limestone. It contains the Giant’s Cave, of national physiographic significance as the ‘finest example in Britain of cavern collapse in action.’
To the south of this area, running in a line south of the Mid-Craven fault, is a series of limestone knolls, green against the darker millstone grit background of Cracoe Fell. These were laid down in shallow warm seas along a former coastline and are known here as the Cracoe Reef Knolls.
This is the classic site for the study of knoll-reefs in the Asbian carboniferous limestone and is a key site for understanding reef communities and their palaeoecology. At Swinden Quarry a reef has been dissected by quarrying and its internal structure revealed. This includes a reef core of thickly-bedded limestones, rather sparsely populated, but with marginal deposits with an abundant shelly fauna including large brachiopods, varied gastropods and occasional bivalves. Intrusive minerals include valuable lead deposits.
Come down the valley and the change of underlying rock can be seen in the darker stone in the field walls. The Millstone Grit outcrops at the well-known Cow and Calf Rocks at Ilkley and forms a rolling dissected plateau. The impermeable nature of the rock produces blanket bogs and mires, and drier areas have wet and dry heaths and acid grasslands.
Cayton Gill Beds stretch north from the Washburn to Masham, muddy sandstones and siltstones which contain many remains of brachiopods and bivalves. Great Almscliff Crag is one of the largest and most massive gritstone tors in the Pennines. It is of importance since it shows the association of sound, and weathered, bedrock, and is within the limits of the last glaciation, unlike many similar features further south.
In mid-Wharfedale the coarse sandstones are known as Addingham Edge and Bramhope Grits. The Otley Shell Beds can be seen exposed in Otley Chevin and at Great Dib Wood there is a natural section through two Namurian sandstones with the Otley Shell Bed sandwiched between. This is one of the youngest horizons to yield trilobites at surface exposures in Britain. It is rich in remains of many of the animals that inhabited the Carboniferous sea.
Glacial lakes filled mid-Wharfedale at one time and deposited sand and gravel, which have been quarried and now form the basis of the Society’s Otley Wetland Reserve, and Ben Rhydding and Knotford Nook gravel pits.