The most important geological feature of the Cumbrian Dales is the Dent Fault, which runs from Kirkby Stephen to Kirkby Lonsdale, a distance of 32km. It is the best known example in the country of a reverse fault and forms a division between two contrasting types of scenery: the horizontal limestone scars of the Yorkshire Dales and the smooth, rounded Howgill Fells formed by the Silurian rock of the Lake District. The rocks on the Lake District (western) side rose up 2400m against the horizontal strata on the Pennine (eastern) side, forcing limestones into a vertical position close to the fault plane. The fault crosses Dentdale alongside Barkin Beck near Gawthrop and up Barbondale. The flat valley bottom near Gawthrop was once a lake, formed by glacial action during the Ice Age.

Sedbergh sits on top of an area of red conglomerate, a reddish rock that comprising pebbles of Silurian age mixed with weathered granite cemented together. It forms the base of the Carboniferous series of strata and is a land deposit that collected in the hollows and valleys of the desert landscape at that time. A significant area of Great Scar Limestone lies to the east of the Dent Fault and underlies Dentdale, Garsdale and the east side of the Rawthey Valley. Above the Great Scar limestone are the alternating beds of the Yoredale series of rocks, limestones, sandstones and shales. They are evident as horizontal scars along the valley sides of Garsdale and Dentdale. Thin beds of limestone form platformed steps down the riverbed over which the River Dee flows. Often the bed is dry and the water flows through underground caverns.

Below the Main Limestone, near the top of the Yoredale Strata, coal occurs and evidence of old pits and shafts can be seen at Garsdale Common. The former workings are scattered near the Galloway Road between Garsdale and Dent stations. Limestone was extracted from small quarries in Dentdale from the Yoredale Series, and cut and polished to produce Dent marble.

During the Ice Age, Cautley Crags on the eastern edge of the Howgills were exposed by the gouging effect of the ice creating the beginnings of a corrie. The valley bottoms adjacent to the Howgills received thick deposits of boulder clay. The Lune gorge was deepened by ice moving from north to south and the western dales also had glaciers that stripped off soil cover and revealed the scars typical of the Yorkshire Dales. Combe Scar to the west of Dent shows the outline of a small corrie with its own moraine of glacial debris.

Dentdale, Garsdale, the Rawthey Valley and Lune Valley are west facing dales in contrast to all the other dales within the National Park. Their rivers, the Dee, Clough and Rawthey, all flow into the River Lune that flows west into the Irish Sea.