All of Swaledale contains rocks that date from the Carboniferous geological time period, which lasted from 360 to 299 million years ago. It is dominated by Carboniferous rocks of the Yoredale Group and in particular the Alston and Stainmore Formations with the Alston forming much of the lower parts of the hills in Swaledale. The Carboniferous rock strata are clearly visible on the valley sides as a repeating pattern of banded rock outcrops or steeper slopes of hard limestone and sandstones interspersed by broader more gentle slopes formed by the less resistant mudstones. Dale-side gills often cut a stepped pattern as they cross the alternating hard sandstones and limestones, for example at Gunnerside and Swinner Gills.
Within the Yoredale Group, which gently dip 10° to the north-east, there are seven limestones, the lowest exposed being the Hardraw limestone on which the village of Feetham is built. The Yoredale limestones are thinner and darker in appearance than the Great Scar limestone that outcrops further south. As each of the Yoredale limestones is relatively thin (most are less than 20m thick), Swaledale does not include the type of scenery that is found in the limestone country further south, and there are few potholes, caves and underground passages. However, at the top of the series of Yoredale rocks, Main limestone (Great Limestone) is found in a prominent band, averaging 23m in thickness to a maximum 40m thickness on Kisdon and is responsible for several prominent scars and limestone features such as the limestone cliffs on Cotterby Scar near Keld, Kisdon and Ivelet scars above Muker and the high scar of Fremington Edge.
The influence of limestone is visible in the vegetation type of the dale-sides, limestone supporting brighter green, species rich grasslands in contrast with the coarser grasses on sandstones and shales. In some areas e.g. on Kisdon side, the underlying geology of alternating bands of strata is reflected in a horizontally striped appearance to the valley side vegetation pattern. The Millstone Grit lies on top of the Yoredales comprising a series of coarse-grained sandstones with intervening shales and occasional coals. The horizontal gritstone strata cap the moors to the north and south of the dale. The different geology is expressed in the landscape by differences in the vegetation and colour changes from the lower and upper parts.