Sea levels in the Carboniferous were constantly changing. When sea level was low, the deltas built out into the sea and swampy conditions prevailed, covered with vegetation. When sea level rose, the deltas and associated forests were drowned and sea life returned with the occurrence of limestones. This cycle happened many times, building up repeating layers of limestone, mudstone, sandstone, fossil soils and sometimes thin coals. These cycles, known as ‘cyclothems’, make up the Yoredale Group rocks of Swaledale. The word Yoredale is the old name for Wensleydale, where these cyclic sequences were originally studied in detail. This contrast and repetition of different sedimentary rocks has produced the terraced hillsides of Swaledale and can be seen in many of the area’s waterfalls.
The Yoredale Group is divided into two main units, the Alston and Stainmore Formations, of which only the Alston Formation is significantly exposed in Swaledale. It is made up of cyclothems, including about eleven named limestones, which can be traced across large areas of Northern England. The highest unit in the Alston Formation is the thick Great Limestone. Research focused on the Great Limestone (Main Limestone), has recognised limestone-mudstone couplets representing millennial-scale environmental changes, suggesting that the muds represented high-frequency sea level rises in addition to the larger scale changes allowing the re-establishment of the deltas. However, the amount of limestone decreases up the sequence, reflecting less frequent flooding by the sea and increased dominance of the large deltas flowing south. Not all cycles are of equal thickness and variations can reflect changes in magnitude of the sea rise or fall (global sea level as a result of glaciations at the South Pole); increased rates of runoff from the land allowing deltas to dominate for longer, as well as local faulting and the shifting nature of delta channels.