Yoredale Group
The Yoredale Group consists of a cyclic succession of rhythmically deposited sediments. These are known as Yoredale cyclothems as Yoredale was the old name for Wensleydale in North Yorkshire where they were first studied. Each cycle ideally comprises, in ascending order, limestone, mudstone, sandstone frequently topped with seatearth, and locally coal. The cyclicity can be observed at a variety of scales and in varying degrees of complexity; the largest cycles occur at the scale of tens to hundreds of metres in thickness. The cycles have been interpreted as the products of alternating periods of relatively slow and rapid relative subsidence of the area, reflecting a complex interplay of marine and non-marine deltaic conditions during sedimentation. During periods of slow subsidence, deltas advanced across the area from the north carrying mud and sand. Periodically, deltas built up to above sea level allowing the growth of swamp forests now represented by seatearths and coal seams. With more rapid subsidence warm, clear marine water spread across the area, overwhelming the deltas and depositing marine limestone. Sea level changes related to repeated growth and melting of southern hemisphere ice-sheets may also have played an important role in the formation of Yoredale cyclothems.
Whereas many cyclothems include representatives of each rock type, others are incomplete with one or more of the characteristic rock units absent. Most commonly absent is the coal horizon. Limestones are the most persistent, and generally the most easily recognised, of the component rocks, although they commonly comprise only a comparatively small proportion of the total thickness of rocks within the succession. The limestones are generally rather impure and contain significant amounts of clay and bituminous impurities, giving them a rather dark grey colour. Most contain an abundance of, mainly fragmentary, fossils. Although certain beds are characterised by rich faunas of fossils such as corals and sponges, it can be extremely difficult or impossible to tell the limestones apart. By convention, each cyclothem is named after the limestone at its base.
Many individual rock units have long had local names, several of which can be recognised and correlated over wide areas. Many of these names recall the locality at which the rock unit is best developed, at which it was first distinguished, or reflect a variety of intrinsic characteristics such as thickness.
The Yoredale Group is divided into three formations, based largely on the relative abundance of the different rock types within the cycles of each division, from the bottom up these are the Tyne Limestone Formation, the Alston Formation and the Stainmore Formation. Limestones generally increase in importance and thickness as the sequence is traced up towards the top of the Alston Formation, and decline in significance thereafter.
