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Carboniferous Layers of the Landscape


Dynamic Deltas

Sandstones of the Yoredale Group often contain sloping parallel layers within individual beds. Known as cross-bedding (or cross-stratification), this feature formed when flowing water caused sand ripples or small dunes to migrate along the base of a river or distributary channel of a delta in the direction of current flow.  The gently sloping layers face downstream and allow a geologist to interpret the direction of flow of ancient currents in the channel. The occurrence of cross-bedding in many of the sandstones provides evidence that during Carboniferous times, rivers and large deltas were predominantly flowing south from northern uplands. The shape of the grains and the sorting and composition of sediment can provide additional information on the history of cross-beds. Roundness of the grains, limited variation in grain size, and high quartz contents are generally attributed to longer histories of weathering and sediment transport. Many of the sandstones contain feldspars that are a common mineral constituent of granites that formed several of the upstanding blocks to the north (e.g. Cheviot Block with Cheviot Granites; Alston Block with Weardale Granite).

Many of the sandstone beds that form part of the Yoredale Group contain a sedimentary structure known as cross-bedding with layers at an angle to the main bedding. Sandstone bed of a deltaic channel at base of Cotterby Scar is commonly encountered in sandstones throughout Swaledale. Note geological hammer for scale.
Cross-bedding is formed by the downstream migration of ripples or dunes in a flowing fluid like a river channel. The fluid flow causes sand grains to move up the stoss (upstream) side of the bedform and collect at the peak until the angle of repose is reached. At this point, the crest of granular material has grown too large and will be overcome by the force of the moving water, falling down the lee (downstream) side of the ripple or dune. Repeated avalanches will eventually form the sedimentary structure known as cross-bedding, with the structure dipping in the downstream direction of flow. © Elizabeth Pickett, 2022 .