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).