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Life in the Tropics


Rivers of Sand

Although dominated by limestone, the Great Scar Limestone sequence also contains sandstones, such as the Ashfell Sandstone Formation and pebbly sandstone within the Brownber Formation, as well as thin siltstones and mudstones throughout the sequence. These represent sediments brought into the sea by rivers draining off land to the north. They herald the establishment of large deltas that would dominate the later part of the Carboniferous.

Reddish Ashfell Sandstone below Ash Fell Edge, showing swirly structures that formed when it was wet sand in a delta that built out into the sea during deposition of the Great Scar Limestone. © E. Pickett

On sandstone (and some limestone) crags or quarry faces where the layering (bedding) is seen in cross-section, you may spot parallel sloping layers between the main horizontal bed surfaces. Known as cross-bedding, this feature formed when flowing water caused sand ripples or small dunes to migrate along a riverbed or sea floor in the direction of the current. The sloping layers face downstream, so these structures show which way the ancient water currents were flowing. Cross-bedding in local sandstone, along with other evidence, tells us that in Carboniferous times sediment-laden rivers were flowing south from northern upland areas.

The formation of cross-bedding on a riverbed by the migration of sand ripples or small dunes. As the ripple or dune moves, traces of its former positions are left in the sand as a series of sloping layers. © E. Pickett
Cross-bedding in Ashfell Sandstone in a small quarry near Smardale Bridge. © E. Pickett
Partly quarried exposure of Ashfell Sandstone at Bents near Brownber, showing cross-bedding and carved initials dating back to the 19th century. © E. Pickett