shelly till



Glacial rafts of chalk on the coast of NE Norfolk


Glacial Rafts

Definition: sediment or bedrock transported en masse by glacier ice

Glacial thrusts at Sidestrand

Glaciers and ice sheets are capable of entraining very large slices of material from the glacier bed. In Scotland the largest glacial erratic blocks are the size of trucks but these are dwarfed by the rafts of bedrock and unconsolidated sediment along the shores of the Moray Firth. The Leavad raft, for example, is at least 500 m long and those on the southern shore reach over 1 km in width.

These rafts match in size the many examples reported from the shores of the southern Baltic and southern North Sea. the glacial rafts around the Moray Firth are striking reminders of the erosive capabilities of glaciers. The concentration of rafts in this area is a reflection of a combination of circumstances:

  • the occurrence on the floor of the Moray Firth of horizontally-bedded rock and Pleistocene sediment
  • a ramp formed by the shoreline that required ice moving out of the Moray Firth to climb uphill.

Although rafts of debris may be frozen on to the sole of cold-based glaciers and be detached from their surroundings, this is not likely to be the case in the Moray Firth. The Moray Firth ice stream was thick and highly erosive, at least at its maximum extent and thickness, and the signs of glacial abrasion on the bedrock of Caithness indicate the passage of warm-based, sliding ice. Merritt et al (2003) suggest that the rafts have formed when high water pressures developed in sand layers within clays and muds and led to the detachment of large slabs of rock and sediment. This requires that the base of the ice is charged with debris and that the basal ice and parts of its bed formed of soft sediment are deforming together. Usually, any slabs detached in this way would be mashed and digested by the movement of the basal layers to give shelly till. The ramp along the Moray shore, though, allowed rafts to be projected more or less whole into the body of the glacier along shear zones within the ice. Although the structure of the raft is often intact, with bedded sediment and layers of intact shells, evidence of its tortured movement is present as faults, folds and other deformation structures.