Rackwick, Hoy

The Bay of Rackwick displays probably the best developed landform assemblage related to ice retreat found on Orkney but its history is not known in any detail. The moraines, meltwater system and sediments relate to three lobes of ice – one pushing in from the Pentland Firth, one moving south past Berrie Dale and Grutfea and another moving southwest past Dwarfie Hamars – which were probably all connected to the main mass of the ice sheet then covering lower ground on Orkney and the adjacent sea bed.

Ice pushing in from the Pentland Firth left a fine lateral moraine at the foot of the slopes of Mel Fea. Large blocks and angular boulders accumulated by rock fall on the glacier surface and small ponds developed at its side. An earlier phase of ice retreat is marked by the hummocky moraine in the recess above Red Geo. The isolated Green Hill is part of a former till ridge. It contains basalt erratics so must be derived either from towards Berrie Dale or from just offshore.

Ice moving south past Grutfea left a series of arcuate end moraines on the valley floor, perhaps deposited on to sands. Till contain basalt erratics and resting on bedrock extends at least as far south as Glen.

Ice moving south from Dwarfie Hamars trapped meltwater lakes against its flanks in which considerable thicknesses of sand accumulated. A major ridge, up to 20 m high, runs parallel to the valley axis and is capped by the thick peat of the Moss of the Whitestanes. Exposure is poor but the ridge appears to be composed of a range of materials, including sand, till and bedrock. It is difficult to interpret its origins because of the lack of information about its internal materials and structure. Its surface appears to carry morainic mounds and erratic blocks yet its northern flank is developed in soft materials, probably sand. The ridge may represent a pile of sediment that accumulated as the ice lobes first separated at Rackwick and which was then later over-ridden by a readvance of ice from the northeast. This readvance left a system of large meltwater channels on the southern face of the ridge. Small scrapes along the channel flanks show deposits of rounded gravel.

The sequence of deglaciation around Rackwick remains to be firmly established.