glacial erratics

glacial stratigraphy







Lodgement till. South Harbour, Fair Isle (HU 203699). Note the mix of clast types and the manner in which the stones float in a matrix of  sand and mud. The brown colour of the till probably reflect the incorporation of debris from Devonian and Permian sandstones from offshore. Photo by Hamish Ross.













A till composed of ripped-up blocks of underlying sandstone on the cliffs at Sandness


Definition: accumulations of unsorted, unstratified mixtures of clay, silt, sand, gravel, and boulders deposited directly from glacier ice.

Till content and structure provides evidence of the mode of formation of the associated glacial landforms and of former directions of ice flow. Till is also of interest where it occurs within sequences of other sediments as it provide evidence of glacial conditions within the stratigraphy.

Till is generally only thinly developed on Shetland and tends to be thickest on lee slopes. Its character and content depends on the rocks over which the ice has passed and the distance of travel. Many tills contain erratics, ranging in size from boulders to sand grains. Locally-derived tills are dominated by local material and often can be traced to nearby outcrops of glacially-disturbed and dragged bedrock. The direction of over-turning and dragging of blocks can often indicate the direction of ice flow.