Detail of a lodgement till on Orkney, with a striated sandstone boulder, an erratic of red granite, grey fragments derived from the Rousay Flags and a red matrix from the Eday Beds.
Thin, locally-derived till with many angular sandstone clasts. Birsay.
Till capping low cliffs in NW Westray
Definition: accumulations of unsorted, unstratified mixtures of clay, silt, sand, gravel, and boulders deposited directly from glacier ice.
Much of the low ground of Orkney is blanketed by a layer of glacial deposits left directly by the last ice sheet. Termed boulder clay in the 19th century on account of its common mix of boulders in a matrix of sand, mud and clay, this material is now referred to as till. Till content and structure provides evidence of the formation of landforms of glacial deposition and of former directions of ice flow. Till is also of interest where it occurs within sequences of other sediments as it provides evidence of glacial conditions within the stratigraphy.
The character and content of a till 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.
In Orkney, till is generally only thinly developed on high ground and tends to be thickest on lee slopes where it can be as much as 10 m thick. The till colour depends on the matrix material. In eastern and northern Orkney the till is red due to the passage of ice across the marls and sandstones of the Eday Beds. Further west the red hues are replaced progressively by browns and greys as more material from the Rousay and Stromness Flags is incorporated. Locally-derived debris dominates but a range of far travelled erratics is also present. Shell debris also occurs, especially in eastern areas.