A to Z of North Wales Geology
T is for..............TILL
TILL, or Boulder Clay, is material which has been deposited directly by glacier ice. Glaciers,
which are flowing masses of ice, will carry within them anything from boulders the size of a
car down to the finest dust. Despite the apparently solid nature of ice, it is suprisingly
plastic and flows quite readily as it hovers on the brink of melting at its base. Material
released from the base of the ice forms deposits of till. In the picture (left) of till
exposed above Deganwy beach, the scale of the material can be judged from the author's bicycle
TILL has a special texture because of the wide size range of the particles which it comprises. Such material can not be deposited by water because transport involving water inevitably separates the sediment into sized categories depending upon the energy of the flow. Only the highest flow rates will move the cobbles, but silt and clay will be moved almost continuously.
TILL will contain some of everything that the glacier was transporting, rather like a conveyor belt upon which rocks were dropped or scooped up as it passed by. This gives us a very useful way of tracing the flow of ice, because distinctive rock fragments will be gathered up along the ways. There has long been good sport trying to recognise the origin of such rocks, since this defines the direction of ice movement from high mountains with ice caps towards lower and ultimately warmer areas. Rocks of glacial origin whose origin is unrelated to the local geology are known as ERRATICS.
TILL in North Wales can be seen to have derived from two conflicting directions. The most obvious direction for ice movement is out of the mountainous interior - Snowdonia and the Berwyns - down valleys which have been deepened by ice flowing from the high corries. Those glaciers laid down a till comprising mainly grey-coloured slaty mudstones, and a variety of igneous rocks ranging from acid volcanic to basic intrusive. In the coastal area, a different material may be recognised. This is a red (when fresh, or yellowish when weathered) clay with sparse rock fragments including distinctive types, such as granite (pictured, right), coarse-grained red sandstone, banded chert and flint. Tracing each of these back to their nearest outcrops shows that ice has moved in a clockwise direction around the Irish Sea Basin - flowing out of Northern Ireland, Southern Scotland and the Lake district - before leaving through the St. George's Channel.
TILL distribution maps show us that pressure from Irish Sea ice has overwhelmed the flow of Welsh ice periodically. Ice was then pushed high up the front of the coastal mountains before a more westerly flow across Anglesey was resumed.
TILL is frequently found at the top of the beach or in river cuttings, where it is actively eroded by the water. Interesting rocks are liberated and will be found among more locally-derived material. See if YOU can recognise the local and foreign types.
©Jonathan Wilkins, 07.00