Material deposited by glaciers is collectively termed GLACIAL DRIFT – It can be sub-categorised into: DIRECT DEPOSITION TILL - Glacial deposits, unstratified and unsorted material (boulder clay) – large fragments of rock of all sizes and shapes randomly mixed together. Can be divided into: 1. lodgement till – deposited at base of glacier due to basal melting – material is smeared onto the underlying rock due to the pressure of the ice as it continues to move 2. ablation till – material deposited as the ice melts away (coarser than lodgement till 2. INDIRECT DEPOSITION a. FLUVIOGLACIAL – picks up load once carried by ice (often then carried beyond limits of glacier – sorted and stratified by the action of water b. ICE – CONTACT STRATIFIED DRIFT – partly sorted by water – roughly stratified – deposited in vicinity of melting ice. GLACIAL DEPOSITION
There are several theories for the formation of drumlins – and it is believed that different drumlins probably form in different ways (our understanding is incomplete. The formation is likely to include one or more of the following:
Formed as ice become overloaded – resulting in lodgement of subglacial debris as it melted out of the basal ice layers and further ice movement streamlines and moulds the material;
As glacier re-advances, previously deposited material is re-shaped
Accumulation of material around a bedrock obstruction (rock-cored drumlins);
Thinning of ice as is spreads out over lowlands resulting in reduction in competence of glacier – debris deposited and streamlined by continuing forward movement;
some believe catastophic floods beneath ice sheets ripple ground moraines into mounds / hollows.
Examples of Drumlins : Hellifield, Ribblesdale – North Yorkshire – Swarms 40m high, 50-500m long – may be the result of ice sheets coming down from the Lake District, overloaded – change of gradient on the lowlands may have caused the ice to deposit the drumlins here. Other examples: New York State (USA) – largest drumlin field – 10,000 drumlins! Eden Valley - Cumbria
These are pieces of rock that geologically are out of place
They can vary from small pebbles to huge boulders.
These rocks were initially supra-glacial debris (either from plucking or from rockfall from weathered slopes above the glacier.
They have then been transported and deposited into an area of differing rock type (hence the geological difference)
Norber Erratics (Yorkshire Dales) – see photo – Blocks of Silurian shale – deposited on carboniferous limestone – erratics have protected the underlying rock from carbonation weathering, resulting in the erratics being perched on pedestal’s of the underlying rock.
Bluish Granite blocks unique to the island of Ailsa Craig (Ayrshire) found on the SW
Lancashire Plain – shows the direction of ice flow and a journey of at least 240km;
Chalk Rafts – transported from the North Sea bed to West Runton