The flanks of the Crag and Tail features are often over-deepened by diverted ice, leading to the formation of lakes. Waverley Station and The Grassmarket occupy sites of former lakes. Over-deepening is also evident beneath Salisbury Crags
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This diagram shows the drumlins in cross section. They are usually found in 'swarms' or 'en echelon'. Sometimes the landscape is called a 'basket of eggs' topography. The steep end is called the stoss and is on the upstream side. The ice dropped till under the glacier and it gradually smoothed the profile. They are elongated features that can reach a kilometre or more in length, 500m or so in width and over 50m in height. | |
| A car
driving down Highway 29 (Green Bay USA) over this undulating landscape of
drumlins.There is still some
debate about how drumlins are formed, but the most widely accepted idea is
that they were formed when the ice became overloaded with sediment. When
the competence of the glacier was reduced, material was deposited, in the
same way that a river overloaded with sediment deposits the excess
material. The glacier may have experienced a reduction in its competence
for several reasons, including melting of the ice and changes in velocity.
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Drumlins along Highway 29 south of Green Bay near Lake Superior in Wisconsin USA. The map shows rounded contour lines indicating low hills (drumlins) and the drainage (streams) lines in blue are in low valleys. Which way was the ice Moving? It looks to me as if it was from south west to north east. | |
| A crag and tail is an elongate hill that has at one end a steep face of ice-smoothed rock and at the leeward end a tapering streamlined tail of till. Possibly an erosion feature - certainly the crag is and it protected the 'tail' from erosion. Certainly shows the influence of geology (KI 3b). | |
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During the last
glaciation ice flowed eastwards (towards the North Sea) in the Edinburgh
area. Volcanic rocks were more resistant to erosion than surrounding
sedimentary rocks and developed westwards facing crags with gentle lee
slopes aligned towards the east. Good examples can be seen below Edinburgh
Castle (a volcanic plug) at the west end of Princes Street and below the
Observatory at the west end of Blackford hill (a lava flow).
The flanks of the Crag and Tail features are often over-deepened by diverted ice, leading to the formation of lakes. Waverley Station and The Grassmarket occupy sites of former lakes. Over-deepening is also evident beneath Salisbury Crags
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