KKNDALL : TUK CILACIER LAKES OF f "LEVEL AND. 
'27 
discharged their waters by the east coast route. Il)uniclale 
is a deep, ratlier narrow valley, forming a V both in j^lan and 
section. It opens northward into Eskdale at Sleights, and is 
separated from the Murk-Esk Valley on the west by Sleights 
Moor, a broad tract of Lower Estuarine grit, which forms a steep 
escarpment on the west, north, and east. This constitutes 
a watershed nowhere falling below 800 feet. On the east. Iburn- 
dale is enclosed by the high moorlands (of the same grit) of L'ggle- 
barnby and Sneaton. which attain an elevation of TOO feet (piite 
near to Eskdale ; and this altitude, or a still greater . aie. is 
maintained nearly to the head of the valley. 
Drift deposits of great thickness occur in the tioor of the 
valley, and to a considerable height up its sides. On tlie advance 
of the northern ice-sheet a lake was formed in Iburndale. tlie 
overflow of which was by a gap in the eastern side of the valley, 
cutting through the 675-foot contour, but not descending below 
the 650-foot contour. From this gap a deep, peat -filled valley. 
Biller Howe Dale, of characteristic section, extends eastward 
into the Jugger Howe Beck drainage, which constitutes the 
true source of the River Derwent. At the upper and stream- 
less end of this channel is an abandoned oxbow separated from 
the main channel by a small hill of solid rock (Plate XL). At 
the maximum extension of the ice the watershed of Sneaton 
!Moor was overridden, and the ice extended to the Iburndale 
overflow. In the stage of retreat the watershed was trenched 
by two well-marked lake-overflows, but now streamless where 
they cross the watershed, and containing a considerable depth 
of peat. 
Robin Hood's Lakes. — The natural amphitheatre of Robin 
Hood's Bay is packed with boulder-clay in its lower parts, and 
an outer fringe of drift on the westerly moorlands consists of 
sand and gravel heaped up in large mounds. The whole of 
the pre-glacial drainage of this area probabh- centred on the 
Bay (Plate X.). 
I At the period of maximum extension the margin of the 
ice-sheet appears to have extended from the head of Iburndale 
along a west-to-east line to the great bend of Biller Howe Dale, 
and thence to have run in a souther!}^ direction past tlie side 
