BY A. N. LEWIS, M.C. 27 



Lake Seal lies in a long, deep valley, decidedly U-shaped, 

 which is blocked by the moraine at the eastern end of 

 the lake. The top of this stands 200 feet above Lake Web- 

 ster, but has been much worn down near its junction with 

 the solid rock of Mt. Bridges. Farther east it is at least 

 another hundred feet higher. This as it stands represents 

 the largest specimen of a moraine in the Park. The moraine 

 bounds the eastern edge of the lake, and curves round the 

 southern shore, abutting on to a ridge running down past 

 Platypus Tarn from a shoulder of Mt. Mawson. The shore 

 of the lake on this moraine is bordered by a beach of water- 

 worn cobbles, testifying to the force of the waves churned 

 up by the winter hurricanes. 



On the Broad River, opposite the end of this lake, can 

 be seen half a mile of diabase outcrops rising 100 feet 

 abruptly from the river, but on the Lake Seal side these 

 rocks are covered with glacial till of the moraine just 

 discussed, and are invisible. It was here that the glacier 

 turned north. The depth of ice must have been tremen- 

 dous, perhaps 1,000 feet. One of the most useful pieces ot 

 investigation in the Park would be to ascertain the height 

 on each side of Mt. Bridges to which the glacier extended, 

 and to ascertain the depth of Lake Seal by a series of 

 soundings. 



The line of glacial till extends over the whole slope 

 from the Broad River to Lake Seal, reaching its highest 

 point some six hundred yards south-east of the lake, whence 

 it drops sharply to the southward into the gully of an un- 

 named tributary of the Broad. 



A spur bounds Lake Seal on the southern side, gradual- 

 ly rising until it becomes part of the configuration of Mt. 

 Mawson. This doubtless has a core of solid diabase, al- 

 though it is deeply overlaid with glacial till. Along this 

 ridge south of the lake, and parallel to the shore, run lines 

 and ridges of this morainal material, perhaps representing 

 lines of lateral moraines, but more probably ridges caused 

 by lateral pressure of the ice. Some of the hollows between 

 them contain ponds. Some of these hollows may have been 

 formed by the imprisoning of large masses of ice in the 

 moraine, the melting of which has caused the surface 

 of the ground to sink. Among these ridges lie whole lines 

 of huge boulders, many exceeding 20 feet in every measure- 

 ment, and often piled on top of each other. Nowhere in 

 the Park are there finer examples of erratics. In one place 



