Vol. 58. ] GLACIBR-LAKES IN THE CLEVELAND HILLS. 475 
and running at different levels for 1 or 2 furlongs. The reason 
for this general absence or imperfect development of beach-lines in 
the Cleveland area admits of a very simple and natural explanation. 
For their production, stability of the water-level during a consider- 
able period is one of the great requisites, abundance of beach-forming 
material the other. The occurrence of successive sea-beaches 
round the coasts of Scotland and Norway does not imply that the 
land rose per saltum, but rather that there were protracted periods 
of comparative repose and beach-formation, followed by periods of 
movement, slow, yet sufficiently rapid to prevent the accumulation 
of a recognizable beach. Again, the beaches are not horizontally 
continuous, but are interrupted where the supply of beach-making 
material was deficient. 
In the case of a glacier-lake, the stability of the level will be 
determined by the constancy of the position and altitude of the 
ice-dam, and the rate of erosion of the channel by which the over- 
flow takes place. The former factor cannot in the generality 
of cases be evaluated, but the latter is much more easily deter- 
mined. It will depend upon the volume of water to be carried, 
the fall, and the nature of the rock at the overflow. In the cases 
of both the Mirjelen See and the Glen-Roy Lakes, the overflow takes 
place (or took place) over hard crystalline rock, which is very slowly 
eroded, and consequently the lake-levels were very stable and the 
beaches are large and well-defined. The overflows of the Cleveland 
Lakes were over Jurassic rocks, for the most part soft shales ; hence 
they were cut down rapidly, and little or no beach-formation 
resulted. In a few cases the sills were cut down on to hard beds 
of grit; and it is at levels coincident with these that the best-defined 
strand-lines occur. 
(2) Deltas.—When a stream carrying sand or gravel enters 
standing water, the detrital materials are cast down in the form of 
a fan, the surface of which 1s just below the highwater or flood-level. 
If debouching into the sea, the fan shelves off into the deep with 
a continuous slope, while a lake-delta usually terminates with an 
abrupt face. 
Mr. Jamieson, and observers who preceded him, found that the 
deltas of streams flowing into the Glen-Roy Lakes were very well- 
defined at the point where they coalesced with the beaches. In the 
Cleveland area such deltas are rarely seen, but some examples 
exist, especially in the valley of the Esk. They occasionally exhibit 
the fanlike form and steep scarp of lacustrine deltas, but more 
often they are simply patches of current-bedded sand and gravel 
occurring isolated at the beach-level where some stream has 
debouched, generally the overflow from another lake. 
(3) Floor-deposits.—The late Mr. Dugald Bell and Prof. 
James Geikie have alluded to the deposits which accumulated 
upon the floor of the Black-Cart Lake; and the late Prof. Carvill 
Lewis made reference to the nature of the deposits accumulated in 
