522 THE FRESH-WATER LOCHS OF SCOTLAND 
2. Barrier Basins. — These may be due to the following causes : — 
(a) A landslip often occurs in mountainous regions, where strata, 
dipping towards the valley, rest on soft layers ; the hard rocks slip 
into the valley after heavy rains, damming back the drainage, which 
then forms a barrier-basin. Many small lakes high up in the Alps 
and Pyrenees are formed by a river being dammed back in this way. 
(b) A glacier. — Li Alaska, in Scandinavia, and in the Alps, a 
glacier often bars the mouth of a tributary valley, the stream flowing 
therein is dammed back, and a lake is thus formed. The best-known 
lake of this kind is the Marjelen Lake in the Alps, near the great 
Aletsch Glacier. The lake varies in area, being sometimes a mile in 
length, and at other times disappearing entirely through a crevasse in 
the ice; in August 1907 it disappeared in one night. Lake Castain 
in Alaska is barred by the Malaspina Glacier ; it is two or three miles 
long and a mile in width when at its highest level, and discharges 
through a tunnel nine miles in length beneath the ice-sheet. The 
famous parallel roads of Glen Roy in Scotland are successive terraces 
formed along the shores of a glacial lake during the waning glacial 
epoch. Lake Agassiz, which during the glacial period occupied the 
valley of the Red River, and of which the present Lake Winnipeg is a 
remnant, was formed by an ice-dam along the margin of two great ice- 
sheets. It is estimated to have been 700 miles in length, and to have 
covered an area of 100,000 square miles, thus exceeding the total area 
of the five great North American lakes: Superior (31,200 square 
miles), Michigan (22,450 s({uare miles), Huron with Georgian Bay 
(23,800 square miles), Erie (9960 square miles), and Ontario (7240 
square miles). 
(c) The lateral moraine of an actual glacier. — These lakes some- 
times occur in the Alps of Central Europe and in the Pyrenees. 
(d) The frontal vioraine of an ancient glacier. — The barrier in 
this case consists of the last moraine left by the retreating glacier. 
Such lakes are abundant in the northern hemisphere, especially in 
Scotland and the Alps. 
(e) Irregular deposition of glacial drft. — After the retreat of 
continental glaciers great masses of glacial drift are left on the land- 
surfaces ; but, on account of the manner in which these masses were 
deposited, they abound in depressions that become filled with water. 
What are called " kettle-holes are evidently spaces originally filled 
by large masses of ice, which melted away after the detrital matter 
was laid down. Often these lakes are without visible outlet, the 
water frequently percolating through the glacial drift. These lakes 
are so numerous in the north-eastern part of North America, that one 
can trace the southern boundary of the great ice-sheet by following 
the southern limit of the lake-strewn region, where lakes may be 
