LAKES IN RELATION TO GEOLOGICAL FEATURES 477 
view of this evidence, the phenomena presented by these plateau basins 
may be satisfactorily explained by the action of land ice. 
VALLEY ROCK-EASINS 
Valley rock-basins are more important topographical features, and 
the question of their origin has excited keener controversy. One 
condition of prime importance in the formation of such basins is the 
production of graded valley floors, reduced to a base-level either with 
regard to the sea or to barriers of hard rock with intervening weaker 
strata. These flat reaches might then be converted into rock-basins 
either by diff'erential crustal movements with or without lateral com- 
pression, or by land ice, which is capable of eroding below the action 
of running water, as suggested by Sir A, C. Ramsay. It need hardly 
be pointed out that aqueous erosion is incapable of producing the 
characteristic phenomena of valley rock-basins. 
The soundings of the Lake Survey have established certain points 
which are highly suggestive in connection with the question of the 
origin of such basins. They show (1) that these depressions are 
U-shaped in cross-section, like the contour of the glaciated valleys in 
which they lie ; (2) that there is a lack of adjustment between the large 
valley rock-basins and tributary streams, the relation between them 
being analogous to that between trunk streams and tributary hanging 
valleys ; (3) that while the large lakes have usually comparatively flat 
floors, many of them have several distinct basins ; (4) that the deepest 
soundings frequently occur where the constriction of the valley is 
greatest ; (5) that the steepest slopes are often found at concave bends 
in the larger rock-basins, where it can be shown that the difl"erential 
erosion of the ice must have been most powerful. 
All these phenomena indicate that valley rock-basins present 
many of the features which are characteristic of glacial action. But 
in addition to these points we will now proceed to show that the 
distribution and form of many of the rock-basins in Scotland are 
produced under complex local conditions dependent on the geological 
structure, pre-glacial topography, and difl'erential ice-erosion of the 
particular regions in which they occur. 
A study of an orographical map of Scotland shows that valley 
rock-basins are almost confined to those highly dissected regions 
where deep through valleys have been established between high 
mountains, and where the cols form low divides or passes in the exist- 
ing watershed of the country. In the section dealing with topography 
we have endeavoured to point out that the westerly and northerly 
flowing streams have, by capture, reversed the drainage and produced 
a series of through valleys. Such features are specially developed 
in the western portions of the Northern, Central, and Southern 
