It is of course possible to have valleys without lakes, and in fact the latter are, now at least, exceptional. There can be no lakes if the slope of the valley is uniform. To what then are lakes due?
Professor Ramsay divides Lakes into three classes:—
1. Those due to irregular accumulations of drift, and which are generally quite shallow.
2. Those formed by moraines.
3. Those which occupy true basins scooped by glacier ice out of the solid rock.
To these must, however, I think be added at least one other great class and several minor ones, namely,—
4. Those due to inequalities of elevation or depression.
5. Lakes in craters of extinct volcanoes, for instance, Lake Avernus.
6. Those caused by subsidence due to the removal of underlying soluble rocks, such as some of the Cheshire Meres.
7. Loop lakes in deserted river courses, of which there are many along the course of the Rhine.