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.

8. Those due to rockfalls, landslips, or lava currents, damming up the
course of a river.

9. Those caused by the advance of a glacier across a lateral valley,
such as the Mergelen See, or the ancient lake whose margins form the
celebrated "Parallel Roads of Glen Roy."

As regards the first class we find here and there on the earth's surface
districts sprinkled with innumerable shallow lakes of all sizes, down to
mere pools. Such, for instance, occur in the district of Le Doubs
between the Rhone and the Saone, that of La Sologne near Orleans, in
parts of North America, and in Finland. Such lakes are, as a rule,
quite shallow. Some geologists, Geikie, for instance, ascribe them to
the fact of these regions having been covered by sheets of ice which
strewed the land with irregular masses of clay, gravel, and sand, lying
on a stratum impervious to water, either of hard rock such as granite or
gneiss, or of clay, so that the rain cannot percolate through it, and
without sufficient inclination to throw it off.

2. To Ramsay's second class of Lakes belong those formed by moraines.
The materials forming moraines being, however, comparatively loose, are
easily cut through by streams. There are in Switzerland many cases of
valleys crossed by old moraines, but they have generally been long ago
worn through by the rivers.

3. Ramsay and Tyndall attribute most of the great Swiss and Italian
lakes to the action of glaciers, and regard them as rock basins. It is
of course obvious that rivers cannot make basin-shaped hollows
surrounded by rock on all sides. The Lake of Geneva, 1230 feet above the
sea, is over 1000 feet deep; the Lake of Brienz is 1850 feet above the
sea, and 2000 feet deep, so that its bottom is really below the sea
leve