glacier, where it was either dropped at the margin or, during the
recession of the glacier, was laid upon the surface as the ice melted
away. The result of this erosion and transportation has been to change
the conditions of the surface both as regards soil and drainage. As
the reader has doubtless perceived, ordinary soil is, outside of the
river valleys, derived from the rock beneath where it lies. In
glaciated districts the material is commonly brought from a
considerable distance, often from miles away. These ice-made soils are
rarely very fertile, but they commonly have a great endurance for
tillage, and this for the reason that the earth is refreshed by the
decay of the pebbles which they contain. Moreover, while the tillable
earth of other regions usually has a limited depth, verging downward
into the semisoil or subsoil which represent the little changed bed
rocks, glacial deposits can generally be ploughed as deeply as may
prove desirable.

The drainage of a country recently affected by glaciers is always
imperfect. Owing to the irregular erosion of the bed rocks, and to the
yet more irregular deposition of the detritus, there are very numerous
lakes which are only slowly filled up or by erosion provided with
drainage channels. Though several thousand years have passed by since
the ice disappeared from North America, the greater part of the area
of these fresh-water basins remains, the greater number of them,
mostly those of small size, have become closed.

Where an ice stream descends into the sea or into a large lake, the
depth of which is about as great as the ice is thick, the relative
lightness of the ice tends to make it float, and it shortly breaks off
from the parent mass, forming an iceberg. Where, as is generally the
case in those glaciers which enter the ocean, a current sweeps by the