In
characteristic localities the loose materials deposited by glaciers may
readily be recognized at first sight, and distinguished from water-worn
pebbles; nor is it difficult to distinguish both from loose materials
resulting from the decomposition of rocks on the spot,--the latter
always agreeing with the rocks on which they rest, while the
decomposition to which they owe their separation from the solid rock is
often still going on. Such _debris_ are found everywhere about
disintegrating rocks, and they constantly mingle with the loose
fragments brought from a distance by various agencies. They are found
upon and among the glacier-worn pebbles, especially where the latter
have themselves been disturbed since their accumulation. They are also
found among water-worn pebbles, wherever the rocky beds of our rivers or
the rocky bluffs of our sea-shores crumble down. In investigating the
character of loose materials transported from greater or less distances,
either by the agency of glaciers or by water-currents, it is important
at the very outset to discriminate between these deposits of older date
and the local accessions mingling with them.
Occasionally we may have also to distinguish between all these deposits
and the _debris_ brought down by land-slides, or by sudden freshets
transporting to a distance a vast amount of loose materials which are
neither ice-worn nor water-worn.
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