INFLUENCE OF DEBRIS ON FLOW OF GLACIERS. 825 
the densely charged bottom layer to the clearer ice vertically 
above, might be gradual or abrupt, according as the percentage 
of débris decreased gradually upwards or had a sharply defined 
upper limit. When the change was abrupt, a plane of shear might 
be established. Whether a glacier charged at its base with débris, 
shall erode or deposit at a given locality depends on its rate of 
flow. The rate of flow is controlled by several factors, one of 
the most important being the percentage of débris contained in 
the ice. If the débris-charged ice in contact with the rocks 
beneath moves at all it will cause abrasion; if so heavily 
charged with débris, however, that it is rigid under the forces 
to which it is subjected, it will remain stationary and not 
only cease to erode but protect the rocks beneath, and lead 
to the accumulation of débris. A glacier may, therefore, erode 
in one portion of its course and in another portion accumulate 
débris in its stagnant bottom layer. Also, a decrease in the rate 
of flow may cause débris-charged ice to stagnate at a locality 
where erosion was previously in progress; while an increase in 
the rate of flow might lead to the removal of a previously stag- 
nant layer. 
If a glacier occupies a valley in which there is a change from 
a precipitous to a gentle slope, the rate of flow on the precipitous 
slope, other conditions being the same, will be greater than 
below, and may be sufficient to carry forward an amount of débris 
which would cause stagnation when the more gentle slope was 
reached. A glacier might, then, erode the rocks over which it 
passed in one portion of its course and farther on, accumulate 
débris in its basal portion so as to cause stagnation, without an 
increase in the amount of foreign material carried. Whether a 
glacier shall erode or deposit, depends, therefore, on a ratio 
between strength of current and the percentage of débris in its 
basal portion. 
Clear ice in flowing over ordinary rocks has but slight if any 
power to abrade them. If débris of the kind commonly present in 
glaciers, is added to the ice, other conditions remaining the same, 
its erosive power will be increased until the percentage of débris 
is sufficient to materially check the flow, and will then decrease as 
