j24 The American Geologist. May, 1892 
Are the Glaciers of North America Advancing or Retreating ? 
The glaciers of this continent have been known for so short a 
time that only small portions of their histories have been read. 
Their study is comprised almost entirely within the past decade 
and has been carried on in such a desultory way that for the most 
part only qualitative evidence as to their advance or retreat is 
available. 
Character of the Evidence: Kvidence of the advance or retreat 
of the ends of alpine glaciers, or of the borders of piedmont 
and continental glaciers, may be obtained in various ways.  (Gla- 
ciers which are advancing sometimes plow into the debris in 
front of them and force it up in concentric ridges, usually with 
the formation of cracks in the soil. The surfaces of the ridges 
formed in this way are frequently covered with vegetation, which 
in addition to their forms and the character of the material of 
which they are composed, serves to distinguish them from termi- 
nal moraines. When a glacier advances into a forest, the trees. 
are broken off and piled in confused heaps about the margin of 
the ice. The upper surface of a glacier is known to flow faster 
than the ice below, and an advance is probably accomplished by 
the upper surface flowing over and burying the ice which rests on 
the ground. For this reason, advancing glaciers usually present 
bold scarps at their extremities, and, in general, are not covered 
with a broad sheet of debris. 
In retreating glaciers the layers of new snow deposited on the 
névé fields and changing to ice as they flow downward, are melted 
before reaching the margins of the ice streams, andthe slow moy- 
ing ice at the bottom is thus left exposed and melts away. The 
retreat is accomplished not by a contraction in the volume of the 
ice-body, but by the melting of its distal extremity. The ice 
which is not covered by fresh layers melts at the surface, and the 
englacial debris which it contains is concentrated in a general 
sheet forming fringing moraines. Whena sheet of debris of this 
character is extensive and covers the lower portion of a glacier. 
from side to side, it indicates that the ice beneath is practically 
stationary and consequently is melting and retreating. The ends 
of retreating glaciers’ frequently have a gentle surface slope, and 
in many instances are so ¢ompletely concealed by debris that the 
actual terminus of the ice cannot be distinguished. When the 
moraines are heavy, however, and especially when they are clothed 
