828 THE JOURNAL OF GEOLOGY. 
thickness and expand. The effects of such a check will vary 
with conditions. 
In the case of a growing glacier, the increasing volume of 
ice above the dam, would cause it to rise and flow over the 
obstruction, which would then become subglacial. If the gla- 
cier was slowly wasting away, its terminus might remain station- 
ary for a time and increase in thickness and then continue to 
diminish, leaving its highly débris-charged extremity to slowly 
waste away and finally leave a terminal moraine. The delicate 
balancing between conditions which cause a glacier to advance, 
and those favoring recession, so frequently to be observed, 
would lead to many variations in the changes induced by the 
congestion of débris, above considered. This process will be 
again referred to in connection with the influence of débris on 
fluctuations in the lengths of ice streams. 
It is frequently stated that terminal moraines are formed by 
the carrying forward of superglacial débris and its projection 
over the end of a glacier. Ridges of débris may frequently be 
seen about the extremities of glaciers, which are receiving addi- 
tions in this manner. Such ridges usually have smooth outer 
slopes and when the ice withdraws from them, the sides left 
unsupported, acquire even slopes, also, owing to the sliding 
down of the material; their crest lines are sharp, but frequently 
undulating in the direction of their length. Terminal moraines 
of this character are in reality aprons of débris, analogous to 
talus slopes at the bases of steep cliffs. 
Moraines of another type illustrated by the great terminal 
which crosses New Jersey, Pennsylvania, etc., have broad, hum- 
mocky surfaces, with basins between, and originate from the 
melting of débris-charged ice. Their irregularities in relief are 
due to the unequal melting of the ice that held the débris, and 
the concentrations of the foreign material in depressions after it 
became superglacial, in the manner now well shown in the broad 
moraine-covered border of Malaspina glacier. Irregularitie 
would also result from inequalities in the distribution of the 
débris while yet englacial. 
Two types of lateral moraines, corresponding in the manner 
