Tee Age in Xorfli An/ericd and Exroije. — I'jiJkdi/. 
109 
companying- table, so that the basal ice stratum, 400 feet thick, 
terminating within the first mile from the front, should con- 
tain 5 feet of englacial drift; the stratum, 200 feet thick, ter- 
minating in the second mile, 2 feet of drift ; the 150 feet of 
ice terminating in the third mile, 1^ feet of drift ; the fourth 
mile's ic3 stratum, 120 feet thick, 1 foot of drift; and the stra- 
tum of 100 feet in the fifth mile, seven-tenths of a foot. The 
amount of englacial drift above the altitude of 970 feet, 
reached at the end of five miles, would be about five feet in a 
thickness of about 800 feet of ice, the upper limit, as before 
noted, being assumed to be 1,760 feet above the land surface. 
The rate of ablation of the ice in the warm summers of the 
Champlain epoch, with alternating sunshine and still more ef- 
ficient rains, probably averaged from two to four inches daily 
during 200 days of the warm portion of each year. In the re- 
maining five and a half months we may suppose that the 
snowfall and ablation counterbalanced each other, while the ice 
advance, though diminished on account of the lower tempera- 
ture, would produce some thickening of the border, When a 
series of years had a small mean rate 6t ablation, the ice 
front remained nearly stationary, giving the conditions neces- 
sary for the formation of a marginal moraine ; but when the 
ablation was more rapid, no belt was occupied by the front so 
long as to be marked by morainic hills and ridges. An aver- 
age ablation of two inches per day during 200 days of each 
year may be assumed as permitting the front to remain on the 
same line, or with advances and recessions not exceeding a 
half mile or one mile from that line. The resulting moraine 
would be heaped irregularly on a belt one or two miies wide. 
Coiuh'tioiis of Morainic Drift AccHiniddtion. 
Ice 
stratum 
GLACIAL 
ASCENT OF ICE SURFACE 
ADVANCE. 
term ina- 
ting in 
success- 
Feet 
Feet 
Mile.s 
per 
Total. 
Ratio. 
daily in 
in 80 
ive miles 
Mile. 
^umm'r 
years. 
1 
400 
400 
1 -.V.i 
2.1 
2.5 
2 
200 
COO 
1:20 
iM 
5.8 
H 
150 
750 
1 ::i5 
5.8 
6.7 
4 
r2o 
870 
1:44 
7.8 
8.7 
5 
KXJ 
970 
1 :5;i 
8.S 
10.4 1 
MORAINIC DRIFT, IX FEET. 
Englacial. 
5.0 
2.0 
1.5 
1.0 
0.7 
Total averaf<e tliickness of moraine from these five miles, 
83.7 feet, if amassed on a belt one mile wide. 
Becoming 
sui)«r- 
glacial 
in :<0 y 'ars. 
Previously 
supjr- 
glacial. 
12. :i 
10.0 
10.0 
8.7 
7.8 
48.9 
10.0 
8.0 
0.5 
5.5 
4.8 
.■i4.8 
To supply the ice by onfiow equival?nt to tlu' al)I;itinn of 
two inches daily in suniiir-r upon th:' fir.>t niilc from the I'ron- 
