Uphain.] 04 [March 15, 
covered, testifyin<;- of gre:it defleetions of the ice currents. Else- 
where thront^hout glaciated regions, the striae of any distrittt ai-e 
goyerallj^ )>arallel, or deviate only slightly from one direction ; 
but here, probably more than in any other area of equal extent 
thnt has ever been thoroughly examined, the courses of glaciation 
have a very wile range, from south-southwest to south, south- 
east, east, and even east-northeast. 
Usually the striae on any small space of the rock exposures in 
Somerville are all parallel, or include only a few exceptional 
scratches crossing the others with small angles of divergence ; but 
occasionally intersecting striae have in nearly equal proportions 
two, three, or mnny widely varying courses. Nearly always 
each of the individual scratches on the rock is straight, extending 
commonly a few inches, but sometimes plainly continuous as a 
single line several feet long. It is evident, therefore, that the 
currents of the ice-sheet were deflected here from one course to 
another and even to several successive courses in so short a time 
that it allowed no great amount of erosion of the rock beneath. 
Several instances of exceptional curving striae which I have 
found on the ledges of Somerville, turning in every case toward 
the east from their southward course, seem to record such devia- 
tion while it was taking place. Most frequently, however, the 
records of different directions of the ice currents are preserved on 
contiguous spaces of the rock, all the striae in one case bearing, 
for example, S. 20° E., while at a distance of a few feet another 
spot may be just as uniformly striated S. 60° E. In such places 
a thin covering of drift on the rock surface with most southerly 
course protected it from erosion by the later glacial current pass- 
ing more eastward. It is thus known that there was a definite 
plane between the moving bnse of the ice with its enclosed drift 
and the undisturbed subglacial drift deposits or ground moraine. 
Anj' accunmlation of the ground moraine, however, after having 
received additions for a considerable time from the overriding 
ice-sheet, was liable, on account of changes in the action of the 
glacial currents, to be partially or wholly jjlaned off and carried 
forward, like the detritus supplied from the bed-rocks, wherever 
they were exposed. 
The courses in which the ice-sheet moved over Somerville have 
been observed in many places, as noted in the following list, 
which is arranged in the geogra|)hic order of the rock outcrojts 
from east to west and south to north. 
