The ridges dividing the two sides of a roche moutonnée are perpendicular to
the general flow of the ice sheets.
At the margins of the ice sheets are rugged periglacial regions.These fea-
tures developed along the tip of the ice and were directly controlled by the
glacier. Cold winds blowing off the ice sheets affected the climate of the
glacial margins and helped create periglacial conditions. The zone was domi-
nated by such processes as frost heaving, frost splitting, and sorting, which cre-
ated immense boulder fields out of what was once solid bedrock.
Erratic boulders (Fig. 202) are glacially transported rocks embedded in
glacial till or exposed on the surface. They indicate the direction of glacial
flow.They range in size from pebbles to massive boulders and have traveled as
far as 500 miles or more. Erratics are composed of distinctive rock types that
can be traced to their places of origin. Indicator boulders are erratics of
known origin used to locate the source area and the distance traveled for any
given glacial till. Their identifying features include a distinctive appearance,
unique mineral assemblage, or characteristic fossil content. The erratics are
often arranged in a boulder train, forming a line or series of rocks originating
from the same bedrock source and extending in the direction of glacial move-
ment. A boulder fan is a conically shaped deposit containing distinctive errat-
ics derived from an outcrop at the apex of the fan.
Glacial drift includes all rock material deposited by glaciers or glacier-fed
streams and lakes, where the greatest thicknesses are attained in buried valleys.
Figure 202A glacial
boulder field,Tulare
County, California.
(Photo by F. E. Matthes,
courtesy USGS)QUATERNARY GLACIATION