Structural and Historical Geology

tions up to 2,000 feet thick. In a few places these flows were warped into gentle
folds and the whole group may have acquired a general easterly dip.

Late Miocene and Pliocene Time

No rocks of this age are known in the area. However, as there is palzon-
tological evidence only for the lower limit of the Endako Group, it is possible
that many of the flows included are as young as late Miocene and Pliocene. This
time was also characterized by areal erosion and down-cutting of streams. The
drainage system that was disrupted by the Endako lavas re-established itself and
many canyons were cut in the Tertiary volcanic rocks. Much Tertiary rock was
eroded, established valleys were broadened, and deep weathering may have
occurred.

Pleistocene Time

During Pleistocene time the area was covered by glacier ice to a depth
of at least 5,000 to 6,000 feet. At its maximum the ice may have moved in a
north-south direction as indicated by indistinct grooves but the last ice movement
was from southwest to northeast.

Recent Time

In Recent time streams have been re-establishing the drainage channels
disrupted by glaciation. No further volcanic activity has occurred.

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