Geology Reference
In-Depth Information
American crustal plate. The early volcanism took place
in several stages beginning with lava flows from a
volcanic chain lying immediately to the east of the
Pacific continental margin. As far back as 40 million
years ago, a number of small low volcanoes, irregularly
spaced along a northwest-southeast belt in the region
of eastern Oregon, deposited a thick accumulation of
andesitic tuffs and lava flows that formed the base of
the Western Cascade mountains. The broad width of
Significant volcanic peaks of the Cascade Range
less noticable, eroded Western Cascades. With exten-
sive outpouring of basaltic lavas, the region beneath
the volcanoes continued to sink, creating a series of
troughs or grabens that extend most of the length of
the range. Recent volcanic activity in the province took
place as late as 200 years ago when black cindery lava
flowed out on the flanks of the younger peaks.
The Ice Ages of the Pleistocene brought
glaciers to the range, with ice carving out glacial
valleys, darning lakes, and eroding the sharp serrated
crests of Mt. Washington, Three Fingered Jack, Mt.
Thielsen, and Mt. McLoughlin. Glacial ice is still an
active geologic agent at many of the higher elevations.
Geology
Volcanic vents in the Western and High Cascades
(after Peck, et al.,1964; Priest, et al., 1983)
Construction of the Oregon Cascade range
began with Eocene volcanism that was triggered by
subduction of the Farallon plate beneath the North
