Travel Reference
In-Depth Information
time, the fissure closed; no new lava emerged. By this time the lava had
formed into a long, linear land mass. Limestone hegan to accumulate after
the volcanic eruptions ceased, and the underwater environments where
these limestone deposits formed were colonized hy early Mesozoic creatutes,
including corals and many species of ammonites.
Wrangellia was enormously long. It extended from Vancouver Island as
far as north Alaska. It may have been even longet: Isolated scraps of rock in
the Hell's Canyon region of Oregon and Idaho show the same telltale se-
quence of rocks used in the identification of Wrangellia.
Both paleontologists and paleomagnetists found that Wrangellia was
even more far-traveled than Mt. Stuart. The origin of the Wrangellia basalt
was somewhere in the
southern
hemisphere. The entite rock body had then
been carried northward by continental drift, eventually to crash into, and
fuse onto, the western coast of North America.
When did this titanic collision take place? In the early 1980s, paleo-
magnetists made extensive studies of Eocene-aged rocks at one site near Mt.
Stuart and at another to the east of Vancouver Island. In both regions, a
great thickness of 50-million-year-old sandstone accumulated in basins near
a mountainous region of high relief. These sedimentary rocks contain palms
and other fossilized tropical plants, which suggests that they too may have
come from more southetly regions. But the world of the Eocene appears to
have been warmer and wetter than that of today—a place where tropical
forests could have existed even at the latitude of what are now Washington
State and southern British Columbia. The paleomagnetic results confirmed
this; none of the Eocene-aged sandstone showed any evidence of drift. These
studies place an upper time limit on the collisions of Wrangellia with North
America: 50 million years ago.
The idea that this long, thin terrane had smacked into North America
sometime in the late Mesozoic had just filtered down into textbooks when,
in the mid 1980s, the whole concept of Wrangellia changed. It began to
grow in the eyes of some geophysicists. The prime mover in this was Ted
Irving of the Geological Survey of Canada. Irving, a paleomagnetist, was
stationed in Victoria and thus had ready access to British Columbia rocks.
