Geology Reference
In-Depth Information
Invertebrate fossils of the late Eocene Eugene Formation
In middle and late Miocene time, voluminous
sheets of basaltic lava from fissures and vents in
northeastern Oregon poured through the Columbia
gorge and into the Willamette Valley where they
reached as far south as Salem. The fluid Columbia
River lavas covered the region of the Portland Hills,
most of the Tualatin Valley, as well as the slopes of the
Chehalem, Eola, and Amity hills. The dark, finely
crystalline, collumnar-jointed basalt ranges up to 1,000
feet in thickness. Near Portland the layers of lava
produced a monotonous, flat landscape with only the
tops of several higher hills projecting above the flows.
After cooling and crystallizing, the lavas rapidly decom-
posed in western Oregon's wet climate so that almost
all of the original volcanic landscape has been thor-
oughly dissected. Dark red soils around Dundee, the
Eola Hills, and Silverton Hills are easily recognized as
decomposed Columbia River basalts.
Interfingering with the Columbia River basalts
in northern Marion and Clackamas counties, 1,000 feet
of clastic sediments, mudflows, and volcanic tuffs of the
Molalla Formation represent the first terrestrial
sediments deposited after the withdrawal of the Oligo-
In the Nehalem Valley fine-grained shallow
marine sediments of the Scappoose Formation are
contemporaneous with the Scotts Mills Formation.
Over 1,500 feet of Scappoose sandstones, mudstones,
and conglomerates were deposited in an estuary or
delta environment covering a dissected landscape.
Miocene
With slow uplift of the Coast Range, the sea
withdrew from the region of the Willamette Valley.
The deep shelf and slope environment became shallow-
er as the basin filled. Little is known about the config-
uration and environment of the Willamette Valley
following the regression of the late Oligocene seaway.
A broad semitropical coastal plain, where lakes ponded
in slight depressions, extended from the ancestral
Cascades out to the present shoreline. Black, silty clays
lying between weathered Pleistocene and late Eocene
Spencer sandstones near Monroe reveal ancient
lakebed sediments that contain fossil pollen from
coniferous as well as broadleaf plants once living
around the lacustrine basin. Much of the pollen from
this lake is from plants now extinct in the Pacific
northwest.
