Geology Reference
In-Depth Information
cene seaway. Mudflows here originated when heavy
rains on the slopes of ancient volcanoes caused loose
ash to flow for miles down valleys. Abundant fossil
flora in these sediments suggest a hilly topography
covered with Liquidambar. (sweet gum), Platanus,
(sycamore), and Carva, (hickory) adjacent to swamps
dominated by cypress.
Ginkgo
and
Metasequoia
com-
monly occured in the warm, rainy climate.
Around 1,000 feet of silt in the Portland and
Tualatin basins, designated as the Helvetia Formation,
sits directly above the Columbia River basalts. Pebbles
of basalt quartzite, granite, along with abundant quartz
and mica, suggest these sediments were deposited by
streams of the surrounding mountains ranges as well as
the ancestral Columbia River.
With margins around Portland and Tualatin in
the north and west and Sandy on the east, a bowl-
shaped structure gently sloped from 700 feet at Sandy
to 250 feet at Portland and Vancouver. Flowing in
a
channel south of its present course, the Columbia
River aided by the northerly flowing Willamette River
emptied into the basin to form a lake and deposit silts
and muds in a delta over 1,300 feet thick. More than
1,500 feet of the Sandy River Mudstone of sandstones,
siltstones, and conglomerates filling the Portland basin
underlie the city today. Later Pliocene Troutdale
gravels from the Columbia River drainage washed into
the lake, covering the mudstone to depths of 700 feet.
Dissection and erosion by rivers and Ice Age floods
removed portions of the Troutdale Formation from the
Portland Hills and surrounding areas.
Pathways of the pre-Troutdale Willamette and Tual-
atin rivers in the northern Willamette Valley. Extru-
sion of the Boring lavas moved the Willamette
River channel northward to Oregon City (after Bald-
win, 1957).
Geomorphology of Lake Oswego (photo courtesy of Delano Photographies)
