May
3, 2008 Willapa Hills-Mima Mounds field trip
On this trip we will examine the
effects of the December 2007 flood along the Chehalis River
and in the Willapa Hills. The effects include stream
channel shifts, flood damage to homes, property and infrastructure; deposits of
sediment and debris (including logs), and landslides. We will also examine the
local geology and assess what role the geology may have played in the hazards
that unfolded. We will also visit the Mima Mounds at the WADNR Mima Mounds
Natural Resource Conservation Area as well as the DNR gravel pit along Bourdeaux Road,
slight south of the NRCA, where a cross section of a Mima Mount and its
underlying gravelly substrate can be observed.
STOP 1: FLOOD DAMAGE ALONG RIVER ROAD.
STOP 2: QUARRY EXPOSURE OF COLUMBIA RIVER BASALT WITH COLUMNAR JOINTING.
STOP 3. EXPOSURE
OF PRE-COLUMBIA RIVER
BASALT ASTORIA FORMATION SEDIMENTARY ROCKS IN
THE CHEHALIS RIVER NEAR DRYAD.
STOP 4. COLUMBIA RIVER BASALT
ALONG ELK CREEK ROAD (Drive by no stop).
STOP 5. LANDSLIDE
IN LINCOLN CREEK FORMATION SANDSTONES ALONG SR6 WEST OF PE ELL.
STOP 6. FLOOD DAMAGE AND
DEPOSITS OF SEDIMENT, LOGS, AND DEBRIS ALONG THE CHEHALIS RIVER
AT CERES
ROAD BRIDGE.
STOP 7A. LANDSLIDE
OF SEDIMENTARY ROCKS AND COLUMBIA RIVER BASALTS ALONG NORTH VALLEY WALL OF CHEHALIS RIVER OFF CERES ROAD.
STOP 7B. DEPOSITS
OF GLACIALLY-DERIVED SANDS AT ABOVE SITE.
STOP 8. MIMA
MOUNDS NATURAL RESOURCE CONSERVATION AREA.
STOP 9. BOURDEAUX ROAD EXPOSURE OF MIMA MOUND.
References
Washburn, A. L., 1988, Mima mounds--An evaluation of proposed
origins with special reference to the Puget Lowlands: Washington Division of
Geology and Earth Resources Report of Investigations 29, 53 p.
Figures
