WALLOWA GEOLOGY
An illustrated rock-hunters
guide to Hurricane Creek
By Ellen Morris Bishop
From youngest to oldest, they are:
For The Chieftain
A
utumn, when streams are
low, is an ideal time to
go looking for geology.
Creeks and rivers carry samples
of every kind of rock in the Wal-
lowas down to us. Tumbled and
rounded, only the hardy ones sur-
vive. But in the dry portions of
stream beds, you can ind virtual-
ly every kind of rock in the Wal-
lowa Mountains.
Each individual stone tells
a story. Some bear witness to
monstrous ancient volcanic ex-
plosions. Others tell of languid
seas and vanished life. There are
faults in some rocks. Ancient
loods in others.
And in each rounded river
rock, there is the haunting
memory of time. You hold in
your hand not just a rock, but
a stream, a beach, a torrid lava
low, all from so long ago that all
human memory fails.
Fortunately, geologists and
rock-hunters are good detectives
and good story-tellers. This guide
will help you determine what
kind of rock you have found, and
what story it tells.
What are the Wallowa Moun-
tains made of?
Five different types of rock
comprise the Wallowa Moun-
tains. Examples of all of them
can be found in Hurricane Creek
and in the ephemeral streams
and glacial gravels that oc-
cupy most of the valley
loor. The photos below are
all rocks from Hurricane
Creek. Each is unique. Each
tells a story. The rocks that
you will ind will be some-
what different, but will belong
to one of these rock types. Happy
hunting!
COLUMBIA RIVER BASALTS: (Igneous, volcanic)
About 14-16 million years old. Youngest rocks in Wallowas. Erupted from vents (issures, or “dikes”)
now exposed in the Wallowa Mountains. These lavas inundated the Columbia Basin. (They are 3 MILES
thick beneath Hanford.) Some lowed all the way to the Oregon Coast. Today, they are exposed at
Haystack Rock, Point Lookout, Depot Bay and many other places. Along Hurricane Creek, Columbia
River basalts occupy the top of Hurricane Divide and form Twin Peaks and Sawtooth. Basalt is not
abundant in Hurricane Creek, and much of what you ind has come from the basalt dikes. ID: Dark
red-brown to reddish on weathered surface. Black on fresh surface. May contain holes (gas vesicles).
‘GRANITES’ (Actually granodiorite and tonalite) (Igneous, plutonic/intrusive)
About 100-130 million years old. Light-colored rocks with “salt and pepper” appearance. Intruded
into the older rocks of the Wallowas. The “granites” form the core of the Wallowas — the Lakes Basin,
for example. Formed by melting of the lower crust and of the older greenstones and sedimentary
rocks. ID: Light-colored, salt/pepper appearance. Sparkly dark minerals. Proportion of light and dark
minerals will vary from one rock to another.
SANDSTONE AND SHALE:
(sedimentary; slightly metamorphosed.)
180-210 million years old. When found as rocks in or along Hurricane Creek,
these are usually quite hard and have a “banded” appearance. They are often
dark gray and/or tan. They form the tops of Chief Joseph Mountain and Hurwall
Divide. They were intruded by hot granite magmas, which “cooked” them, fusing
the sand-grains and clays and producing a harder rock than the original sand-
stones and shales.
LIMESTONES:
(AKA, Marble) (sedimentary, but metamorphosed
to a soft marble in many cases.)
220-230 million years old. These rocks formed as parts of reefs or
extensive banks of carbonate sand (Think Bahamas Banks). They
may have a banded or “marbled” appearance, but often are simply
a soft, rounded gray or white rock. The enlarged crystals of calcite
that form the rock may provide a “sparkly” appearance. Some mar-
bles may contain recognizable fossils. Because these are soft rocks,
they tend to be well-rounded and relatively small.
VOLCANIC GREENSTONES:
(Igneous, volcanic rocks that have been metamorphosed.)
230-270 million years old. Formed as lava lows and related processes on
volcanic islands far of the Idaho coast a long, long time ago. Metamor-
phosed during collision with North America about 120 million years ago
and also by heat of Wallowa granite intrusions. Today these rocks are the
foundation of the Wallowas and are very prevalent in Hells Canyon. Dark
gray-green, or gray with a greenish caste. Along Hurricane Creek, many
greenstones were initially formed as volcanic debris lows, explosive ash
eruptions, and stream sediments on the ancient volcanic peaks. Others
were actual lava lows. Their appearance is highly variable, but they all have
a characteristic yellow-green to gray-green color. May include lighter yel-
low-green veins, blobs or stripes.
Wallowa.com | 27