Geology Reference
In-Depth Information
The sinuous lava tube system of the Saddle Butte
lava field includes caves and collapse trenches.
Saddle Butte caves are particularly hazardous
unweathered tuffs are greenish. Weathered exposures
for exploration because of the tendency for roof
display shades of yellow, brown, or reddish brown
portions to fall. Walking across the thin lava shell over
common on the badland topography. Particularly
a tube is dangerous, and none of the caves are safe to
striking along the river and at the reservoir are the
enter.
sheer 100-foot high cliffs of pink rhyolitic lava. Con-
struction of the Owyhee Dam covered the vents from
Owyhee River and Succor Creek Canyons
which these lavas originated.
The Owyhee plateau rises from 2,100 feet
Between episodes of intermittent volcanic
above sea level where the Malheur River enters the
eruptions, animals and plants flourished here through-
Snake to 6,500 feet at the top of Mahogany Mountain.
out the Miocene. Fossil shells, bones, and plant re-
Dissecting the dry desert plateau of southeast Oregon,
mains, especially those of the famous Sucker Creek
canyons of the Owyhee, Malheur, and Snake rivers, as
flora, give clues to the Miocene environment. Beauti-
well as the smaller creeks have been deeply cut into
fully preserved leaves of oak, birch, maple, willow, pine,
some of the most scenic landscape in Oregon. Rock
laurel, and sycamore signify a moist temperate climate
formations exposed in walls of the river canyons tell
of more than 20 inches of rainfall annually in contrast
the geologic story of this region from Miocene volca-
to the arid environment today. Ancestral horses, deer,
nism that took place 15 million years ago through the
camel, and the pronghorn antelope fed on the extensive
Pleistocene Ice Ages only a few thousand years in the
grasslands and forests. Elephants lived on the wooded
past. The oldest rocks in the Owyhee canyon are
slopes, and aquatic rhinoceroses along with giant
colorful ash and lavas of the Sucker Creek Formation
beaver inhabited the wetlands.
violently expelled from fissures on the plateau during
A brief period of erosion and uplift that pro-
the middle Miocene. Much of the ash was washed
duced low mountains was followed by thick sequences
down the hillsides to be deposited in stream valleys.
of Owyhee basalts interbedded with red and brown
Ash and thick layers of basalt were covered over by
tuffs. This eruption blanketed more than 1,000 square
additional accumulations of lava. The fine-grained
miles with hot volcanic material about 15 million years
layers of the tuffs are usually thin and nearly white,
ago. The main layers of Owyhee Basalt are visible near
whereas the coarse-grained, massive layers of fresh
Mitchell Butte, below the Owyhee Dam, at Hole-in-
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