|Geology of Silver Falls State Park
A History of Ocean Beaches, Lava Floods, Buried Forests, and Volcanic Ash Falls
Situated at the western fringe of the Cascade Foothills, Silver Falls State
Park offers a glimpse into the geology of this heavily vegetated area. Ex-
posed rock in waterfalls and creek banks reveal three major episodes of
Oregon’s geologic history:
Episode 1) Sandy Beaches, Episode 2) Massive volcanic lava floods,
and, Episode 3) Cascade-style volcanic ash deposits. Stream erosion
along Silver Creek has slowly chewed its way upstream, creating the
waterfalls and exposing rocks from these three different periods.
Episode 1: A Sandy Beach!
Below the rest of the rock in the park is sandstone
from when the Silver Falls area was part of
the Oregon coast. This rock only outcrops along
Silver Creek in the very northwest portion of the
park, an area that can not currently be accessed
by trails. This sandstone was, however, quarried
by the Civilian Conservation Corps to build the
Stone Shelter in the South Falls day use area.
Feel free to visit the Stone Shelter and look at all
of the seashells exposed in this rock!
Episode 2: Basalt Lava Floods!
The black rock forming the cliffs at the waterfalls includes the Wanapum and
Grand Ronde Basalts, part of a series of massive lava eruptions that are
collectively known as the Columbia River Flood Basalts. During the period
from 16.5 to 15.3 million years ago, the
ancestral Cascade mountains had very
few eruptions. Instead the volcanic activity
happened further east in Washington and
Oregon, resulting in extremely thick basalt
lava flows, the same kind of rock that forms
at Hawaiian volcanoes today! Some of these lava flows followed the ancestral
Columbia River Valley into the Silver Falls area. At least 8 separate flows are
exposed at Silver Falls.
Episode 3: Volcanic Ashfall
Capping the basalt are deposits of volcanic ash similar to what we see in
a Mt. St. Helens eruption today. The rock formed from this ash is called
Volcanic Tuff and tends to be easily eroded. The hills in the eastern
portion of the park are made of this tuff. It was deposited sometime between
15.3 and 7.5 million years ago.
A Buried Forest
The amphitheater walkway at North Falls
is in relatively soft sandstone and mud-
stone, sediments that were deposited by
a stream in between the basalt flows. A
forest was able to grow on this sediment
and was, in turn, buried by the next lava
flow. In the North Falls amphitheater
you are walking in the root zone of this
ancient forest. If you look up at the ceiling you may notice round holes with
blackened material inside them (a flashlight helps). That blackened material
is the charred remains of the trees buried by that lava flow. This charred
wood now forms a weak form of petrified wood that may rarely be found in
the park. You may look for this petrified wood, but remember the park has
a strict no collecting policy regarding all of our rocks. We want to make sure
our rocks are there to share with future visitors as well!
The Civilian Conservation Corps preferred to use local rock in the
construction of many of their buildings, including those at Silver Falls. The
sandstone from Episode 1 was quarried to build the Stone Shelter in the
South Falls day use area. Most of the rest of the structures in the park were
built using the black basalt from Episode 2. Its durability makes it good for
forming waterfall cliffs and for making buildings that can stand up to the
Beeson, Marvin H., 1985, Regional correlations within the Frenchman Springs Member
of the Columbia River Basalt Group: New insights into middle Miocene tectonics of
northwestern Oregon, Oregon Geology, Vol 47. No. 8. Oregon Department of Geology
and Mineral Industries, pp. 87-96.
Bishop, Ellen Morris, 2004, Hiking Oregon’s Geology, 2nd edition. The Mountaineers
Books, Seattle, WA.
Freed, Michael, 1979, Silver Falls State Park, Oregon Geology, Vol. 41. No. 1. Oregon
Department of Geology and Mineral Industries, pp. 3-10.
Norman, Elizabeth Storm, 1980, Geology of the Columbia River Basalt in Silver Falls
State Park, Oregon. Unpublished bachelor’s of science in earth science thesis ,
Portland State University. 43 p.
Information gathered by Grant D. Smith - 9/2007