Geology of Silver Falls State Park
A History of Ocean Beaches, Lava Floods, Buried Forests, and Volcanic Ash Falls

    A Brief Geological History
    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!








    Good Building Stone
    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
    Oregon rains.

    References
    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
The Stone Shelter is currently
undergoing restoration as a skilled
stone mason, restoration carpenters,
and team of park employees use the
same techniques that the C.C.C. did
when building it 70+ years ago.