The amphitheater walkway at North Falls is in

relatively soft sandstone and mudstone, sedi-

ments that were deposited by a stream in be-

tween 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 amphi-

theater 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 bur-

ied 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!

Geology of Silver Falls State

Park

A History of Ocean Beaches, Lava Floods, Buried Forests, and

Volcanic Ash Falls

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 Unpublished bachelor’s of science in earth science thesis , Portland State University. 43 p. Brochure made by Grant D. Smith - 9/2007

A Buried Forest!

A buried tree in the ceiling of the North Falls Amphitheater

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 struc-

tures 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.

North Falls

Twin Falls

Middle North Falls

Drake Falls

Lower North Falls

Crag Falls*Elbow Falls*

Diagram Not To Scale *Elbow & Crag Falls cannot be reached by trails

Fern Ridge TuffBetween 15.3 and 7.5 Million yrs old

Wanapum Basalts15.3 Million yrs old(at top)

Alluvial Sand & Mud(and buried trees)

Grande Ronde Basalts16.5 to 15.6 Million yrs old

Scotts Mills Sandstone26 Million yrs old

Situated at the western fringe of the Cascade Foothills, Silver Falls State Park offers a glimpse into the geol-ogy of this heavily vegetated area. Exposed rock in waterfalls and creek banks reveals 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.

A Brief Geological History

Below the rest of the rock in the park is sand-stone 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 1: A Sandy Beach!

The black rock forming the cliffs at the waterfalls includes the Wanapum and Grande 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 ances-tral Cascade moun-tains 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 Ha-waiian volcanoes today! Some of these lava flows fol-lowed the ancestral Columbia River Valley into the Silver Falls area. At least 8 separate flows are exposed at Silver Falls.

Episode 2: Basalt Lava Floods 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 Mil-lion years ago.

Episode 3: Volcanic Ashfall

A Buried Forest!Good Building StoneA Brief Geological HistoryEpisode 1: A Sandy Beach!Episode 2: Basalt Lava FloodsEpisode 3: Volcanic Ashfall

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