Geology
One hundred and fifty million years ago, Oregon did not exist. Not until plate tectonics separated North America from Europe and North Africa and pushed it westward did the continent acquire, bit by bit, what became the Pacific Northwest. Over many millions of years, the continent collided with and incorporated islands, reefs, and other exotic terranes. Part of the last major exotic terrane acquired by the North American Plate during the Eocene lies under the Tryon Creek watershed. The terrane consisted of a chain of seamounts that by 34 million years ago was being uplifted to become the Oregon Coast Range and the Tualatin Mountains (West Hills). The easternmost exposure of the basalts of this terrane is in Waverly Heights, near Milwaukie, across the Willamette River from Tryon Creek, and this formation underlies most of Tryon Creek State Park.
Between 15 and 16 million years ago, in the Middle Miocene, eruptions of Columbia River basalts from volcanic vents in eastern Oregon and Washington flowed across much of northern Oregon, sometimes reaching the Pacific Ocean. Although these basalts have been mapped in the West Hills under Marquam Hill, Hoyt Arboretum, and the steepest slopes of Forest Park, they flowed around but did not completely cover the Waverly Hills Formation in the Tryon Creek watershed.
Starting about 3 million years ago and continuing at least through the late Pleistocene, extensional faulting of the Earth's crust led to eruption of small volcanoes in the Boring volcanic field. This field extended roughly from Portland and Tualatin on the west to Battle Ground, Washington, on the north to Sandy and Boring on the east. Two of these volcanoes, Mount Sylvania and Cook's Butte, are in the Tryon Creek watershed. The Mount Sylvania eruptions included ash plumes and lava flows that covered some of the Waverly Heights Formation and Columbia River basalts.
About 15,000 years ago, cataclysmic ice age events known as the Missoula Floods or Bretz Floods originating in the Clark Fork region of northern Idaho inundated the Columbia River basin many times. These floods deposited huge amounts of debris and sediment and created new floodplains in the Willamette Valley. Over long stretches of time between the great floods, dry winds deposited silt. At elevations above 300 feet (90 m) in the Tryon Creek watershed, wind-blown silt covers the lava, while at lower elevations sand and gravel cover the bedrock.
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