The Geology of the West Hills

The Geology of the West Hills

Let’s consider for a moment what we’re walking over. What are the Tualatin Mountains made of and when were they formed?

Sticking to the soles of your boots is a yellow-brown clay commonly referred to as Portland Hills Silt that was deposited here during the last ice-age, about 14,000 years ago. But below that silt lays a powerful geologic history that defies our current preconceptions of Portland and the Tualatin Hills

Imagine a time before the Cascade Mountains arose and basalt lava welling up like molten wax could flow unimpeded from Eastern Oregon all the way to the Pacific. When these “flood basalts” flowed into the Portland area more than 16 million years ago they even temporarily blocked the Columbia River, diverting it via Salem and forcing it into the Pacific near Lincoln City. Repeated floods of molten basalt rock continued to inundate the region, with each layer being eroded by time and reduced to clay. The red crust frequently encountered on exposed basalt in the Tualatin Mountains dates back to a time when our area enjoyed a tropical climate. Then about eight million years ago the tectonic forces that pushed up the North Cascades also folded these basaltic flows into the Tualatin Mountains, the Coastal Range and the present coast line.

During this period and extending as recently as two million years ago successive waves of waterborne sediment known as the Troutdale Formation washed up against the base of the Tualatin Hills depositing quartzite, schists and granites scoured from the bed of the Columbia River, as well as sandstone eroded from the Cascades.

Towards the end of the Troutdale depositions until only a few hundred thousand years ago, volcanoes began to erupt across the lower Willamette Valley. Most of the taller isolated hills in Portland derive from this period. Referred to as the Boring Volcanoes, they produced basaltic lava that can easily be distinguished from Columbia River Basalt by its gray color. Look for this it along the ridge tops and west slopes of the Tualatin Mountains.

The 32 Boring volcanoes in the Portland area had barely cooled when our ancestors arrived on the scene – just in time to witness the horrific Missoula Floods that repeatedly burst through an ice dam in Western Montana sending 500 cubic miles of water in a gargantuan torrent across eastern Oregon and Washington. As this towering mass of water piled through the Columbia River Gorge at nearly 65 miles an hour it rose to a height of 500 feet and five miles across. The ground would have literally shaken as this cataclysmic eruption stripped away top soil, cut huge canyons into the mountain sides and literally tore away at the bedrock of the Columbia River

Lest we think that the great cataclysmic tumult that forged this savage place has relented in the face of human endeavor, the 1980 eruption of Mount St. Helens gave ample warning that the primeval forces rule this country still. Look upon these lands with awe and wonder, for they are nothing less than awesome and wonderful.

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