Marys Peak isn’t just the highest point in the landscape in the region. It’s a living monument to the geological forces that shaped — and are still shaping — the entire Pacific Northwest.
“There’s not a lot of places on earth where you can see the whole landscape, where you can see all the elements of a subduction zone, from one spot,” said Bob Lillie, a retired Oregon State University geology professor and the author of a new book about the mountain titled “Oregon’s Island in the Sky: Geology Road Guide to Marys Peak.”
Located in the Siuslaw National Forest, Marys Peak rises to an elevation of 4,097 feet above sea level, making it the highest point in the Oregon Coast Range. On a clear day, the summit offers views of the Pacific Ocean to the west and the Willamette Valley and Cascade Mountains to the east — all key pieces in the region’s geological jigsaw puzzle.
As Lillie explained on a recent visit to the mountain, Marys Peak stands on the edge of a subduction zone, where two vast segments of the earth’s crust collide. The Juan de Fuca Plate, which underlies part of the Pacific between Northern California and Vancouver Island, is bumping into the North American Plate. The denser, heavier oceanic crust is plunging underneath the more buoyant continental crust in the process geologists call subduction.
As the oceanic plate dives deeper underground, increasing heat and pressure force out hot water, which melts the rock above it to create magma. Some of that molten rock eventually erupts at the surface, gradually building up the High Cascades volcanoes such as Mount Hood, Mount St. Helens and Mount Rainier.
Marys Peak, Lillie said, is composed of rocks that formed far out in the Pacific in the distant past, beginning with a layer of fine-grained lava called basalt that erupted from sea-floor vents about 55 million years ago. Over the next 20 million years, sand and mud were deposited on top of the basalt and compressed into layers of sandstone and shale.
Roughly 35 million years ago, this section of ocean floor ran head-on into the North American Plate. At about the same time, magma was rising up through the earlier sea-floor deposits to form a hard layer of gabbro, similar to basalt but harder and more coarse-grained.
As the two plates ground together, a portion of this geological layer cake was scraped off the top of the oceanic crust and piled up to form the Oregon Coast Range and the Olympic Mountains in Washington. In this area, however, a chunk of that crust ran into the Corvallis Fault, which further lifted it and thrust it eastward over the Willamette Valley. This is the part we know today as Marys Peak, McCulloch Peak and the hills of the McDonald-Dunn Research Forest.
That uplift helps explain why Marys Peak today is the highest point in the Coast Range, but only in part. Like other parts of the range, much of Marys Peak has eroded away over the millennia — including about 10,000 feet of relatively soft sandstone and shale. The removal of those strata, however, has left the mountain capped with a 1,000-foot-thick layer of gabbro, which is much more erosion-resistant.
Today, a drive from Philomath out Highway 34 and up Marys Peak Road takes curious visitors past visible examples of all of these rock types, exposed in road cuts, quarries and outcrops at various points along the way. In his new book, Lillie lays out a six-stop itinerary that illustrates the geological processes involved in building up the mountain and tearing it down again.
“We call it the dynamic duo: uplift and erosion,” he said.
During a stop at Parker Creek Falls, 6.8 miles up Marys Peak Road from the Highway 34 turnoff, Lillie pulls some rock samples out of his car to make a point.
“These rocks,” he said, holding up chunks of basalt, sandstone and shale collected on the mountain, “were probably deposited two to three miles below sea level.”
Then he turns and points to the waterfall, cascading down a steep slab of dark-colored gabbro on the side of the road — part of the hard-rock layer on the mountain’s upper slopes.
“You went from two to three miles below sea level to two-thirds of a mile above sea level (today). And you also lost about two miles due to erosion, so altogether five miles’ worth of rock. Without erosion, you’d have Mount Everest.”
Other stops on Lillie’s driving tour highlight tilted layers of shale and sandstone, showing how the strata were folded after being uplifted and then thrust eastward along the Corvallis Fault; spherical basalt “pillows” formed during ancient seafloor eruptions; undisturbed sedimentary layers that remain horizontal after being lifted up from the bottom of the ocean; and a roadcut that exposes a gabbro dike cutting vertically through shale and sandstone.
The high point of the trip, literally and figuratively, comes after a half-mile walk from the upper parking lot to the summit of Marys Peak. The hard gabbro cap supports a thin layer of soil, creating the spectacular meadows that erupt in colorful wildflowers in late spring as the snow disappears from the upper slopes. Fringing the meadows are extensive groves of noble fir, a cold-loving species that once grew extensively throughout the region but has slowly retreated to higher ground as the climate gradually warmed since the end of the last ice age.
If the weather is good, you can look to the west, gazing out over ridges receding in the distance to the blue Pacific, where the mountain had its birth, then turn your eyes to the east, across the fertile and populous Willamette Valley to the High Cascade summits — all visible reminders that the Pacific Northwest is part of an active subduction zone.
Lillie, who has made this pilgrimage hundreds of times, likes to ponder some of these things while he’s taking in the view from the top. The same geological processes that created this dramatic landscape, he notes, can also wreak great destruction, as in the 1980 eruption of Mount St. Helens or the potentially devastating Cascadia subduction zone earthquake geologists believe could strike the region in the next several decades.
“You learn to take the beauty with the beast, the good with the bad,” Lillie said.
The dense forests, open meadows and sweeping vistas of Marys Peak make the mountain a popular destination with all sorts of recreational users, from picnickers and hikers in the summer months to snowshoers and cross-country skiers in the wintertime. From the mid-1940s to the mid-1980s, thousands of area residents flocked to the mountain each summer for an annual barbecue known as the Marys Peak Trek. And long before that, the peak served the native Kalapuya people as a spiritual refuge, a place for vision quests.
“Being a high point, it’s a special place,” Lillie said. “It’s an important part of our history and our culture.”
But if it’s to remain an important part of local culture, Lillie said, Marys Peak will need our help.
Today, radio towers serving numerous communications networks compete for space on the summit. Growing numbers of visitors threaten to damage the sensitive meadow flowers. And rapidly accelerating global warming, fueled by the accumulation of greenhouse gases in the atmosphere from the burning of fossil fuels, could someday kill off the last remaining noble firs.
That’s one of the reasons Lillie wrote “Island in the Sky.” By helping other people understand the dynamic processes that created Marys Peak, he hopes, they’ll be inspired to take action to protect the mountain he loves.
“You can think about these things on the top of Marys Peak,” Lillie said. “A place like Marys Peak can make you appreciate there are things worth fighting for.”
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