Table of Contents


General Introduction


Impressions of the North Cascades
Essays about a Northwest Landscape

Part I: Landscapes of Memory

Ancient Aires and Rock Romancing

My first exposure to the geology of the North Cascades was on a field trip with Peter Misch, the legendary University of Washington professor who, in his sixties, was still outlasting his graduate students on the ski slopes. Driving up the Skagit River Valley in his battered blue van, Peter provided a running commentary which gave familiar landmarks a whole new meaning in a geologic context. The towering cliffs of Sauk Mountain became an ancient sea floor thrust onto the continent by tectonic forces of unimaginable power. Diobsud Creek was no longer mundane, but rather a marker of the Straight Creek fault—a San Andreas-like structure that changed the nationality of rocks near Stevens Pass by moving them across the Canadian border to the vicinity of Harrison Lake. Fortunately the last movement on this fault was about forty million years ago, so the International Boundary Commission will not have to adjudicate.

By the time the trip was over, I had filled a notebook with revelations confirming my decision to abandon a career in art and architecture and start a romance with rocks. What I learned after years of study in the Skagit region was just how complicated a relationship with the rocks of the North Cascades can be.

Nowhere is this better illustrated than in the sorry saga of the Skagit Nuclear Power project. In the late 1970s Puget Sound Power and Light announced plans to locate a nuclear plant about 8 miles east of Sedro Woolley at a site on Bacus Hill declared to be the safest and best of hundreds of localities evaluated. The Bechtel Corporation, one of the most experienced engineering firms in the world, was hired to demonstrate the geologic feasibility required for site licensing by the Nuclear Regulatory Commission. The company spent millions on geologic research and hired several famous geologists as consultants. But in the end all this money and expertise did not come close to providing an adequate understanding of geologic features in and around the Skagit Valley. Because seismic safety could not be assured, the site was abandoned at a cost that customers of the utility continue to pay.

Geology also played a role in Seattle City Light's decision to abandon the Copper Creek Dam site and to curtail a plan for raising the height of Ross Dam on the Skagit River. Much research has been done by a long list of geologists. In addition to Peter Misch and his pioneering studies, others playing a major role in reading the rocks of the North Cascades include Ned Brown at Western Washington University, Joe Vance and Bernard Evans at the University of Washington, Bob Miller at San Jose State University, and Roland Tabor, Ralph Haugerud and Wes Hildreth of the United States Geological Survey. And then there are the legions of graduate students and field assistants who have put their hearts and minds on the line to thrash up a creek bed full of devil's club or lead a 5.10 route up an icy rock wall to get a close-up look at a critical outcrop.

So after years of additional research, including dozens of graduate theses and hundreds of technical publications, do we know enough about the North Cascades for a concise description of the materials and processes involved? The answer is no. But what I can offer in this brief essay is a progress report given in the form of a narrative field trip to three spectacular locales, each of which illustrates an important aspect of the geologic evolution of the North Cascades. The first will be a hiking traverse from Austin Pass to Lake Ann, revealing some "explosive" new data on the volcanic history of Mount Baker. The second will be a hike from Schreibers Meadow to Park Butte where we will examine some of the most ancient and some more recently formed rocks in the North Cascades. And, lastly, we will venture into the depths of the Skagit River Gorge where we will discover evidence of the cataclysmic events which formed the deepest, darkest roots of the mountain range.

Austin Pass to Lake Ann

Our first excursion begins at the end of State Route 542, the Mount Baker Highway, runs 55 miles east of Bellingham into the heart of the North Cascades. From here a summer-only road runs through the Mount Baker Ski Area to Austin Pass, where we park and put on our day packs for an 8-mile round trip to Lake Ann. This is an unusual Cascades hike because it begins with a steep descent of the headwall of a glacial cirque. From the trail we can see that the cirque lies between two steep ridges. To the north is Shuksan Arm. On the south is Kulshan Ridge, which is a finger of lava pointing toward the summit of Mount Shuksan, looming bright and white on the eastern horizon. The outcrops at Austin Pass and the surrounding promontories of Table Mountain and Kulshan Ridge are composed of andesite, named for the Andes Mountains in South America. This particular andesite has a polka-dot pattern of white feldspar crystals set in a gray background. The gray is formed by a mass of microscopic crystals and glass. As the proportion of glass increases, the background gets darker, eventually becoming jet black. Another variety of andesite has a red background produced by the oxidation of iron; this rock has literally rusted.

When the lavas around Austin Pass erupted, about 300,000 years ago, they flowed down valleys which have long since been removed by erosion. In fact, the flat top of Table Mountain at 5,742 feet was once in the bottom of a lava-filled valley. It now stands as a high plateau because the lava was much more resistant to erosion than the surrounding valley walls. Geologists call this process topographic inversion, and Kulshan Ridge is another example. It seems astonishing that the landscape could change to such an extent, but where geologic time is involved there is no such thing as the Rock of Ages.

The history of the Earth, according to some, can be read from the record of the rocks like the pages in a book. This may be partially true of places like the Grand Canyon, but about a quarter mile down the Lake Ann Trail is an outcrop demonstrating why the reading is so difficult in the Cascades. In a small streambed that crosses the trail, two distinctly different rock types occur. Upstream on the left is more of the andesite we saw above. On the right is a mottled green rock that belongs to the Chilliwack Group. This was originally part of a basaltic sea floor that existed 250-300 million years ago, possibly somewhere in the southeast Pacific. By the time the andesite erupted 300,000 years ago, the basalt had been cooked into a metamorphic greenstone, mashed against the North American continent, uplifted, and eroded to form a valley through which the lava flowed. Between the two rock types is an unconformity, a gap in time, tantamount to ripping most of the pages out of the rock record, leaving only a chapter near the beginning and another near the end of the story. What happened in between? We will have to go elsewhere to find out.

The switchbacks continue down the cirque headwall toward the headwaters of Swift Creek. Emerging from the forest onto the meadows of the valley floor, we see a jumble of rocks that rises at about a forty-five-degree angle to merge with the cliffs of Kulshan Ridge to the south. This is talus, blocks of rocks that have been pried off the ridge by gravity and freeze-thaw action. This freeze-thaw process involves in numerable cycles of water freezing to ice in fractures. Like pipes in a cold snap, the rock cracks and is slowly but surely pushed over the edge to tumble onto the slopes below.

As the trail wends its way through the talus, we discover that some of the blocks are greenstone rather than the andesite above. Careful scrutiny reveals there is a brigade of greenstone blocks that extends all the way across the valley. This is not talus, but rather a rockfall and avalanche that originated to the north, high on Shuksan Arm. The hurtling mass filled the valley floor and ran up the opposite side, creating a dam and a temporary lake which filled with sediments. The dam has now been breached, leaving only a few blocks on top and a flat valley floor as evidence of the catastrophe. Campers in the valley should cast a wary eye on the fractured rocks above and pray for seismic stability.

Continuing down the trail we reach a bridge over Swift Creek and discover that the rock here is neither greenstone nor andesite. Close examination reveals a mosaic pattern in tones of pink, white, gray, and black. The U.S. Geological Survey has informally named this rock the granodiorite of Lake Ann Stock. Granodiorite is composed of four minerals: the pink is potassium-feldspar; the white, sodium feldspar; the gray, quartz; and the black, either biotite or hornblende. This is a stock because the granodiorite extends only about 2 miles across the landscape—from here to Lake Ann (geologists name rocks for places where they are well exposed). The total area of exposure is only about 10 square miles—much less than the 40 square miles required to be designated a batholith like the Chilliwack Composite Batholith, which crops out from the Skagit River to the Canadian border. If stock and batholith are terms that do not fit into your memory bank, you can call this rock either a pluton or plutonic rock, after Pluto, the Greek god of the underworld—where these kinds of rocks crystallize from a molten state.

Plutonic rocks are common in the Cascades for the same reason that there has been lots of volcanic activity and earthquakes in the Pacific Northwest—the Cascadia Subduction Zone. We live on an active margin, where the Juan de Fuca Plate (a geological name for the sea floor) is moving towards a head-on collision with North America. But approximately 200 kilometers offshore, the sea floor takes a dive and moves under the edge of the continent. As it descends, it is transformed by metamorphism and, at a depth of about 40 miles, begins to suffer serious dehydration. The fluids expelled trigger massive melting in the sea floor slab and in overlying mantle rocks. Some of this molten rock (magma) makes it to the surface as volcanic activity. The rest is stuck in the crust to become the plutonic part of the story, which has been playing almost continuously in the Cascades theater for about 50 million years now.

What is most remarkable about the Lake Ann Stock is that it is only about 2.5 million years old. While inconceivably ancient on a human time scale, to a geologist this age is virtually instantaneous, and makes the Lake Ann Stock one of the youngest exposed plutons in the world. When the granodiorite crystallized, it was probably located at least 2 miles below the surface. So as we wade in the creek and cool our feet, we are walking on rock that required more than 10,000 feet of erosion to be exposed. This equates to an average uplift rate of about a half inch per year. This rate may not sound like much, but it is close to a world-record pace in mountain building.

Beyond the bridge, the trail continues through the woods until we reach the junction of the Baker Lake Trail. From here it is all uphill to Lake Ann. The first few switchbacks are still in the Lake Ann Stock, but as we traverse into another cirque below Lake Ann, yet another rock type appears. This stuff is bizarre. It looks as if somebody painted black and white stripes on a piece of cardboard and then, in a fit of rage, twisted it into a crumpled mess. The truth is, the stripes are layers of graphite- and quartz-rich sediment deposited on a sea floor approximately 150-160 million years ago. These layers sat peacefully for millions of years until they reached a plate margin (not necessarily North America) about 120-130 million years ago. Here the sediments were jammed down a subduction zone and squeezed into a metamorphic rock type called phyllite. At the same time the sea floor was converted to metabasalt. Then about 90 million years ago the plate smashed into the margin of North America, most likely at the latitude of Baja California. The phyllites and metabasalts were regurgitated by the Shuksan Thrust Fault, which involved even more twisting and recrystallization. Ultimately these rocks hitched a ride on a northward-moving oceanic plate and traveled as much as 2,500 kilometers to their present location, where the phyllites crop out along Shuksan Arm and the metabasalts comprise the upper massif of Mount Shuksan, which rises above us in all of its calendar-image glory.

So now it is time to reverse course and head back toward the trailhead where evidence of one of the most calamitous events in geologic history can be found. Between the bridge and talus slopes below Kulshan Ridge, we leave the trail and head about a quarter mile cross-country to the head of a rapidly deepening canyon. As we pick our way down the drainage we see that the walls of the canyon are composed of light gray to buff-colored material that looks like ash. It is ash, and in places it is more than 3,000 feet thick. The age of the ash has been measured at 1.15 million years (give or take 10,000). It apparently formed in a single explosion of incredible proportions. This focal point of the eruption was the Kulshan Caldera, where a section of earth about 4 miles long and 5 miles wide suddenly collapsed into an underlying mush of molten rock. This same process produced Crater Lake from Mount Mazama and has been responsible for the greatest volcanic eruptions ever observed on Earth. A comparable event at Krakatau, Java in 1883 cooled the entire planet to such an extent that all-time record-low temperatures were recorded for three subsequent winters in Europe and North America. A remarkable aspect of this caldera collapse was that it apparently happened beneath a sheet of glacial ice, so abundant steam was mixed with the ash, making the eruption even more explosive. Because of glacial erosion, we see few of the deposits from this eruption outside the source area, although a one-foot-thick deposit called the Lake Tapps Tephra can be found near Tacoma. It is mind boggling to consider the effects on the Puget Sound region if a similar eruption were to occur sometime in the future. I would not want to be living within 100 miles of the caldera, but there is always the hope that a strong wind would blow much of the ash toward uninhabited areas to the northeast.

Returning to the trailhead, we retrace our steps through time from the rockfall, which occurred less than a few thousand years ago, to the greenstone which flowed as lava hundreds of millions of years ago. As humans we have no basis for comprehending the pace of geologic time. In his book, The Dragons of Eden, Carl Sagan compared the age of the Earth to one calendar year and made the observation that on this geologic clock the time span since the birth of Jesus would be less than the last second before New Year's Day. Another way of putting it is that if you live to be 100 years old, your lifetime measured against the age of the Earth would be less than one snowflake on the 10,778-foot summit of Mount Baker—certainly a perspective to ponder.

Schreibers Meadow to Park Butte

Our next excursion is about 120 miles by road, back to Bellingham, down Interstate 5, up Highway 20 (the North Cascades Highway), and along the Baker Lake Road to U.S. Forest Service Road 11 up the southwest flank of Mount Baker. The parking lot at the end of this 8-mile, dust-bowl route is one of the most popular places to be in the North Cascades and parking is limited, so come early during the summer or face a lengthy jaunt just to get to the trailhead. Crossing the bridge over Sulphur Creek and traversing Schreibers Meadow, we are surrounded by subtle evidence of two very recent catastrophic events related to Mount Baker. The first is a tree-covered hill to the south that rises a couple of hundred feet above the meadow. We depart the trail and head for the hill, grazing on seasonal berries and slapping at seasonal insects as we go. Being good campers we have brought along a small shovel which we use to dig an inconspicuous hole in the hillside. Beneath the thin soil a surprise awaits. This hill is composed of cinders and is the source of the ash layers we observed on the road coming in. The Schreibers Meadow cone formed during the most recent flank eruption on Mount Baker sometime between 7,600 and 12,000 years ago. We won't be able to get a precise age on this eruption until some one finds a well-preserved piece of charcoal among the volcanics. This eruptive event also featured a lava flow which moved down the Sulphur Creek Valley and probably reached the Baker River. Another young lava flow with an age of about 11,000 years can be seen at Crag View, just northeast of Schreibers Meadow.

Turning to the trail, we are now walking on evidence of the second catastrophe. The topography with lots of little lakes and hummocks is also a clue. Checking one of the drainage ditches along the trail, we see none of the ash or cinders that make up the cone. Instead, the surface is littered with blocks of andesite embedded in a mangled matrix of volcanic debris. This material originated high on the slopes of Mount Baker, probably near the rim of Sherman Crater. Here a large sector of the mountain collapsed and crashed downslope, first as a rock avalanche and later as a debris flow when the rock disintegrated and mixed with snow and ice. Another portion of this debris flow, which occurred approximately 6,800 years ago, roared down the Middle Fork of the Nooksack River and ran all the way to Bellingham Bay.

It takes little imagination to picture the devastation such an event would cause, and looking upward toward the east side of the crater, we can see a large mass of crumbly rock poised on the rim above Baker Lake. Geologists have named this outcrop Lahar Lookout, after the Indonesian name for a volcanic debris flow. All it would take to bring this down would be a steam blast or an earthquake. With this thought in mind we hurry across the meadow and into the woods on our way toward the aptly named Rocky Creek. Here again boulders clutter the landscape. However, snags of dead trees reaching skyward through the debris tell us that whatever happened here was not very long ago. A good guess is that it was a jokulhlaups. This is an Icelandic word for "ice burst flood." These happen when water accumulates under ice, like the Easton Glacier, which presently resides about a mile up slope. Considering the nature of Mount Baker, the water was probably produced by a steam vent under the glacier. When enough fluid had collected to float the terminus of the glacier upward, the torrent rushed downslope, destroying everything in its path. We pick up the pace a bit—this hike is turning into the Valley of the Shadow of Death!

A swaying suspension bridge presents a more immediate hazard—the embarrassing possibility of a clumsy fall into Rocky Creek. Then the ascent begins. Gently at first, we surmount morainal ridges formed less than 100 years ago when the Easton Glacier extended to at least this point. One piece of evidence for global warming is the rapid retreat of the Easton Glacier—more than a mile per century—which is matched or exceeded by many other ice sheets in the Cascades. Past the moraines the switchbacks get serious as we climb nearly 1,200 feet in little over a mile to Morovitz Meadows. The upper part of the ascent is one of the world's most dramatic transitions in landscape. The trail flattens out and we emerge from an Alaska yellow cedar and silver fir forest to the spectacle of an emerald-green meadow overarched by the brilliant white prominence of Mount Baker filling the skyline. Somebody starts singing the title song of The Sound of Music and it seems appropriate. For those who can tear their eyes off the skyline, there is a good geologic story in the ditches again. Interspersed with the mineral soil and peat that has collected over the centuries, there are black and cream-colored layers giving testimony to the recent volcanic history of the Pacific Northwest. The relatively thick black ash just below the surface is the product of the most recent eruption of Mount Baker which probably occurred in 1843. Other layers are from eruptions of Glacier Peak, Mount St. Helens and even Mount Mazama (now Crater Lake).

From here the trail diverges—the right branch leads up the climbing route to Railroad Grade and the left goes on to Park Butte. We choose the climbers' route because designated campsites, with a glorious view of Mount Baker and the meadows, are along the ridge just ahead. After setting up camp, the obvious afternoon excursion is a trip up to climbers' camp and beyond to the Deming Glacier overlook. A short hike brings us to the edge of the Railroad Grade. There are conflicting stories about the origin of this name, but the most plausible is that from a distance the lateral moraines formed during the rapid retreat of the Easton Glacier look like a pair of rails running up the mountainside. However, it is no gentle gradient up close on the edge where we now stand. The slope drops almost vertically to the floor of the great cavity drilled out of the mountain by the glacier when it was at its maximum stand in the late 1800s. We pick our way carefully upward along the knife edge of the moraine until we pass the present terminus and come alongside an expanse of blue-gray ice. Occasional cracks and groans are heard as the ice grinds downslope at the ponderous rate of a few inches per day. Those who carefully venture onto the ice can peer down bluish depths of crevasses that form where the glacier flows over ridges in the bedrock. Some of them seem bottomless, but the maximum depth of fractures in temperate glaciers is only about 100 feet. Below this the ice flows like silly putty to quickly heal any crack that opens.

From here we head cross-country through frost-shattered blocks of lava toward the Deming Glacier overlook, yet another extraordinary viewpoint. After about a mile of stumbling through scree and edging our way along steep, rocky inclines, we reach the edge of the abyss. This glacial trough makes Railroad Grade look like a drainage ditch. The terrain is so steep that the contours on a topographic map form a dark smear. Okay, the distance is not quite as far to fall as at Glacier Point in Yosemite, but it is definitely more spectacular because the neck-twisting views are not only down but up and all around. Even better, this is glacial geomorphology in action. You can almost feel the power surging in the immense mass of blue-white ice that looms directly above and plummets to the depths below. We wait, watch, and listen until the setting sun paints a rosy alpenglow, and then scurry back to our campsite in the meadows below.

We rise with the sun. Our agenda today is to follow a footpath familiar to thousands of Cascades hikers—the Park Butte Trail. Our purpose, however, is not the ascent, but reading the rocks and landforms along the way. The first geologic theme of note is a rounded ridge that runs across the lower end of Upper Morovitz Meadow. This is yet another moraine, one which formed at the terminus of a small cirque glacier and once completely enclosed the open end of the meadows. It has now been breached by stream erosion, but on the lower slope of Castle Crags we find laminated clay deposits telling us that if we had been here a few centuries ago we would have been swimming in a lake at least 10 to 20 feet deep.

Mount Baker from Sauk Mountain

As the trail rises from the meadow, we encounter outcrops that look very different from the lava flows we have seen elsewhere. These rocks are twisted and gnarled, with a grain almost like that of wood. The rocks look ancient, and they are—they belong to the Yellow Aster Complex, the oldest unit in the Cascades. Dating is scattered and imprecise, but some parts of the Yellow Aster may be more than a billion years old. Their origin is problematic. We know these rocks are found only in the vicinity of the Shuksan Thrust Fault runs from here to Lake Ann and beyond. The current theory is that the Yellow Aster Complex represents the sliced and diced remnants of an ancient continent shuffled into the deck of the Cascades during some episode of mountain building lost in the mists of time and complex geology.

Onward and upward the trail winds through large talus blocks that have fallen off the cliffs rising above. At first glance this rock looks just like the andesite we have seen elsewhere, but radiometric dating indicates it is more than 700,000 years old and thus part of the early history of Mount Baker volcanism. The only visible clues that these flows are older are the presence of a few tiny green crystals of olivine and the topographic inversion that occurs here and at Cathedral Crags to the north.

Crossing a plateau at the top of the inverted lava ridge, we reach Pocket Lake—a textbook example of a cirque and tarn. There have been no detailed studies of the glacial history here, but it is a reasonable guess that glacial ice covered most of the landscape until the end of the Little Ice Age in the late 1800s.

Finally we reach the summit of Park Butte, occupied by one of the last, once numerous, Forest Service lookouts (this one is maintained in excellent condition by the Skagit Alpine Club). Toward the east is Mount Baker and Railroad Grade. To the west we see the U-shaped glacial valley of the Nooksack Middle Fork and a jagged mountain range that has an unusual orange color. This is the Twin Sisters Range which consists mostly of serpentine and dunite, rock types rarely found on the surface of the Earth. Indeed the Twin Sisters is claimed to be the largest single mass of this kind of rock in the world. The current geologic interpretation is that this rock was originally located 40 or more miles below the surface in the Earth's mantle. Like the Yellow Aster Complex it was thrust up to its present position by movement on the Shuksan Thrust Fault.

From Park Butte we also get a good look at the two volcanoes which developed to the southwest of the Kulshan Caldera. Looking directly north, the peaks of Lincoln and Colfax carve a dark, jagged edge on the skyline. These are part of the Black Buttes, a volcano active between about 500,000 and 300,000 years ago. The Black Buttes is composed predominantly of pyroclastic andesite, which indicates that most of the eruptions were explosive. However, there are at least a few extensive lava flows of the same vintage (for example, Table Mountain). What we see now at Black Buttes is only a portion of the original cone. Glacial erosion has removed the top and ripped into the sides, so at the head of the Deming Glacier we can see the vent structure in the core of the volcano.

Mount Baker is much younger, probably less than 50,000 years old, with the youngest flow less than 1,000 years ago. It also represents a change in volcanic venue because it is centered about 2 miles northeast of Black Buttes. However, because it is about twice the size of the Black Buttes, the two cones overlap, forming a saddle between the peaks. Between the summit (Grant Peak at 10,778 feet) and Sherman Peak to the south is Sherman Crater, where a sudden increase of steam vent activity occurred in 1975. This led to intensive monitoring of the mountain and the closure of Baker Lake, a popular recreation area. We now think the steam is probably due to a change in the plumbing system of the vent which allows meltwater to trickle to a greater depth where rocks hot enough to produce steam reside. There is no evidence that magma is moving upward to cause an eruption. It is unfortunate that the "cry wolf" aspect of the Mount Baker closures may have been responsible for the blasé attitude of the public and some government agencies that resulted in considerable loss of life during the 1980 eruption of Mount St. Helens. We now know enough about how Cascades volcanoes work to identify the steam activity at Mount Baker as non-threatening. But there can be no doubt that Mount Baker will erupt again. When it does, the Park Butte lookout could be an awesome and relatively safe place from which to watch it happen—unless the wind is blowing in a southerly direction or there is a south-directed lateral blast similar to Mount St. Helens.

One purpose of this excursion has been to justify the whimsical "Ancient Aires" part of the title. Despite considerable research effort, the Yellow Aster Complex remains a mystery. We do not know where it came from or where it has been on its torturous travel to end up as isolated fragments along great faults in the North Cascades. In fact we will probably never know the whole story of these rocks. That is the joy and frustration of geology. The joy is the opportunity for creative interpretation. Within the road rules of geology, I can say that the Yellow Aster came from the ancient supercontinent of Pangea and once existed at the geographic position of Australia. As Pangea broke up, some fragments became attached to the sea floor of the Proto-Pacific ocean and were transported like blocks on a conveyor belt across the sea to become North American immigrants. The frustration is that I cannot prove this is true. Rock types found in the Yellow Aster are not that uncommon and could match many other locales of similar age. Geologists have only a few small remnants of these ancient rocks, so reconstruction of the Earth's early history is as much an art as a science.

The Skagit River Gorge

Our last excursion begins in Newhalem, a village occupied mainly by people who run the Skagit River Hydroelectric Project for Seattle City Light. When I was doing my Ph.D. research on the Skagit Gneiss, this was my home for several summers. I ate (overate) at the Gorge Inn, swam in the old quarry down the road, climbed the rock walls north of town, and generally developed a special feeling for a dwelling place surrounded by mile-high mountains.

Driving east from Newhalem on the North Cascades Highway, we abruptly enter the Skagit River Gorge, one of the deepest canyons in North America. An ideal place to observe Gorge geomorphology is from the pullouts above Gorge Lake. Here we see that the bedrock walls descend from the adjacent peaks in a broad U-shape, except for the very bottom where a V-shaped notch has been cut. The notch in the bottom is the result of recent downcutting by the river. But the bulk of the canyon must have been cut by a glacier. One possibility is that ice pushed through here about 22,000 years ago during a period of extensive alpine glaciation called the Evan Creek Stade. Elsewhere this event was responsible for cutting the valleys now filled by Lakes Chelan, Keechelus, and Kachess. Another possibility is the continental ice sheet that crossed the Canadian border heading south about 20,000 years ago. During the maximum extent of this ice sheet about 15,000 years ago, the Puget Sound was filled with ice as far south as Olympia, and there is good evidence that a lobe of ice pushed up(!) the Skagit Valley at least as far as Newhalem. Landforms also indicate that the continental ice sheet pushed down the Skagit at least as far as Ross Dam. Whether or not these two segments linked up through the Gorge section is still a matter of speculation.

Another subtle, but significant, geologic feature to be seen from this turnout is the preponderance of boulders. They choke the tributaries and fill the bottom of the valley below Gorge Dam. (Incidentally, the reason there is almost never any water flowing here is that City Light converts the entire flow of the Skagit River to dollars by running it through a tunnel to the electrical generators in a powerhouse at Newhalem.) But back to the boulders, which are the verification that Newton's Laws apply here. The sheer walls and canyons are paths of potential energy that lead naturally from precipice to abyss. Potential becomes kinetic when rocks free-fall as blocks, shattering into progressively smaller splinters as they go. The splinters accumulated on the slope are merely resting a geologic moment, waiting for a downpour of rain or a slide of snow to trigger a massive mobilization. Rocks, soil, plants, animals, anything in the potential path turns into a slurry that hurtles downslope as one of the most destructive forces in nature. This is a debris torrent and anything in its way is flat out of luck. There have been fatalities in the Cascade River Valley to the south and several have rumbled into Diablo Lake near the North Cascades Environmental Learning Center.

Enough of doom and disaster and on to more metamorphism. Driving up the valley and across Thunder Arm, we reach John Pierce Falls. Just beyond the north end of the bridge is one of the better rock outcrops to be found anywhere. No gold, silver or diamonds, but another intriguing story of the North Cascades is readable here. Stand on the far side of the road and look hack at the rocks in the outcrop. You will see black and white and gray stripes, but on a much larger scale than those at Lake Ann. The black parts are generally twisted and consist of tiny flakes or chunks all stretched out in the same direction. The flakes are biotite and form a rock type called schist. The chunks are hornblende and form amphibolite. The schist and amphibolite are interlayered with light layers of quartz and feldspar crystals with a few flakes of biotite that have also been stretched into a rock called pegmatitic gneiss. The schists were originally sand and clay and the amphibolites used to be lava flows. By studying the composition of garnets and other rare crystals in nearby outcrops, we have learned that these rocks were once buried as deep as 80,000 feet within the earth! But there is no good evidence that this burial was related to subduction. So we have a major enigma here, and with every good geologic enigma comes the controversy of competing hypotheses.

One hypothesis is that about 90 million years ago this part of North America was the site of a collision with a microplate named Wrangellia (for the Wrangell Mountains in Alaska). In this collision, North America took the dive so that the Skagit rocks along the margin were buried to great depths where metamorphism occurred. An alternative is that the collision of Wrangellia was a sideswipe rather than a head-on. In this scenario, the burial of the Skagit was due to the loading of enormous amounts of granitic magma in the upper levels of the crust. The evidence available does not make a compelling case for either story, and it is possible that both are partially correct. We need a few more hardy graduate students who are willing to thrash through the devil's club and risk their lives on vertical walls to someday gather the definitive pieces of evidence.

So much for vignettes of the history of the North Cascades—what about the future? We have evidence that these mountains are still rising, perhaps as much as 1 millimeter per year. Multiply this by the geologically brief time interval of a million years and we have the potential for several thousand feet of uplift (minus erosion). We also know from satellite geodesy that convergence of the Juan de Fuca Plate with North America continues at an average rate of about 40 millimeters per year, so we can expect volcanism to continue, probably with greater frequency than during the last century. We can also anticipate great earthquakes because neither subduction nor uplift is a smooth process. The last magnitude 8 (or possibly 9!) earthquake in the Pacific Northwest was in 1700 A.D. Since then the subduction zone has locked in place, building up strain energy. Sometime in the next few hundred years—maybe much sooner—the oceanic plate will suddenly plunge 30 feet under the continent all at once and anyone climbing a Cascades peak will have to beware of falling rocks as the whole region shakes violently in another magnitude 8-plus earthquake. The largest (magnitude 7.2) historic earthquake in the Pacific Northwest occurred in 1872, with an epicenter near the Skagit Gorge. This was probably related to uplift, and quakes of similar size will undoubtedly happen in the near future.

Glaciers in the North Cascades are mostly in the unhappy condition of rapid retreat. This too will pass. We are currently in the latter stages of an interglacial period and climatic models point to another Ice Age within the next 10,000 years. Over the short term, global warming due to the greenhouse effect may shift the climate of the Pacific Northwest to a humid, subtropical condition. This would substantially increase the rates of stream erosion, flooding, and landslides in the mountains.

Finally, the long-range forecast for these mountains is that they will be gone. In about 20 million years the Cascadia Subduction Zone will have been "transformed" into a San Andreas-type fault and volcanism will shut down along with the compressional forces that cause uplift. The compensation for this loss is that Baja California will have moved northward on the Pacific Plate so that it will provide a great offshore view from the mountaintops left and we can contemplate the intriguing possibility of a future North Cascades Multinational Park.


The main impression to be conveyed by these field trips is that the only thing constant about the geology of the North Cascades is that it is constantly changing. We have seen evidence for many different incarnations of the landscape from the sea floor rocks of Mount Shuksan to the Himalayan-like collision zone recorded in the Skagit Gorge. The changes also vary immensely in rate from the supersonic blast of the Kulshan Caldera to the creeping crystallization of the Lake Ann Stock that might have taken 100,000 years or more to complete.

There is also the shifting paradigm of geologic science. When I first traveled with Peter Misch back in the 1960s, there was no such thing as plate tectonics, and even continental drift was viewed with great skepticism. The permanence of the continents and ocean basins was a prevailing concept and geology was interpreted accordingly. Now when I take students on field trips into the North Cascades, the simple message is that there is no such thing as terra firma. The older rocks of the North Cascades are foreign imports that have probably traveled thousands of kilometers from their birthplace to occupy this special part of North America. Most of the rocks younger than about 50 million years are bona fide Yankee residents, but sometime in the geologic future they too will become restless and move on to other parts of the world.

So as we sit on a high ridge looking over the magnificent vista of the North Cascades, we need to realize that this landscape will exist for only a brief moment in geologic time. We can count ourselves fortunate to have been here when a unique combination of tectonic forces and geomorphic agents created a mountain range of incredible beauty and diversity for us to appreciate and seek to understand.

SCOTT BABCOCK is a professor of geology at Western Washington University who has no expectation of ever completely understanding the rocks of the North Cascades. Appreciation, however, comes easily and is happily shared with others who venture into these magnificent mountains.

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North Cascades Conservation Council
P.O. Box 95980
Seattle, WA 98145-2980