Mountain with snow

Mountains & Plateaus of Cascadia

July 10, 2023
Roberta Hoffman

By David McCloskey
© July 2023,
Cascadia Institute

Living here we are stunned by high rugged Mountains and huge open Plateaus. Each is impressive in themselves, but taken together they become truly extraordinary….

When we say “Mountains,” and when we say “Plateaus,” we conjure them separately, as self-contained and static, behaving as good nouns should. And when you’re in the Mountains, they’re so all-encompassing you think that’s all there is forever … and the same out on the Plateaus….

However, when moving out of the Mountains down onto the open vast Plateaus, the transition can be so quick it’s headsnapping! One day a door opens and you enter another world…. Our Mountains are so vertiginous and densely packed its hard to believe that the vast open long level stretches of the great Interior Plateaus are even possible. And yet the transition is right there—so abrupt from one to the other—for example, going over the Central Cascades between Mt Washington and Three Fingered Jack just west of Sisters, Oregon we pass thru one of the most densely packed set of contour changes in elevation, precipitation, and vegetation around, matched only perhaps by The Tetons down to Jackson Hole, Wyoming, or the Chilcotin Plateau just behind the massive vaulting Waddington Ranges…. Everything changes seemingly all at once—like waking up in another world….

That these two radically different worlds stand side-by-side is hard to fathom. But the plain truth is that what we mentally separate is already joined in the actual landscape. For the two are tightly paired—high Mountains leaping up from huge flat Plateaus, and Plateaus floating up in between high Mountains—an unlikely juxtaposition that characterizes Cascadia…. And from the Plateaus you can watch the ragged rolling edge of the world’s horizon-line dancing on distant mountains for hundreds of miles. What those white-caps rolling in the distance mean to folks toiling down on the hot, dry, sandy Plateaus can only be told in poetry…. Its the clasp between these two opposites, and what they mean, taken together, that we shall explore here….

We should also note that as landforms Mountains and Plateaus constitute the majority of the area of Cascadia. To get a quick sense of the scale involved—the Columbia Plateau covers about 32,100 square miles, larger than Maine, and also larger than the states of Rhode Island, Delaware, Connecticut, New Jersey, Massachusetts, and New Hampshire combined (one can only wonder what this world would be like if it also had twelve Senators to be its voice in the U.S. Congress!?) Some say that the Fraser Plateau is even larger. And while the Cascade Volcanic Arc stretches from Mts Lassen and Shasta to Silverthrone about 750 miles long, the massive B.C. Coast Mountains stretch almost a thousand miles from border to border, while the Northern Rocky Mountains run over 1,400 miles from the Wyoming Ranges to the Yukon border, while the whole length of the enigmatic Rocky Mountain-Tintina Trench runs twice as long as that…. Such massive geological structures are visible from space….

So, in the Northern Cordillera we are abundantly blessed with spectacular Mountain Ranges and huge Plateaus. As Mountains and Plateaus come tumbling out across the landscape, what kind of pattern do they form? How do they differ, and how do they relate? While Plateaus and Mountains are commonly viewed as opposite kinds of landforms, one wonders if they also belong to one another in some way? And while each can be regarded as self-contained static features, wholly separate from everything else, seen in regional context they become dynamic relational systems. Here we try to open up those inner articulations. What Story do they tell?

What Are Plateaus?

All our Plateaus in Cascadia are found east of the regional divide—namely, the Cascades and B.C. Coast Mountains. In this sense, these Plateaus of the Interior are “beyond the mountains,” but because they fill in gaps between the Cascades-Coast Mountains to the west and the Rocky Mountains to the east, they are more accurately described as “between the mountains.” Interior = Intermontane Plateaus.

Some quick-study basic definitions and distinctions. A Plateau is like a “plate” from which it receives its name—generally open flat land, slightly cupped or incurving, as if creased in the center and unfolded flat outwards. They are slightly cupped because they slope downward from Mountains on either side. So plates and plateaus go together geographically as well as etymologically.

Is a plain or basin, for instance, the same as a plateau? No—a plain is just that—flat—a plane, perhaps slightly sloping down to the ocean as in a coastal plain, but generally all surface and no depth. Plains may also become basins. Whether depositional, erosional, or structural, plains are mostly laid down in layers because something fluid—water, lava, sediments, even wind-blown loess—swirled and flowed around, and came to stand still there over time in layers.

While a plain is usually a lowland, a plateau is usually an upland. A plateau is a relatively flat elevated tableland opening out between mountains on the sides, with its own characteristic morphology and life-forms. Nor are the Plateaus of Cascadia mesas, but rather large intermontane volcanic plateaus—this is crucial to their origin and nature. Further, they are not really deserts at all, neither barren nor lifeless, but rather “cool steppes” covered with grasslands, sagebrush, and related herbs and lichen soil crusts….

Plateaus of Cascadia

Using the Cascadia2 Map (2023), we begin with the place itself tracking these major Plateaus from south to north:

  • Klamath & Modoc Plateaus—headwaters of the Klamath River, in southcentral Oregon and northwest California,
  • Oregon High Lava Plains in central and eastern Oregon,
  • SnoCap Plateau—headwaters of the Deschutes River, in central Oregon along the eastslope of the Cascades,
  • Snake River Plain—southern Idaho along the track of the Yellowstone Hot Spot,
  • The Great Columbia-Umatilla Plateaus in central WA and northcentral Oregon,
  • Nicola Plateau—headwaters of the Nicola River, in southcentral B.C.,
  • Chilcotin, Thompson, Nazko, & Cariboo Fraser Plateaus—headwaters of these rivers in southcentral B.C.,
  • Nechako-Fraser Plateau—headwaters of the Nechako River in central B.C.,
  • Spatsizi Plateau–headwaters of the Skeena in northern B.C.,
  • Stikine & Nahlin Plateaus—headwaters of the Stikine River in northern B.C.,
  • Dezadeash Plateau—headwaters of the Alsek-Tatshenshini in the Yukon.

Each of these places enjoy their own distinctive character and context, and deserve to be understood in those terms and more widely known.

The Great Axes Converging

Note how this place is composed—how this flying arc of landscape curves and narrows from south to north, especially in the leading lines of the SeaCoast and the Mountains, the two great axes of Cascadia. On the region’s southern stretches from Northern California all the way over to the corners of Wyoming and Utah, the gap between the SeaCoast and Continental Divide widens, including the Klamath and Modoc Plateaus, the Oregon High Lava Plains, the northern edge of the Great Basin, and the long curving arc of the Snake River Plain—track of the Yellowstone Hot Spot—over to The Tetons and Yellowstone, about 750 miles from coast to crest. But going north along the Continental Divide, the space in-between contracts—basically down to the great Columbia Plateau. Until by the Fraser Plateau the overall gap between the axes closes to about 350 miles wide … eventually squeezing out everything else in between as Mountains tumble into the Sea with the Icefield Ranges in SE Alaska/Yukon, in the very corner of the Gulf of Alaska. (All this records the moving signatures of “deep time”in formative geological processes). In short, when Mountains hit the Sea directly, then all the intermontane Plateaus are squeezed out….


Mountains and Plateaus may seem opposites, starkly contrasted contradictories having little in common. Mountains are tall, plateaus flat, one is jagged, the other smooth, one thick, the other thin. Its the difference between enclosed vs. open, sheltered vs. exposed, the full vs. the empty. Moist green vs. arid, dry brown, tree covered vs. bare and comparatively barren…. Mountains rise up as vertical gashes, while plateaus appear as horizontal flat plates—one leaps up, while the other lies down. Mountains form linear belts, while plateaus are oblong, spreading out like blobs. If Mountains are thick rock crunched together, a juxtaposition of shattered planes rotated and stacked like a cubist painting, then Plateaus are that same plastered and crumpled canvas stretched out flat and thin….

Different Worlds = EcoRegions

Because they constitute different worlds, they may be designated as individual “EcoRegions.” These are coherently bounded and work integrally in depth on many levels simultaneously. Each has its own distinctive character and context, its own configurations of animating energies and forms, its own origin and nature, its own history, rhythms, textures, colors, light, sounds, aromas, its own life-forms, creatures, its own “spirit of place”–in short, its own Story.  Each of them constitutes an “EcoRegion” in its own right, recorded on the “EcoRegions of Cascadia” small map inset on the Ish River-Salish Sea Map (2022).

Relations Within Cascadia BioRegion

The question then becomes: how do these separate “worlds” interrelate? Without Mountains would Plateaus even exist? Recall that Mountains and Plateaus constitute the majority of the area of Cascadia, and taken together form a rhythmic regime playing out across the regional landscape, part of the essential character of Cascadia, a song it sings on and on….

We can also grasp this character by its absence—that is, when the context changes.

Once we’re caught up in a rhythm unfolding we know what to expect, and are caught up short when it changes. For instance, on the far side of Cascadia in either direction we enter radically different worlds. To the west is the vast wet wild North Pacific Ocean, while to the east stands the vast ramp of the High Plains sloping down from the Rockies to the Mississippi lowlands. When we run out of rhythmic alternations of Mountains, Valleys, and Plateaus—when the song changes—we know we have left Cascadia…!

Both far sides are more uniform, whereas Cascadia is radically diverse internally. Looking across the whole bioregion from coast to crest—that is, all the way over the Continental Divide at Yellowstone , Glacier National Park, or Columbia Icefields—Mountains & Plateaus go together in Cascadia by an internal necessity, as closely entwined as Mountains & Valleys, Basin & Range, the Sea & the Shore…. Indeed, the many intertwined Mountain Ranges & their intermontane Plateaus rise as pairs with integral inner connections…. How do they thread together to form the larger “BioRegion”?  They belong to one another in many ways….

Inner Connections

There are several clues to the inner connections between Plateaus and their Mountains in Cascadia….

First, recall that a Plateau is shaped like a slightly cupped plate sloping downwards from Mountains; hence, the more formal definition that a Plateau is an elevated tableland enclosed by Mountains on the sides… So, Mountains bound Plateaus … as the Columbia Plateau is bounded by the Selkirk, Palouse, and Clearwater Ranges of the Rocky Mountain margin on the east, and the Cascades on the west… Or as the Fraser Plateau is bounded by the Columbia Mountains on the east and the Chilcotin and Lillooet Ranges of the B.C. Coast Mountains on the west….

Sometimes the abrupt wall is quite dramatic as Mountains leap up straight out of the Plateau, as with the Homathko Ranges in the Chilcotin area near Chilko and Tatlayoko Lakes. In other places, lava flows lap-up valleys in the eastern valleys of the Cascades, as around Ellensburg. Indeed, its their enclosing arms which call forth platforms in the middle. By their very location and shape, these platforms stand “in-between” the surrounding walls of Mountains. So, Plateaus are rimmed by Mountains, as at the same time Plateaus bound Mountains!

Second, these dry Plateaus are themselves creatures of the Mountains. For our Plateaus are not true deserts, which are primarily latitudinal or “zonal” (that is, staying within the horizontal zones stacked poleward influenced by prevailing latitudinal wind patterns) formed by semi-permanent high-pressure systems such as the California High or the Great Basin High hovering over the SW deserts (Mojave, Sonoran, Chihuahuan). Rather, in Cascadia, massive “Rainshadows” are formed by the high Mountain Ranges themselves standing in the way of prevailing westerlies blowing off the North Pacific. Here “the Pacific Wave” climbs the wall of steep Mountain Ranges, dropping copious amounts of precipitation on windward sides, while starving leeward sides. So, our cool grassland steppes are created by lengthy azonal Rainshadows” projected by undulating orographic pressure-waves lifting and sinking eastward. These pressure-waves continue rhythmically rising & falling across the entire region–their shadows are as deep as the mountains high….

In this dynamic, remember the Cordillera is composed not just of the Mountains themselves, but everything in between, esp. their Valleys and inter-montane Plateaus. Here many Mountain Ranges—perhaps one hundred fifty or more named ones—rise in long N/S or NW/SE trending chains with Plateaus emerging as the “in-between” forming the distinct grain of the land. This belted pattern is largely due to great tectonic processes of accretion of oceanic islands and mini-continents, and their subsequent subduction and collisions with the continental margin and each other. Such dynamics form a laminated landscape deeply furrowed from west (younger) to east (older). And in the process creating an alternating series of windward and leeward sides intercepting marine breezes splayed across the grain of the land. “Rainsheds throw Rainshadows,” and the taller the one, the deeper the other… These alternating series of “Rainsheds” and “Rainshadows” repeat in long undulating waves, a truly Cascadian rhythm….

Third, Mountains literally create the Plateaus! Again, recall that these Mountains are formed by the great pressures and torque of tectonic processes, and then these Mountains create the Plateaus….

Note that these elevated platforms rising 2000-3000-4000’ and more are often filled to the brim and over-flowing. When you drive up the Fraser River Canyon from Hope, B.C.  for instance, you climb, climb relentlessly for 150 kilometers north up the long incline onto the vast interior plateaus; going in reverse you literally feel yourself falling, falling down off the edge of the high plateau which had become your world, returning to sea level at Vancouver. Farther north the abrupt descent off the Anahim-Dean Plateau westward thru Heckman Pass down down thru the terrifying gravel switchbacks of the infamous grade known as “The Hill,” relentlessly down down down to Bella Coola … feels like slip-sliding off the edge of the world!

Now, Mountains create Plateaus by filling their “in-betweens” with detritus—sloughing off loose and eroded materials sliding down toeslopes and then washing away, and, above all, by massive volcanic fill. Igneous processes here include vast basalt floods, andesitic explosions of rock and tephra, plus huge volumes of incandescent rhyolitic ash and pumice clouds, etc. For instance, the Chilcotin Plateau stretches out like a vast shining basalt sea. The Oregon High Lava Plateau is filled with basalt flows, andesite explosions, rhyolite, and deep pumice fields, with great black glassy ropy obsidian flows in the caldera at Newberry Volcano. The Snake River Plain (Plateau) is filled with alternating layers of rimrock-forming black basalts, and rounded heaps of multicolored rhyolites, characteristic of the bi-modal volcanism of a Hot Spot Track….

Many don’t realize that the depths of volcanic fill can sometimes be enormous, ranging from hundreds to thousands of feet and more. For instance, the massive Columbia River basalt floods in different phases pumped out over three hundred separate flows, each layered on top of the other, covering by some estimates as much as 105,000 square miles, reaching a maximum of about 16,000 vertical feet around Pasco, WA–that’s over three miles thick! The cumulative weight of all that heavy black basalt depressed the land surface, forming the present bowl-shaped Columbia Basin. (Besides “normal” volcanism, there has also been a strong presence of “Hot Spot” activity such as with Yellowstone, the Anahim “Hot Spot,” etc.)  In short, Plateaus fill up as Mountains fall down … so Mountains both bound and fill their Plateaus!

Fourth, while most are arid, our cool steppe Plateaus are surely not deserts regarded as the absence of water, for they carry great waters—regional rivers flowing thru them include the Klamath, Deschutes, John Day, Grand Ronde, Owyhee and Malheur, the Snake River and many tributaries such as the Clearwater, the great Columbia River and its many tributaries such as the Kootenay, Flathead, Spokane, Yakima, Klickitat, etc., the great Fraser and many of its tributaries such as the Thompson, Chilcotin, Cariboo, Nazko, Nechako, etc., and the great rivers of the north such as the Skeena, Stikine, and Alsek-Tatshenshini…. In short, almost all the great rivers of Cascadia! (And contrary to the desert presumption, with abundant hydroelectricity and irrigation the Columbia Plateau, for example, has surely become “a well watered plain,” capable, as with the Snake River Plain, of tremendous agricultural productivity.)

Indeed, this is one of the distinguishing characteristics of these elevated Plateaus—the presence of major regional rivers coursing thru, often running all the way from the Continental Divide to the Pacific Ocean, forming a veritable tree of waters…. Our plateaus carry this great “Tree of Cascadian waters,” silver threads running from mountains thru mountains to the sea, binding the entire region together from the inside…. While Mountains bound Plateaus, Plateaus bind Mountains to the Sea….

Regional Fabric

Neither Mountains and Plateaus are simple opposites nor contradictories so much as reversible pairs, co-implicative, the “other side of the other,” each calling forth the other….  It doesn’t make sense to treat them separately, for Plateaus imply Mountains, and vice versa—they belong to one another, weave a wider fabric. (What we have severed let us help heal by recognizing a prior integrality. Time to let go of fixed inherited abstract logics, as we embrace a new geographically grounded eco-logic!)

If we look deeper, we find Cascadia is more than a fixed set of features like a single Mountain range, and more a flow or even flying arc of energies, forming a whole rhythmic regime, a place of places tumbling and intertwining together… Braided in some places, an imbricated weave in others–both form the living fabric of Cascadia…!


In sum, our Mountains and their intermontane Plateaus rise as braided pairs in Cascadia, each calling forth the other, forming a distinctive fabric within the landscape where:

  • Plateaus are rimmed by Mountains,
  • Following the grain of the land, a whole alternating series of “Rainsheds & Rainshadows” emerge created by Mountains,
  • The Mountains create elevated Plateaus thru erosive deposition and massive volcanic fill,
  • They carry major regional rivers forming a great “Tree of Waters,” binding Mountains to the vast Sea beyond….

Mountains & Plateaus are the song the place sings.
Together they form alternating pairs, intimately intertwined,
an ongoing ondographic rhythm playing across the laminated
landscapes of Cascadia….

1 Comment

  1. Denise Lassaw

    Beautiful and sexy geology prose poem. 44 years intwined in Alaska’s mountains and rivers without end, now in the Cascadia system I have yet to know intimately.

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