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Cycles and Seasons

Bedrock: The First Story

On the trail that connects the Logan Pass visitor center to Hidden Lake overlook there is a shallow pond. Near Hidden Pass, it collects its meltwater from the Continental Divide and sends it down the shallow gorge that drains the Hanging Gardens; as a waterfall it plunges into the upper St. Mary Valley where it becomes Reynolds Creek; joined by other tributaries, it continues its long journey to Hudson Bay.

The surface of this pond is seldom still, for the wind treats it like a sea. Because the water is shallow, the wave action wrinkles the bottom mud into ripple patterns, mimicking the churning waves.

I like to come here early in the morning. Sometimes, arriving before the wind awakes, I catch reflections of the surrounding mountains. Beyond the low bench of Logan Pass the Garden Wall begins, running northward with the Divide. In the eastern valley the pitched peak of Going-to-the-Sun hunkers in the morning light like a tensed warrior. To the south, the incisor Bearhat, beautiful cloud cutter of Hidden Lake Valley, juts above the nearby saddle of the pass. But over this place, standing as fresh monuments to an age of ice, tower the cliffs of Clements and the pyramid Reynolds.

I am sitting on a wedge of red rock. Its surface exhibits a wrinkled pattern identical to the ripples in the soft mud of the shallow pond. The distance is not great; with a stick I could reach out and touch the mud. Yet this represents a gulf no bird can fly, for between the ripples of this rock and the ripples of this mud lie billions of vanished mornings, a constellation of years.

These red, green, tan, white, black and purple bands of rock that layer Glacier's mountains comprise the oldest unaltered sedimentary rocks on Earth. They were laid down in Precambrian time, more than a billion years ago, when life was just beginning, as the deposits of an inland sea.

For millions of years, sand, mud and carbonates washed into the ancient sea, compressing the lower layers into mudstones and limestones, building up a sediment thickness that may have been as much as 10,000 meters (see metric conversion table on page 136).

When we look at the sharp contours of Glacier's mountains, we see the evidence of uplift, overthrust and glaciation. But on the geologic clock these are recent events a mere eyeblink of time ago. For the vast majority of years, the rocks lay undisturbed and level beneath the sea and land.

To understand better the tremendous time scale these rocks represent, we need a way to visualize the vast collection of years. If we were to make a movie of these geologic events, we would first need to determine how many years each minute should represent. Since the Pleistocene lasted about 3,000,000 years (its four ice ages sculpting the present muscle of this land), let us make each minute portray a million years. To chronicle these rocks we will then need a film 60 hours long!

Not until the fifty-seventh hour of our film will the Mesozoic lowlands begin to bulge with the coming Rocky Mountain chain. During the long preceding hours we would have seen little else but sea—with drawing, advancing, deep and shallow; yellow, green, and brown with great colonies of algae. Unseen below the water, lava has spilled out occasionally on the sea bottom; once, it intruded between the rock layers below, forming the conspicuous, 60-meter-thick band of black diorite that we see to day on many mountain faces in Glacier.

During this time of initial uplift an amazing process is going on deep under ground. A major fault has developed, fracturing the buckled layers of rock. A vast mountain plate begins to slide eastward, over-riding and submerging the rock layers to the east and opening the wide trench that is today the North Fork Valley. Known as the Lewis Overthrust, this gigantic earth-force has created an unusual situation: ancient rock strata lying atop recent rock strata.

Now less than 3 minutes of film remain. The arrival of the ice is imminent. We look at the landscape of featureless mountains and wonder at the dramatic difference that this last 3 million years will make. We do not see the familiar forests and lakes, the savage peaks, and the broad, deep valleys of this present land. These mountains are gentle, arid, and shallow-valleyed. The vague outlines are there; we recognize the general alignments of the drainage systems, the bloated domes from which sharp peaks will be cut. The mountains are connected to one another by blunt ridges and smooth saddles, and the shadows they cast are dull, dunelike.

Suddenly the ice is there, filling the landscape, with only the mountaintops protruding. Four times in these last 3 minutes of film the ice sheets advance and retreat, each time leaving an altered landscape. Strange lakes and forests fill the gaps between the glacial invasions. Then we see the mountains we now know come into being rapidly, as if the land were being hacked into shape by giant cleavers.

After this flicker of Pleistocene time, the film ends, the forests return, and familiar lakes shine beneath the sun again— these lakes and forests we had thought to be timeless.

Up springs the morning wind from Hidden Valley, making the nearby alpine fir branches whiz with its passing and shattering the perfect reflection of Bearhat Peak on the pond. From where I sit, it is a short distance to Hidden Pass; so I leave the pond and walk to the overlook to see again the fine basin quarried by an ancient glacier.

Hidden Lake, deep, far below, so blue, fits into its cliffed, crooked valley like a polished boomerang. Closely ringed by ridge and peak—distant Sperry Glacier and pointed Gunsight peering up from the southern jumble, and broad Bearhat impossibly close—this lovely lake is almost lost amid such sharp proclamations of rock. Its outlet gorge gives a narrow view across the angled, hidden valleys of Avalanche and McDonald, past the pyramid of Stanton, to the low, faraway undulations of the Whitefish Range.

Glaciation is a cruel master of mountains, biting deeply into their bulk and leaving sheer, spectacular contours when the glaciers disappear. The landforms here attest to their power, everywhere exhibiting the effects of glaciation.

In eating back the mountain headwall, alpine glaciers formed rounded depressions, called cirques. Unlike the narrow clefts left by running water, these broad, deep basins look as though they were made by ice-cream scoops gouging into the rock. Hidden, Ptarmigan, Iceberg, and Avalanche Lakes sit in well-developed cirque basins, and many mountains are dimpled by the beginnings of other cirques—the conspicuous amphitheater on the south shoulder of Heaven's Peak, for example.

Occupying all major drainage systems, glaciers modified the contour of the valleys, changing them from their narrow, stream-cut V-shapes into broad U-shapes. Into these wide main valleys, waterfalls plunge from higher, smaller valleys. Like rivers, flowing glaciers have tributaries. Lacking the ice mass and cutting power of the main glaciers, these tributary ice fingers could not bite as deeply into the bedrock. When the ice melted, hanging valleys were left stranded high above the main valley floor. Hidden Lake sits in one of these hanging valleys, and from it Hidden Creek plunges 750 meters into Avalanche Basin toward McDonald Creek.

On my many previous visits to this pass I have been too busy enjoying the wildflowers, the weather, or the scenery to realize what an open textbook of glaciation is everywhere displayed.

I stand here on a small saddle of a pass. Wherever glaciers met, passes, or cols, were created. A high, notched pass like this one (or Swiftcurrent or Gunsight) reveals recent connections. Broad, lower passes, such as Logan, resulted where the ice early overran the mountain ridge and had a chance to work longer.

Where two glaciers worked on opposing sides of a ridge and failed to meet, they formed an arête—a thin, steep-walled remnant resembling a saw blade. Another ice age would probably consume the park's many thin arêtes, such as the Garden Wall and Ptarmigan Wall; but it would also create new ones from existing ridges.

Further testimony to the sculpting power of ice is presented by Mt. Reynolds, looming to the east. The most dramatic feature of a glaciated landscape is the pyramid-shaped mountain called a horn—and Reynolds is a perfect example. Horns were formed when three or more glaciers cloaked the mountain, excavating its sides toward its core and gradually transforming its original domed shape into a sheer-sided peak. Glacier has many remarkable horns, from the sleek spire of St. Nicholas in the south to exquisite Kinnerly in the northern Kintla valley.

Sperry Glacier stares back at me from the flank of Gunsight. Glaciers found in the park today are not remnants of the last ice phase, which ended here about 8,000 years ago, but are newly formed, having come into existence some 4,000 years ago. They reflect a cooling trend in the present climate.

Shrinking steadily from their period of greatest extent in the middle of the last century, these modern glaciers finally stabilized in the late 1940s and since then have shown only a slight increase in area.

Movement distinguishes glaciers from icefields, and the movement of ice is a force on as well as a feature of a landscape. A glacier excavates by abrading and plucking at the rock. Alternately melting and freezing, ice at the headwalls plucks out blocks of rock. Ultimately the rocks are deposited along the sides or at the feet of the glacier as moraine debris. But as they move in the grip of the ice, they constantly abrade the rock surfaces they encounter. Polished rock beds of past glaciers show striations—grooves gouged by rock fragments imbedded in the moving ice.

Flow rate of a glacier depends upon the thickness of the ice and the degree of slope. Under tremendous pressure, ice becomes plastic, like thick taffy. Unlike kilometer-thick continental glaciers, which may move a hundred meters a day, small alpine glaciers seldom progress more than two or three centimeters per day.

Although a glacier moves, it gets nowhere if in a state of equilibrium—when annual melting equals annual accumulation. Snow mass gained at the sun-shielded headwall is usually lost as melt at the exposed snout. Glaciers such as Sexton or Weasel Collar, whose snouts perch on cliff edges, also lose mass by calving. Thunder you hear on a late-summer day near such a glacier may actually be the sound of ice pushed off from the lip of a cliff.

Walking back to the visitor center, I suddenly stop where the trail skirts the steep moraine of Mt. Clements. From the opposite side of the moraine five mountain goats have appeared. Spotting me on the trail below, they also halt. But before I can get to my camera they are off in a stiff-legged gallop, running in single file along the crest of the moraine to the distant safety of the mountain face.

Moraines are ridges of rock debris piled up along the edges and terminuses of glaciers. Like a bracelet lying against the wall of this mountain, the circle of steeply plied debris marks the extent of a small, recently vanished glacier. Ghost of the power that once resided here, a stagnant icefield lies beneath the confining walls of the moraine. The recent accumulation of these rock fragments is a mighty accomplishment, attesting to the force of moving ice.

Reaching the mountain wall, the goats scramble upward to a ledge, sending scree streams pouring from several clefts. Encountering a narrow, steep snowbank, they do not hesitate but continue across the slope. Above the rock fingers of this peak the gathering clouds grow black. A sudden crack of thunder hurries me down the trail.

Although geologically young, the Rocky Mountains in Glacier are composed of soft sedimentary rocks that are easily assailed by the many agents of weathering and erosion. If not rejuvenated by continual uplift, these magnificent peaks will glimmer but briefly in the long memory of the planet.

Already the sharp countenance of this land is being softened by the ongoing forces of erosion. Chief among these is water, which attacks the mountains everywhere. In addition, frost action continually exploits rock fractures, breaking down blocks of rock into talus and scree. Avalanche and rockfall sweep down the slopes. Layers of softer rock erode quickly, undercutting more resistant rock and creating overhangs which gravity, in time, will collapse.

The lashing rain catches me on this sun-and-storm-contested pass. Ice, gravity, wind, and especially water—all attack a land that dares the clouds.

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Last Modified: Sat, Nov 4 2006 10:00:00 pm PST

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