Glacial Landforms in the Sierras

Because moving ice is infinitely more erosive than moving water, most of the landforms in the higher altitudes in the temperate zone have been more influenced by glaciers than by running water. The Sierra Nevadas are a case in point.

Although the Sierras were not a part of the great sheet of ice covering the Western part of North America, it still had extensive glacier action.

“When the ancient ice was squeezed into narrow river valleys, it speeded up. It tore at the floor and slopes, changing the V-shaped profile with gentle slopes characteristic of stream erosion into deep U-shaped troughs. A superb example is California’s Yosemite Valley, which experienced widespread glaciation — though not a full-scale ice sheet – that affected the Sierra Nevada about 20,000 years ago.” (Glacier p29)

U-shaped and V-shaped valleys

In our 3D Topo sculpture of Kearsarge Pass there are examples of both the U-shaped glacier valleys and the V-shaped erosional valleys. Our sculpture represents an area about 4 miles wide by 10 miles deep. The back third is the Western Sierras. We see no erosional valleys, only glacial valleys. Bubbs Creek has a U-shaped bed with steep walls.

It seems evident that during the last Ice Age that snow covered the Sierras permanently year round. This snow pack turned to ice, which kept packing down with the increased snow and no melting. When it became deep and heavy enough, it began to move. Because the Western Sierras were so shallow compared to the Eastern Sierras, the glaciers could form and flow, while on the steep Eastern Sierras, the glaciers had a harder time collecting and forming because the angle was too sharp.

Additionally the flat Owens Valley at 4000 feet creates an updraft of heat, which dissipates clouds, reducing rainfall on Eastern side. This is called the rain shadow effect. In addition to reducing moisture, this heat also melts glaciers. Hence on the Eastern side one sees these U-shaped valleys down to about 8000 feet, at which the sharp transition occurs to small V-shaped valleys. The strong sense is that the glaciers stopped at this point. Possibly this was 20,000 years ago when it reached its peak expansion and then began retreating back up the hill.

There are areas on the Eastern side where there is an V-shaped erosional valley beneath the U-shaped glacier valley. Two items worth commenting upon; the first is that the erosion is very shallow indicating that the rock is very hard, which it is, granite. The second item worth noting is that if the local glacier had dissolved by 8000 years ago, and that this is all the erosion that has happened by running water in 8000 years, that running liquid water is a minor force compared to running frozen water in the Eastern Sierras. Water on granite is nothing compared to ice on granite

Ice vs. Water or Frost Riving, simply shattering

The creeks that occupy the glacier valleys are puny in comparison with the U-shaped valleys. It is easy to sense that Charlotte Creek, Bubbs Creek and Independence Creek could in no way have sculpted these huge U-shaped valleys. The creeks are so small because the runoff on the hard granite is so fast and the erosion so minimal that it has a hard time accumulating into a full-scale river. Additionally the tops of the mountains retains snow year round, hence the streams are fed by dripping snow rather than by torrential rainfall. When there is an exceptional warm spell after heavy snowfall, there is a real danger of flooding. This is because most of the flora is shallow rooted because of the hard granite underneath and the generally gentle drip system of the snow pack.

On the other hand, the destructive and adhesive power of ice is awesome.

“Bedrock lying at the head of the newborn glacier, and under it, already has been jointed or fractured by centuries of frost action—the alternate expansion of water through freezing (about 9 per cent by volume) and then contraction through thawing.” (Glacier, p29)

This is called frost riving and is an incredibly powerful force. As a liquid it fills up an area and then under a decrease in temperature, it expands, shattering granite, milk bottles left out or in a freezer, or simply put anything but itself.

While ice shatters as it expands from freezing, it also grabs the debris that it has created. As it melts and then refreezes, it absorbs all that it encloses into the glacial mass.

“The glacier’s icy fingers probe into the fractures and adhere to the rock fragment, which are then carried along by the moving mass of ice. The rocks, now frozen in the bottom layers of the glacier, pry loose further fragment, which also are incorporated into the ice.” (Glacier, p29)

 

Thus the ice leaves no survivors. Nothing can resist its expansion; also nothing is left behind. If you have ever touched something frozen and left behind skin, then you know how adhesive ice is. Thus it breaks down granite and then uses this same granite to scour the sides and bottom of its path.

The Lovely Cirques, Arźtes & Tarns of the Eastern Sierras

The nature of water to expand and contract when it freezes and thaws, combined with its scouring tendency creates a landform called a cirque.

“Thus, the floor, headwall and sides of the hollow are gradually scooped away. The result is a circular steep-sided basin that geologists call a cirque. Cirques sometimes grow quite large—up to a mile in diameter, with walls several hundred yards high.” (Glacier p29)

 

We have many cirques in our topo-sculpture. Each of the Eastern Sierra Lakes is at the bottom of a cirque: i.e. the two Golden Trout Lakes, The Parker lakes, and Robinson Lake. The most spectacular of all is the terrace of lakes including Matlock Lake and Slim Lake. This plateau-like area includes 5 lakes and rises 800 meters, over 2500 feet. Even the southwest side of Heart Lake, on the main path to Kearsarge Pass, rises almost 500 meters, over 1500 feet. These are all cirques formed by glacial action.

 

“If cirques scallop all sides of a mountain, they may bite into it so deeply that only a single jagged peak or horn, remains. The classic example is Switzerland’s Matterhorn, but thousands of other peaks are glaciated horns, including the highest of them all, the 29,028-foot Mount Everest.” (Glaciers, p29)

 

We also have a lot of horns in our small section of the Eastern Sierras. Black Mountain, Mount Rixford and its unnamed twin peak are all horns. This also explains why the peaks feel so good on the fingers. It explains why the unnamed peak on the ridge next to Kearsarge Pass that began my odyssey downwards had the sculpted bubble gum effect. The glaciation has the effect of creating U-like shapes. Hence the convex sides of these horns fit perfectly with our concave fingers. The bubble gum shaped by my preteen fingers took on the shape of a horn for the exact reverse reason that my fingers are concave, having expanded to most efficiently fill the space.

 

“The retreat of the ice leaves a rugged tableau, dominated by carved peaks called horns and sharp, ragged ridges known as arźtes. Dramatic hanging valleys carved by tributary glaciers join the deep U-shaped valleys that mark the wake of the glacial mainstream.” (Glacier, p32)

 

Our topo-sculpture has many arźtes also. The Kearsarge Ridge, the east-west ridge that extends 6 miles in our sculpture alone, is an arźte. It must have been a huge glacier that filled up that basin. There are smaller arźtes acting as divides between any of the consecutive lake systems. There is one between the Parker Lakes and Sardine Lake, between Sardine Lake and the Golden Trout Lake system, then between there and the Big Pothole Lake system and then between these and Robinson Lake.

A sidelight: when creating my topo-sculpture, these narrow ridges would regularly give me grief. The topo pieces for the arźtes were so long and narrow that they would break apart and need to be recut more than once. This was especially true of the arźte between the northernmost Golden Trout Lake and the Parker Lakes. This was one of the areas that convinced me to go on to the 20-meter foam core increments. One weird little segment sat awkwardly on top of another weird shape. Assembling those pieces, building one on top of the other, was aggravating because of the long and narrow theme. Plus at this time I was still shaving the ¼ inch foam core. But the finished product - Whew! It is still amazing. I love to touch it, to feel its rugged stability, its concavity to my convexity. What a rush!

Now while there are the cirques which are glacier oriented and move downhill through openings, like erosional valleys, there are also the pothole lakes which are created more by the frost riving of individual lakes rather than by glacial action. Big Pothole Lake, Little Pothole Lake, both aptly named, and actually most of these high Sierra lakes have sunken through frost riving. Big Pothole Lake has actually sunk 40 meters below its surface. When the lakes freeze they expand to fill the geography in circular fashion, as ice tends to do. Then when these lakes thaw they shrink down into the hole. The higher up the lake the deeper the freeze, the greater the relative expansion and contraction of the lake. These holes allow the lakes to freeze and thaw. These high lakes left behind by glaciers are also called tarns.

Ice Sheets & Glacial Valleys

“The second act in the ancient drama of glacial erosion occurred when the streams of ice descended from the mountains into the plains and major river valleys, merging into larger valley glaciers and enormous sheets.” (Glacier, 29)

The ice sheets occurred north, especially in Canada. Although the northern ice sheet did not extend into the Sierra Madres, too far south, (the summers were too warm), there were streams of ice that merged. In our topo-sculpture, there was one glacier river that went from the Kearsarge Lakes on over to Charlotte Lake and down where Charlotte Creek is now; we will call this the Charlotte Glacier. There was another glacial river that created a U-shaped valley where Bubbs Creek and Vidette Meadow are now; this we will call Bubbs Glacier. At one point Charlotte Glacier, the higher elevation glacier, broke through the granite arźte between the two glaciers. This is where Bullfrog Lake is now. Now Bubbs Creek, instead of following the ancient glacier path from Kearsarge Lakes to Charlotte Lake, took a shortcut through the place where the Charlotte Glacier broke through. Instead of starting in the Kearsarge Lakes, Charlotte Creek starts in Charlotte Lake. Hence the latter rivers were not the cause of the erosion but instead followed the path of glacier erosion. Initially there were probably two creeks, which ran parallel to each other and then joined after Mount Bago, where they join now. The Ice Age glaciation broke the wall between the two valleys. Now the upper river jumps down into the lower river at the break instead of continuing in the original river valley - the erosion between the two river valleys.

As mentioned the Western Sierras has a shallow enough slope to support the flow of these glacier rivers. In the ten thousand cuts necessary to make all the hundreds of topo forms necessary to create the relief map it became quite evident that these huge glacial rivers left ripples in the surface of the rock. This was true of both the Charlotte Glacier and the Bubbs Glacier. The undulating surface of the Kearsarge Pinnacles, the remaining ridge dividing the two glacier rivers, is striking. But rhythmic undulation occurs over the length of both glacier rivers. Perhaps this occurred through the annual melting and thawing cycle of the glacier. Or perhaps it reflects the dry and wet cycles that extended over a period of years. There is one last explanation that I like. These ripples reflect periods of glacial surges followed by steady growth.

Whatever the explanation for these ripples, these glacial rivers scoured the granite of the High Sierras. They flowed and eroded the landscape, perhaps along compression lines, maybe the places that compressed when the Smartville Block began smashing into the side of the North American continent, hundreds of millions of years ago.

Glacial Debris

We’ve been speaking of the ways that glaciers transform the landscape when they are expanding. They grow into the land surrounding them. When they start moving they take this land with them to scour the ground and walls that they pass over and through. These glacial rivers as they expand and flow transform the landscape in major ways that we’ve examined. When these glaciers start retreating they also transform the landscape. Not only do they drop off huge chunks of ice, which might turn into lakes, they also drop an incredible amount of debris, which as we shall see, might even be called mountains.

Drift is the term used for the debris deposited by glaciers. This term is a throwback to the Biblical Flood, when debris drifted all over the planet. There are two types of drift, till and outwash. Till, Scottish for ‘obstinate land’, is debris that has been dropped from the ice itself. This is where the erratics came from and it is a chaotic jumble of non-sorted material. Outwash is debris that is carried from the glacier by meltwater. Meltwater is the water from a melting glacier and is used by some Scandinavian countries as a source of drinking water. Outwash is highly organized sediment just as any liquid stream deposits, heavy, big and dense on the bottom, light, small and aerated on the top. Outwash is much better soil for vegetation for this reason.

Whenever a glacier retreats it drops the debris it has accumulated during its growth. Remember that a glacier is a conveyor belt for the debris it picks up along its path. The ice and what it freezes onto are always flowing towards the snout, the terminus, of the glacier. Hence when a glacier stops, it immediately begins dropping the accumulated debris.

When the terminus of the retreating glacier pauses in its movement, it drops debris. When this debris forms a ridge, it is called an end moraine. When it is the furthermost end moraine, it is called a terminal moraine and represents the glacier’s furthest advance. When the glacier’s retreat is continuous, it drops a steady accumulation of till, the unsorted stuff. This is called ground moraine. A transverse moraine is formed on the sides of a glacier, in the direction of the glacier flow. It is the drift that is dropped along the side of the glacier as it retreats. A medial moraine is the drift dropping between two glaciers

One of the most striking features of the Kearsarge Pass experience was the terraced lakes. These were the terraced lakes that tricked me. I had never experienced these before and misinterpreted my road maps to fit former conceptions. In Europe they call these terraced lakes, paternoster lakes. Anywhere glaciers have gone, they leave lakes behind. First they scour out a U-shaped valley. Then upon retreating they drop debris called moraines, which cause a ridge. The ridge contains the meltwater from the retreating glacier – resulting in a lake. Additionally they leave large ice blocks behind, which may be covered by the glacial debris. When the ice blocks melt and freeze, they erode the land around them into small round lakes called kettle holes or kettles. These lakes are especially prevalent in Wisconsin and Minnesota. Bingo! Big Pothole and Little Pothole Lakes are certainly examples of this type. What is a pot but a kettle. Hence the mechanism by which the glacier creates lakes is erosion combined with deposition.

Before moving on to the analysis of the Sierra landforms left by glaciers, we will mention one other type of glacial debris, icebergs. Many people, including myself, thought that icebergs were merely the breaking up of huge ice lakes and oceans in the Arctic north. I envisioned the process as similar to a lake freezing over in the winter - then thawing in the spring - and breaking up into chunks. The size and irregularity were attributed to the excessive cold of the north. Actually icebergs are another product of glaciers. The glacier flows into the ocean. The water eats away at the bottom. A huge ice sheet forms over the water. When it can no longer support its weight, it breaks off, and sinks into the water as an iceberg, an ice mountain. This process is called calving. A huge sea of icebergs surrounds Antarctica. This was the first obstacle to exploring its interior. They come from Antarctica’s glaciers flowing into the ocean.

 

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