The Airliner Chronicles: There Were Glaciers Here...

Have you ever wondered about recessional moraines? I guess not too many people have pondered such a deep question, but on my recent flight over the Sierra Nevada, I was excited to see what was without a doubt the best example I've ever seen of this feature.

Recessional moraines are the piles of glacial till (loose debris) that form around the terminal margins of glaciers that are in the process of melting back over decades or centuries. Quickly receding glaciers might not leave such ridges, but if the climate briefly stabilizes, the moraines will form around the end of the glacier. Such moraines can be hard to see at times. There are several prominent moraines in places like Yosemite Valley and Kings Canyon, but they are covered by forests and are thus difficult to see from above. When the reservoir holding back Lake Thomas A. Edison in the upper drainage of the San Joaquin River was constructed, the forest cover was removed. During the winter, the lake is mostly drained, and with the low sun angle on snow, the moraines stood out in sharp relief.

The moraines were mapped by Joseph Birman in the 1950s as Tioga stage glaciers, dating back to around 20,000 to 13,000 years ago. This was the last major glacial episode to affect the Sierra canyons, and was responsible for most of the lakes and glacial polish that can be found in the mountains today.

Moments later we were flying along Mono Creek leading to the Sierra Crest, where I could see a multitude of classical glacial features. In one view I could see all the features I am always trying to sketch on the chalkboard in my classes illustrating the erosional features of alpine glaciation. I've labeled some of the most obvious features below.

As uncomfortable and inconvenient as flying can be, I was having a pretty good time!

The Airliner Chronicles is one of my on-again/off-again serial features, which is usually updated whenever I fly somewhere.

Vagabonding across the 39th Parallel: In the Former Realm of Glaciers...Part II, on Trail Ridge

It was the morning of our last day of an all-too-short visit at Rocky Mountain National Park during our vagabonding journey across the 39th parallel. We were now turning towards home, with the intention of crossing the Continental Divide on Trail Ridge Road. It's one of the nation's most spectacular drives, on par with Tioga Pass and Tuolumne Meadows in Yosemite, the Beartooth Highway near Yellowstone National Park, or the Going to the Sun Highway in Glacier National Park. It was another day in the former realm of the glaciers, as we drove from the end of the Pleistocene ice rivers to their source at the Continental Divide.

We made a first stop at Moraine Park to see a non-thunderstorming valley (it had been a wild night in the rain). A turn towards the mountain crest (above) gave us a fine view of a u-shaped valley, and a hanging valley. Hanging valleys develop when a larger glacier cuts a deeper trough than a smaller tributary glacier. The valley floor of the smaller glacier sits at a much higher elevation, often resulting in waterfalls or cascades where the drainage enters the deeper valley. In the picture above, the hanging valley is on the upper right hand corner (click on the photo for a larger view).

Rocky knobs in the middle of the valley that have been shaped by the grinding action of the ice are sometimes called roche moutonnées. The name refers to rock "mutton" or fleece (different sources disagree), presumably because the rocky outcrops looked like grazing sheep from a distance. The ice grinds and smooths one side of the rock, and plucks and tears at the other end, producing an asymmetrical outline.

The linear ridge in the picture above is one of the vast piles of debris left behind along the margins of the glacier that filled this valley, which is called Moraine Park. These ridges are called lateral moraines. A terminal moraine also once blocked the end of the valley, forming a shallow moraine lake. Over time the lake filled with sediment and evolved into the meadow we were strolling along. The picture below provides a perspective on the size of the lateral; it's big.

It was time to start up Trail Ridge, the premier park road that traverses the divide between Big Thompson Canyon and Fall River and crosses three passes, Iceberg, Fall River and Milner, to reach the headwaters of the Colorado River on the west side of the park. The road reaches 12,183 feet (3,713 meters). The route provides a bird's-eye view of the birthplace of the glaciers that scoured the high country.

From our first stop on the road, Many Parks Curve, we had a nice panorama of the lateral moraines we had just left in Moraine Park. The moraines surround the linear meadow in the center of the photo below. The park service website for Rocky Mountain has a marvelous interactive page on the glaciation of the park that offers this particular panorama with labels of the glacial features, and a view of this landscape as it would have appeared during the height of the ices ages.

The road climbs relentlessly and we arrived at the next viewpoint, Rainbow Curve. In the distance on the flank of MacGregor Mountain we could see a feature that is often mistaken for a glacial feature, a group of exfoliation domes (below). It would be easy to imagine glaciers overriding and smoothing off the summit of these granite monoliths, but these peaks stood above the ice rivers. The rounded shape of the domes results from unloading, the expansion of the granite as erosion removes the heavy overlying rock. Cracks form parallel to the surface of the rock, causing corners and edges to fall away.

We were so high up now we might as well have been flying (see the photo below). Looking down we could see the other major glacial moraine complex at Horseshoe Park and the long winding switchbacks of Trail Ridge Road on the right. On the left side one can see the tan colored deposits of the "Alluvial Fan", which formed in 1982 when a poorly built earthen reservoir damming Lawn Lake gave way, pouring nearly 30 million gallons of water down the small side canyon. The surging waters filled Horseshoe Park for a time, and killed three people.

Like Moraine Park, Horseshoe Park was once a shallow lake dammed up behind the terminal moraine of the glacier. When the lake filled with sediment, Fall River flowed across a nearly level surface, and the water took a meandering course across the meadow complex. The meander loops are constantly changing as faster flows cause bank erosion on the outer edge of the loops (cutbanks), and deposition on the inside of the loop (point bar). The loops get larger and larger until they breach the meander neck and form a cutoff.

By the time we reached Forest Canyon overlook, we were truly in the land of glaciers. Looking across the canyon, we had an excellent view of the birthplace of a glacier, a curving bowl-shaped valley called a cirque. The cirque in the picture below is on the flank of Terra Tomah Mountain (12,718 feet; 3,876 meters). Cirques develop when enough snow falls on a yearly basis that some survives the summer melting. Year after year the snow accumulates into a permanent snowbank that has been compressed into solid ice. When the ice reaches sufficient thickness, it behaves plastically and starts to flow slowly away from cliffs, plucking out loose boulders and destabilizing the slopes above. Eventually the ice eats into the mountainside enough to form the bowl-shaped valley.

The snowbanks filling the bottom of the cirque in the picture above are not glaciers; the climate has been too warm these last few centuries to allow a glacier to persist at this elevation. Several small glaciers can be found in Rocky Mountain National Park, and we could see what looked like a small one tucked into a higher cirque in Hayden Canyon (below). The bouldery debris below the icefield can be considered a terminal moraine, although the ridged appearance of the rocks may indicate the presence of a rock glacier, a moving icefield that is more or less completely buried by rocks. The park service site mentioned previously offers an interactive interpretation of the sights seen from the Forest Canyon overlook.

Glaciers in alpine environments tend to form extremely rugged terrain (as seen above), but Trail Ridge Road follows a long ridge with rather muted topography characterized by gentle slopes and swales that were never glaciated.  These boulder-strewn fields preserve a relict landscape of the terrain that existed before the ice age glaciers sculpted the ridges and valleys below.

The conditions are fierce at this elevation, with extreme cold for most of the year, and occasional hurricane-force winds. Trees cannot survive here, the only vegetation being low perennials and grasses that can tolerate the short growing season. This kind of tundra environment is more characteristic of northern Canada and Alaska.

At the highest part of the road we passed a herd of elk relaxing in the sun. Once we crossed Milner Pass, we started descending into the Kawuneeche Valley, the headwaters of the Colorado River. A spectacular ridge forms the western side of the valley, the Never Summer Range. The Never Summers are composed in part of much younger plutonic (granitic) rocks than the ancient metamorphic rocks forming the core of the park. The rocks formed in Cenozoic time (25-30 million years ago) during a time of profound change in the American west. The San Andreas fault was beginning to develop, the Sierra Nevada were beginning to rise, and volcanism was sweeping across Nevada and Utah.

We were considering staying at park's other campground at Timber Creek, which seemed to have plenty of openings for some reason (we were able to stay the previous two nights on the east side only because of cancellations by others). We also noticed that although the views of the Never Summer Range were spectacular, something was terribly wrong with the forest. The trees were dead...everywhere. What happened here? That will be the subject of the next post...

A Convergence of Wonders, Day 9: Into the Depths of the Crust, and of Time

We've been traveling through the Pacific Northwest and northern Rocky Mountains on a class in geology and archaeology for the last nine posts. Yesterday we made our way south from Glacier National Park over a corner of the Great Plains. Today (that is, June 23rd) we would be headed someplace different: down to the deepest part of the Earth's crust, and into the depths of geologic time. We were going to have a look at some of the oldest rocks on the planet.

How does one get to the base of the Earth's crust, or even into the mantle? Given that the base of the crust is 15 or 20 miles beneath us, and the deepest tunnel ever dug is 2 1/2 miles, one cannot walk or ride there. What we have to do instead is find a place where the crust has been brought up to us. Such a place is the Beartooth Mountains on the Montana/Wyoming border near Yellowstone.

In late Cretaceous and early Cenozoic time, around 70-50 million years ago, the crust in the Rocky Mountains was being twisted and deformed by an errant and misguided slice of Pacific Ocean crust that had somehow become trapped sliding along the base of the continental crust until it reached Montana and Wyoming, where it was finally able to sink. The mountain-building event, which formed much of the Rocky Mountains (including the mountains around Glacier National Park), is called the Laramide Orogeny. The rocks of the Beartooth Mountains were pushed up and over Cretaceous sedimentary rocks. Way, way up. The rocks originated in the deepest part of the continental crust, and these rocks are old. Very, very old.

Our first stop was within the Stillwater complex, a unique sequence of rocks that may have originated in the deepest parts of the crust, and which may have had an ultimate source in the Earth's mantle very close to the outer core. The Stillwater complex is a layered intrusion, a pluton composed of various kinds of peridotite and gabbro (the rocks are composed largely of the mineral olivine, which is also known as the gemstone peridot). It formed 2.7 billion years ago, making these rocks almost the oldest we would see  on the trip (more in a moment). Such complexes are quite rare at the Earth's surface, and contain an interesting mix of rare elements and minerals. We were parked near the Stillwater Mine, which is actively extracting platinum, chromium, and other rare metals. The mine dumps include some nice samples of magnetite, olivine, pyrrhotite and other interesting minerals.

By early afternoon, we were done with the Stillwater, and headed to Red Lodge for a class in the park. The students were listening with rapt attention, they said. Their closed eyes made it easier for them to concentrate on the meanings of the words they were hearing. That's what they said, and since students in my classroom are always saying the same thing, it must be true...

Red Lodge marks the beginning of one of the most remarkable roads in the United States, the Beartooth Highway. From an elevation of about 5,600 feet, the road climbs to the summit region of the Beartooth Plateau at just short of 11,000 feet. It is a marvelous place to see the work of glaciers, but even more stunning is the age of the rocks that the road is built on.

The rocks are composed of metamorphic rocks like gneiss, schist, and quartzite, with an occasional intrusion of granitic rock. The rocks formed between 2.7 and 3.3 billion years ago, which makes them very old (more than a billion years older than anything in California), but remarkably, fragments in the quartzite are even older! Zircon is a very tough mineral that resists being destroyed by erosion or metamorphic activity. Grains of zircon survive the Earth's recycling process that tends to destroy almost any other mineral. Quartz is another durable mineral, but it cannot usually be dated easily, but zircon can be dated. Grains of zircon in these mountains have been dated at 4 billion years. For comparison, the Earth itself is 4.6 billion years old. These grains in these rocks are the most ancient objects I've ever held that didn't fall to Earth from space (meteorites are generally leftovers of the origin of the Solar System and are the same age as the Earth).

The Beartooth Highway provided the most spectacular glacial features seen on our trip outside of Glacier National Park. The picture above shows a wonderful example of a U-shaped valley. Glaciers tear away at the walls of a valley, unlike a river, which only erodes the valley bottom. Glaciers cannot turn corners well, so the U-shaped valleys tended to be very straight. Hanging valleys, smaller glacial troughs that couldn't cut to the same level as the trunk glacier, are seen high on the main valley walls.

The Beartooth Mountains take their name from the "fang" seen in the picture below, beyond the head of the circular valley called a cirque. These bowl-shaped valleys in the highest reaches of the mountains were the origin point for the glaciers (snow would blow off the highest summits and ridges, so glaciers couldn't form on them, but in the shaded cirques instead). Sharp knife-edged ridges between glacial valleys are called aretes (not pictured).

It was strange to drive from summer to winter in the space of an hour. The road had opened to traffic only a week or so before we arrived.

The summit plateau provided a wonderful panorama of the Beartooth and Absarokapre-European period.

As we drove out of the Beartooths towards tiny (and somewhat unfriendly) Cooke City, we had a nice view of Pilot Peak, an outstanding example of a glacial horn, a spike of rock that has been plucked by glaciers from three sides or more.

Driving through the Lamar Valley in the late afternoon, we were reminded of just how big the snowpack was this year, and how big the flooding danger was. The road was being undercut by the surging river.

We had arrived in Yellowstone National Park! We didn't have much chance to explore, as the sun was nearly down, and our camp was on the other side of the park, at Madison. And Yellowstone is a big park. Our explorations would start in the morning...