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...

In the last post, we saw Rocky Mountain National Park turn black, white, and shades of gray during an intense thunderstorm. It was another day on our vagabonding journey across the 39th parallel, a trip that explored the geology across Nevada, Utah and Colorado last July. I've been expounding on our discoveries off and on ever since. We managed to score a campsite for a second night in Rocky Mountain National Park, which gave us time for a short hike into the realm of the glaciers.

Our campsite the first night was in a rocky gorge just downstream of the glacial terminus in Fall River Valley at Aspenglen. The previous day we had experienced a bit of magic on a walk around Bear Lake, one of the most popular stops in the park, and we decided we wanted to explore a bit higher, closer to the origins of the glaciers that once existed in the park.

The short hike to Nymph and Dream Lakes looked like it would do the trick. It shared a trailhead with Bear Lake, but climbed a few hundred feet in a mile to a pair of tarns, rocky basins scoured out by glaciers during the Pleistocene Ice Ages. It didn't take long to arrive at the lily-filled Nymph Lake.

An entire corner of the lake was covered with some kind of water-loving flower with strange stringy petals, whose identity I leave to you, the reader. This is because I am a geologist, who despite knowing hundreds of mineral and rock names, cannot remember the names of more than a dozen flowers or so...

The skyline above the lake was dominated by the tower of Hallett Peak (12,713 feet; 3875 meters), a reasonable example of an almost-horn. A glacial horn is a spiky kind of mountain peak that is surrounded by cliffs caused when glaciers plucked away at the base of the mountain. The Matterhorn in Italy/Switzerland is a famous example. Hallett Peak has indeed been plucked, but a considerable area of the original unglaciated surface still remains on the highest part of the mountain.

From Dream Lake a little higher up the trail, the profile of Hallett was clearer. One can see the u-shaped valley to the right of the peak. Rivers only erode the bottom of a valley, and mass wasting adjusts the valley walls into an V-shape. Glaciers scour the valley walls and the valley bottom, giving the valley a more circular aspect.

I was entranced by another flower (yellow this time; identification welcome!) along the trail.

Walking down the trail, I was more aware of the sweeping view we had to the south towards Longs Peak (14,259 feet; 4346 meters), the highest peak in the park. Glaciers actually had a challenge with the rocks of Rocky Mountain National Park. They consist mainly of hard granite and metamorphic rocks of Proterozoic eon, mostly between 1.7 and 1.4 billion years. The glaciers repeatedly tore away at these rocks during a series of glaciations starting about 2 million years ago and ending only about 13,000 years ago. There is clear evidence of three episodes, but independent climate records (on the ocean floor, for instance) suggest there were many more. A mere handful of glaciers remain, covering only a few acres.

We hiked back down the trail and saw the storm clouds gathering again. We were starting to appreciate just how sunny our hike had been as a second huge thunderstorm enveloped the region. This one was actually more fierce than our storm the previous day, and lasted for hours. We found that our tent had a slight vulnerability to heavy rain, and we watched helplessly as the tent filled with water. Finally, around midnight the storm let up (we had retreated to the car), so I grabbed a towel and cleaned things up as much as I could. In the morning we discovered that our campsite had a stunning view of Longs Peak that we couldn't have seen during the storm of the previous night. Strange lenticular clouds drifted around the summit of the mountain in the early morning.

It was time to cross the Continental Divide and start our homeward journey...in the next post we tackle the Trail Ridge Road.

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...