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.

Yosemite as an Open Air Classroom: Siesta Lake

I headed up to Tuolumne Meadows this last weekend to have a last look at the Yosemite National Park high country before the roads are closed for the winter. The landscape certainly looked the part, the oaks and aspen were bright yellow, some early snowpatches covered the peaks, and there was a cold wind blowing; it was a delightful day!

I made a short stop at one of my favorite teaching sites, Siesta Lake, a short distance from White Wolf Campground on the Tuolumne Meadows Road, about 10 miles up the hill from Crane Flat. Why is it special?

Studying the effects and timing of glaciation in Yosemite National Park can be problematic, especially if you confine your studies to the floor of Yosemite Valley. The moraines are subtle, and some are missing because of postglacial erosion. The whole valley floor is covered by relatively thick forest, and ironically, it's hard to find many exposures of solid rock to observe glacial polish, striations and grooves (they are there, but you have to know where to look). The valley is not even the right "shape" (like a 'U') because a post-glacial lake filled the valley floor with sediment. Mass wasting has greatly modified the valley walls.

Other parts of the park offer some fine learning opportunities. Siesta Lake is a great place to see the evidence of three glacial stages with a few hundred yards of the road. As can be seen on the geologic map of the park (USGS Miscellaneous Investigations Series I-1874 by Huber, Bateman and Wahrhaftig, 1989), the lake is formed behind the moraines of a small glacier that came down the ridge at the headwaters of the South Fork of the Tuolumne River. The glacier was probably never more than a mile or two long, so the moraines from three different glacial stages are a short distance from each other. In the second photo above, the tills of two stages are in the same picture, with the younger Tioga deposits in the foreground (the angular bouldery deposits) and the older Tahoe deposits on the ridge. The Tahoe boulders are considerably more rounded and weathered. The Tioga stage lasted from around 20,000 to 13,000 years, while the older Tahoe stage took place somewhere between 65,000 and 160,000 years ago (kick the beehive and dozens of Pleistocene researchers will buzz about and start arguing about the specific dates).

A walk up the slopes behind the lake reveals barren granite, with polish, striations, chatter marks and other signs of glacial scraping. My students are usually able to figure out the general relationships and sketch them out on a simple geologic map.

It's a great open-air classroom!