Piles of rock tell the story of a dying Andean glacier

I grew up in a place that was covered by miles of ice just 20,000 years ago.

XKCD Ice Sheets.

I’m used to living with the ghost of the Pleistocene ice sheet, and searching for its telltale signs on the landscape. The geology field trips in which I learned to recognize those signs were some of the happiest days I’ve ever had. Really. Especially with this guy. It was like I had been living for years in a country where I didn’t speak a single word of the language and then after a few hours of study was I able to understand the conversations around me.

Needless to say, my heart goes all a-pitter-patter at the sight of suggestive piles of rock, a boulder that doesn’t quite match the local geology, and parallel scratches on bedrock. I learned to ski on a terminal moraine from the last ice age. I spent long moments imagining myself standing at the edge of that great ice sheet, staring straight up for 2000 meters at a vast white wall not at all unlike THE Wall. (It’s absurd that my blog is just 2 days old and already I’ve referenced the geekiest thing in the world.)

But for all my enthusiasm about glaciers, there is a distinct lack of them in Ohio. The ice retreated from the mid-western United States long ago, and it won’t be coming back any time soon. Can you imagine what it was like the first time I laid eyes on a glacier? I’ll spare you the misty-eyed details, but there was squealing. I’ve been told it was coming from me.  All that, and I was really just looking at a tiny remnant glacier nestled in a north-facing cirque in Glacier National Park.

So let me introduce you to my new friend, the outlet glacier that drops down from Nevado Jatunriti (also known by its somewhat less magical colonial name, Chumpe), a 6100 m high peak in the Cordillera Vilcanota.

A nice view at 5100 m above sea level

An outlet glacier on the south side of Nevado Jatunriti, Cordillera Vilcanota, Cusco, Peru. Taken from the lateral moraine, March 2013. Note the triangular peaks on the glacier terminus on the right side of the photo. Accumulation from snow is being outpaced by ice lost to sublimation. That ice is about to disappear forever. This river of ice is turning into a river.

And just for good measure, this is me getting my first view of the above.

Photo by T. Seimon

First view of the glacier up close. The skeptical and amused look on my face suggests I was about to drop an F-bomb.

All around the receding (read as dying) end of this glacier is geomorphological porn, or at least, textbook-worthy examples of post-glacial landforms. So let’s dive right in and get to learnin’ after a moment devoted to semantic solidarity: moraines are jumbled, tortured heaps of rocks and boulders deposited along the sides (lateral moraines) or at the end (terminal moraines) of a glacier. In the photo of me above, I’m standing on a 100 meter high lateral moraine from a time when that glacier was much more impressive, with ice to the top of my lookout. That time, it turns out, was the Little Ice Age. Keep reading!

Here’s the end of the outlet glacier and its debris field.


As glaciers recede, they retreat from the end and also get thinner (like deflating an air mattress). The lateral moraines tell the story of the ice thinning.

The red lines show the lateral extent of the snout of the outlet glacier in 2013. The yellow lines show the Little Ice Age maximum. The Little Ice Age was a brief period of global cool temperatures and glacial advance from ~1850-1900 a.d. The green lines show a moraine from an older, bigger glacial period, perhaps even the Pleistocene (the last ice age).

Receding glaciers tend to leave huge debris fields behind. Often, they leave a series of large terminal moraines perpendicular to the direction of the advance as they advance and retreat in cycles, bulldozing the debris into piles along their advancing edge until they finally retreat for good. There are big Pleistocene terminal moraines throughout the glaciated portions of the United States.

Our outlet glacier didn’t do much of the bulldozing thing. See the curved separation between the brown lobe of debris and the green vegetation of the wetland? No huge pile of debris. No terminal moraine. What does that mean? Maybe it means the ice rapidly retreated from its farthest excursion down the valley, without advancing again for many years. Can I get a geologist to weigh in?

If you look closely, small terminal moraines are evident farther up the debris-field. The cool part: those moraines have been dated!  These lines in the piles of rocks mark temporary advances and respite from what is likely an unstoppable retreat. I’ve outlined the moraines and their ages below. The date indicates the year that the ice was last at each moraine.

Dated images show steady recession of this alpine glacier in the Cordillera Vilcanota, Cusco, Peru.  Many of the moraines marked here correspond well with the ice extent in the dated images. The fuschia line isn’t a moraine, just an average position for the ice from the 1960-1980 period when there was apparently a rapid advance and retreat. This figure was adapted from Seimon et al. 2007.

So much from a few big piles of rocks. A geologist could tell us 100+ other fascinating things from this landscape. So, if any geologists would like to join me in the field…I shall give you all the nicest tent spots with plenty of alpaca and vicuña dung to soften your bed, and exchange a bit of frog catching for rock hounding.  Thanks for reading!

Literature cited: Seimon, Tracie, et al. (2007). Upward Range Extension of Andean Anurans. Global Change Biology vol. 13, 288–299


One response to “Piles of rock tell the story of a dying Andean glacier

  1. Pingback: Tropical Glaciers: Super-Sensitive Climate Indicators | Expedition Frog Blog·

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