CO₂ Fell So Antarctica Could Rise

A misconception that has been circulating around the internet since (I assume) long before I made my Twitter account, is that through Earth's geologic past CO₂ and climate change have never been connected. At least not in a way where CO₂ drives climate change. But any robust analysis of the geologic record reveals a pretty simple truth: CO₂ is an important "control" of climate change, and to deny that role is to leave major gaps in our understanding of the climate system. The gap I will be filling in this post is the initiation of permanent ice sheets in Antarctica some 34 million years ago. A time when CO₂ fell, and Antarctic ice expanded to the continental scale.

Let's avoid oversimplification. CO₂ is not the only driver of climate change. There are many, many others and they are very, very important. For example, many of the long-term (think on the scale of millions of years) climate changes seen in the geologic record cannot be explained without considering the tectonic changes to the world's continents and ocean basins. As continents move around like adrift puzzle pieces, they separate and join ocean basins. These changes reconfigure ocean circulation, and hence climate.

The birth of continental-scale glaciers in Antarctica was for a long time believed to be the result of the formation of the Antarctic Circumpolar Current ("ACC" for short). Today, this current is very important for keeping warm water away from Antarctica. 65 million years ago, Antarctica and Australia were connected like North America and South America are connected today. The water on one side of these combined continents cannot flow to the other side, inhibiting circumpolar ocean circulation. But after 65 million years ago the continents began to separate, eventually allowing for the circumpolar flow of water around Antarctica. Kennett (1977) concluded that this separation of Australia and Antarctica, forming the ACC, was the kick the climate system needed to grow sea ice in the Southern Ocean 34 million years ago, and permanent ice sheets 15 million years ago.

Kennett was not far off the mark with this theory. Around 15 million years ago marked the formation of the East Antarctic ice sheet. However, we now know from sediment cores sampled from near Antarctica that permanent glaciation began 34 million years ago (Barrett, 1996). What does this have to do with CO₂?

In 2003, DeConto and Pollard approached this problem from a modeling perspective. They wanted to see what effect, if any, atmospheric CO₂ had on glaciation in Antarctica. A good question to ask because in 2003, the reconstructed atmospheric CO₂ record had a gap from 40 million years ago to 25 million years ago -- which happens to be the timeframe of the study question.

Reconstructed partial pressure of CO₂ from 55 million to 0 million years. Note the gap in data. DeConto and Pollard (2003)

Pretty inconvenient, but the researchers pressed on. They considered three key factors: (in no particular order) the first being orbital cycles -- which are responsible for the waxing and waning of ice sheets in the current geologic period, the second being a gradual decrease in atmospheric CO₂ which they suspected was happening during that gap in data, and ocean circulation.

A benefit to modeling studies is the ability to control certain parameters of the climate system in order to test a certain scenario important to your study question. This is what DeConto and Pollard do in their study. With a decrease in CO₂, they modeled two Antarctic glaciation scenarios through a 10-million-year experiment. The first scenario, in their model, they did not allow for the ACC to form. In the second scenario, they did allow for the ACC to form. For both scenarios, atmospheric CO₂ concentrations fell.

Model runs in the DeConto and Pollard (2003) study. With the Drake Passage open (ACC formation) glaciation started early. With the Drake Passage closed (no ACC formation), glaciation started later. Glaciation are shown by a decrease in the change in sea level. From DeConto and Pollard (2003).

The authors of this 2003 study found that, as atmospheric CO₂ declined, permanent ice sheets formed in Antarctica in both scenarios. But, to the deserved credit of Kennett, CO₂ had to decrease further in the closed Drake Passage (no ACC) scenario than in the opened Drake Passage (ACC formation) scenario. This means that, for the formation of permanent ice sheets in Antarctica, CO₂ played a primary role; and the opening of the Drake Passage played a secondary (but still important!) role.

Of course, models are not everything. In 2003, key information about this time in Earth's history -- and Antarctic history -- remained a mystery. Modeling needed to be more robust, and we needed a better understanding of how CO₂ changed during this time.

In 2005, Pagani et al found that atmospheric CO₂ did decrease during this time, which was a very strong indicator that DeConto and Pollard were right. Then came Galeotti et al. (2016), which confirmed DeConto and Pollard's findings. Galeotti and colleagues studied an ocean sediment core off the coast of the East Antarctic ice sheet in the Ross Sea. Cycles in the type of sediment through the core reflect glaciation changes through orbital cycles. By seeing how these sediment cycles changed through the core, they figured out how extensive the Antarctic ice sheet was from 34 to 31 million years ago.

This 2016 study found that prior to 34.8 million years ago (high CO₂), Antarctica had a pretty small ice sheet. It did not extend all the way to the coast like it does today.  This smaller ice sheet was more sensitive to orbital cycles, waxing and waning according to amount of solar insolation it received. After 34.8 million years ago (low CO₂), however, Antarctica had a more extensive ice sheet and became less sensitive to orbital cycles. The authors of this study determined that at 34.8 million years ago, atmospheric CO₂ fell below 600 ppm, which they believe to be a threshold for extensive Antarctic glaciation.

Many things are responsible for climate change and the size of ice sheets. But to deny the important role of atmospheric CO₂ is to limit our understanding of the Earth's climate system. CO₂ is an important driver of climate change. And as we increase atmospheric CO₂ into the future, we threaten the ice sheets we have today.

Ice Sheet Memory Foam Mattress

Ice sheets are beautifully complex systems, but it's easy to think of ice sheets and the environment as a one-way relationship: the environment changes, then the ice changes -- environment to ice. But ice sheets have an influence on the environment just as strongly as the environment has an influence on them. This post will talk about an interaction that I have been obsessed with (and nerding out over) recently: glacial isostatic adjustment (GIA, for short).

Imagine the most comfortable bed you can possibly sleep on. The type of mattress that you just sink in to and is the hardest goodbye on a Monday morning. Glacial isostatic adjustment is like that, but a several-kilometer thick ice sheet is you, and your mattress is the Earth's mantle.

When you lie down on a mattress, you exert your weight on that mattress -- compressing the inside of your beloved foam rectangular prism -- causing the surface on which you rest to sink. Ice sheets on continents are doing the same thing, just on a larger scale. Ice sheets are big. And, as you would expect from that detail, they are also very heavy. So heavy, in fact, that they weigh down the upper mantle beneath them. This causes the ground to subside.

"Thickness of the Ice Sheets" - xkcd comic

The Earth's mantle is deformable, and we can think of the mantle as having a viscosity. Viscosity is the measure of how resistant a substance is to deformation. A substance that is very resistant to deformation is described as having a high viscosity, while low viscosity fluids are less resistant to deformation.

When falling onto your mattress after a long day, think about how far you sink into it, and how quickly. If it's fast, that mattress isn't effectively resisting your force. For the load of an ice sheet, this would mean the mantle below has a low viscosity. The mantle quickly adjusts to the stress applied by the ice sheet. Alternatively, if you slowly sink into your mattress, that's like an ice sheet above a high viscosity mantle. The adjustment is slow.

Your alarm blares, and now you have to get up and get ready for the day ahead of you. You muster up the courage to sit up, then get off your bed completely ... just slowly. Depending on the type of mattress you have, it will rebound as you get up at varying speeds. Memory foam mattresses rebound slowly. Very slowly. Other mattresses might rebound quickly, maybe a second or two after getting up. For an ice sheet (or lack thereof), this is called post-glacial rebound.

Rate of bed elevation change (geoid rate) in [mm/yr]. The bed elevation is rising (blue) in areas that are either losing ice (West Antarctica) or was once covered with ice (North America, Eurasia). (From http://grace.jpl.nasa.gov)

In my next post, we will explore the implications of glacial isostatic adjustment on ice sheets and the world as a whole. Until next time!

--

Some Sources:

ICE5G-D:

• Peltier, W. R., R. Drummond, and K. Roy (2012), Comment on “Ocean mass from GRACE GRACE and glacial isostatic adjustment” by D. P. Chambers et al., J. Geophys. Res., 117, B11403, doi:10.1029/2011JB008967.
• Peltier, W.R., 2004. Global Glacial Isostasy and the Surface of the Ice-Age Earth: The ICE-5G(VM2) model and GRACE, Ann. Rev. Earth Planet. Sci., 32, 111-149.
• A, G., J. Wahr, and S. Zhong (2013) "Computations of the viscoelastic response of a 3-D compressible Earth to surface loading: an application to Glacial Isostatic Adjustment in Antarctica and Canada", Geophys. J. Int., 192, 557–572, doi: 10.1093/gji/ggs030.

ICE6G-D:

• Peltier, W.R., Argus, D.F. and Drummond, R. (2018) Comment on "An Assessment of the ICE-6G_C (VM5a) Glacial Isostatic Adjustment Model" by Purcell et al. J. Geophys. Res. Solid Earth, 123, 2019-2018, doi:10.1002/2016JB013844.

Caron-2018:

• Caron, L., Ivins, E. R., Larour, E., Adhikari, S., Nilsson, J., & Blewitt, G. (2018). GIA model statistics for GRACE hydrology, cryosphere, and ocean science. Geophysical Research Letters, 45, 2203–2212. https://doi.org/10.1002/2017GL076644

xkcd comic: https://xkcd.com/1225

West Antarctica Is Vulnerable

The West Antarctic ice sheet is the ice sheet in Antarctica that is the most vulnerable to climate change. But the ice sheet itself melting isn't quite how the ice sheet will add to sea level rise. It might be weird to think about, but glacier ice doesn't always need to melt to add to sea level rise! And this makes communicating the future of Antarctica difficult to communicate.

It's important to note that there is melting involved in Antarctica's contribution to sea level rise. Especially in West Antarctica. But it's not the grounded ice (land ice, glaciers) melt that is responsible for West Antarctic ice loss.

Mass balance is an important term for glaciology, but certain types of mass balance don't account for everything that happens to the ins and outs of glaciers and ice sheets. Surface mass balance is the mass balance (net ice gained or lost) at the surface of glaciers and ice sheets. Snow can fall on the glacier adding to the surface mass balance, ice can melt or sublimate off the glacier reducing surface mass balance. It's like being an observer at the front door of a hotel: you can get a rough estimate of how many people are in the hotel at a time by tracking how many people leave and enter through the front door.

But some folks at that hotel might be parked behind the hotel, and they have to leave through the back door. But you can't see the back door! And if you're only paying attention to the people coming and going through the front door, you might not notice the hotel becoming vacant.

Surface mass balance of Antarctica and Greenland from 1989-2009. Notice that Antarctica mostly has a positive surface mass balance. Figure from Van den Broeke et al., 2011.

Observing Antarctica through surface mass balance is like watching people enter and leave through the front door of a hotel. We might not be capturing what is actually happening to the ice. Because of that back door, it is entirely possible for a glacier or an ice sheet to have a positive surface mass balance and still undergo thinning and retreat.

How is this possible? Antarctica, particularly the West Antarctic Ice Sheet, is vulnerable to changes that do not depend on the surface mass balance -- like a back door. And these changes occur underneath the ice, and on the margins of the ice sheet itself. At the margins of ice sheets like the West Antarctic ice sheet is the very important system between glaciers (grounded ice) and ice shelves (floating ice, attached to glaciers). The physical mechanism of total mass loss is the dynamics that occur within this system.

When I say total mass loss, I am talking about total mass balance. Total mass balance involves processes at the surface and processes below the ice that may contribute to ice thinning or thickening. Front doors and backdoors.

Antarctic scientists are concerned with the melting at the base of ice shelves. Ice shelves alone cannot contribute to sea level rise, however, because they are already floating in water. But ice shelves do play an important role in glacier dynamics at ice sheet margins, dynamics that can cause total mass loss.

Think of glaciers as flowing rivers. Glaciers flow toward the ocean. Attached to many important glaciers in Antarctica are ice shelves. Think of them as dams. They push back against the flow of the glacier in a way that they regulate how quickly ice flows towards the sea. This relationship exists due to Newton's third law of motion. Every force is negated by an equal and opposite force. The glacier pushes on the ice shelf, and the ice shelf pushes back (put very simply).

The boundary between the glacier and the ice shelf is called the grounding line. This line separates the grounded ice (ice than can contribute to sea level rise) and floating ice (ice that can't). Ice that passes the grounding line adds to sea level rise. By melting ice shelves, we weaken their ability to push back on the flow of the glacier. This allows more ice to cross the grounding line.

Antarctic ice mass change tracked through GRACE, a NASA mission that observes gravitational differences across the Earth's surface and time. This shows that the West Antarctic ice sheet is losing mass, despite a positive mass balance. From NASA

And when more ice crosses the grounding line, we get sea level rise. Going forward, the grounding lines in Antarctica will be very important. But scientists are currently working on finding out what the future will hold for the ice sheet. And sometimes to answer questions about the future, we have to look to the past. But that's a different post...

Sources and Whatnot:

• Antarctic Ice Sheet surface mass balance (antarcticglaciers.org)
• An introduction to Glacier Mass Balance (antarcticglaciers.org)
• Antarctic Ice Loss 2002-2020 : GRACE Tellus (nasa.gov)
• Van den Broeke, Michiel R., et al. “Ice Sheets and Sea Level: Thinking Outside the Box.” Surveys in Geophysics, vol. 32, no. 4-5, 22 June 2011, pp. 495–505, https://doi.org/10.1007/s10712-011-9137-z.

Surface Strain Rates and Rock Glaciers

One of the most beautiful Colorado trails I've hiked was the Spruce Creek trail, passing the Mohawk Lakes, and ascending a glacier-carved cirque. I wrote about the glacial geology of this place in this blog post. At the end of this cirque is a debris-covered glacier (also called a rock glacier) that may or may not still be active.

To recap, a rock glacier is a glacier that has a surface covered in debris -- mainly from rockfalls and avalanches from nearby mountains. This means that beneath its rocky outer shell is a frozen core of ice nougat. But what makes that inner core of ice active as opposed to inactive? Glacier ice is considered "active" when the following things are true:

1. First and foremost, the ice needs to stick around for multiple years. The ice needs to be perennial in a sense that you could revisit the pile of ice years later and it could possibly still be there. Of course, no glacier is truly permanent.
2. The ice within the glacier needs to form from the compaction from snow into ice. Essentially, if the ice at some depth below the glacier's surface isn't being crushed by the weight of the ice above it -- enough to turn fluffy snow into hard ice -- it's not a glacier. So, that snow field that you see at the top of that mountain every August isn't a glacier. It's just a pile of snow.
3. A glacier needs to show evidence of internal deformation currently taking place. Meaning that more than snow being compacted to ice and perennialism, the ice needs to be flowing and deforming. The glacier ice crystals need to be rearranging themselves under pressure.

The pressure within a glacier, also called stress (i.e.: a force per unit of area across a cross-sectional surface) is the result of the weight of the ice. Stress, \(\tau\), is typically measured in pascals, which is a unit of pressure. With enough stress acting on the ice, the ice within the glacier will deform. How much the ice deforms (stretching or compressing) is called strain. Strain, \(\epsilon\), is represented as the ratio of a glacier's dimensions after deformation to its dimensions before deformation. Strain is therefore dimensionless.

Glaciologists, however, are more interested in the strain rate \(\dot{\epsilon}\) which has a dimension of [time]\(^{-1}\). Fundamentally, stress and strain rate are related through a power law famously known as Glen's Law:

\[\dot{\epsilon}=A\tau^n\]

Where \(A\) is a quantity dependent on the temperature of the ice and \(n\) is some constant (typically equal to 3).

Sometimes in glaciology, it is important to test the strain rate at the surface of a glacier. For example, if you want to determine whether or not the Spruce Creek rock glacier is still active, finding the surface strain rate (if any) would be ideal. A glacier like the Spruce Creek rock glacier, however, (if moving) moves very slowly. Thankfully, way back in the 1950s, a field method that allowed scientists to observe very small surface strain rates was developed. And this method was adopted (and slightly modified) by scientists in 14-year-long study particularly on the Spruce Creek rock glacier, from 1985 to 1999.

The original method described by Nye (1959) is to simply place a number of stakes on the surface of a glacier. These stakes should be grouped into squares with 4 stakes at each corner and 1 stake in the center.

The ideal stake setup pattern. From Nye (1959).

These squares should be distributed along the length of the glacier. Then, for some period of time, the length of the sides and diagonals of the squares are measured and recorded. The strain rate is found by using the ratio of the final length \(l_2\) and the initial length \(l_1\) across a time interval of \(\Delta t\).

\[\dot{\epsilon}=\frac{1}{\Delta t}\ln\left( \frac{l_2}{l_1} \right)\]

With 4 sides and 4 diagonals for each square, each square yields eight surface strain rates. Nye reduces this to four surface strain rates by averaging the strain rates for parallel sides. For example, the measured strain rates for the length of \(b_1\) and \(b_2\) would be averaged to one value.

The surface strain rates for 4 directions at every square from the Nye (1959) study. I boxed the strain rates for each direction in red.

To sum it up simply, however, the distance between each stake is measured and used to calculate the surface strain rate.

This method was essentially used in the 1985-1999 study on the Spruce Creek rock glacier in Colorado. However, the rock part of the rock glacier didn't exactly allow for the use of stakes. Leonard and others (the folks who studied the rock glacier) used boulders instead of stakes, since there is no lack of boulders on the surface of the rock glacier.

The strain rates measured from a 14 year study (1985-1999) of the Spruce Creek rock glacier. Data from Leonard et al. (2005).

	Strain Rate (\(10^{-4}\) yr\(^{-1}\))
Maximum	14.7
Minimum	0.1
Mean	2.8\(\pm\)0.3

An important observation from the data above is how small the strain rates are. The mean strain rate is 2.8\(\times 10^{-4}\) yr\(^{-1}\). That is 0.00028 [years]\(^{-1}\)! This means that over the course of the study period, the Spruce Creek rock glacier was deforming, just very slowly. In terms of the surface velocity of the glacier (unit of distance per unit of time calculated on three transects of the rock glacier's surface), the flow averaged from 4 to 7 centimeters per year (Leonard et al., 2005) from 1991 to 2000.

So, why the slow flow? Leonard and others list several possible explanations:

1. The Spruce Creek rock glacier has a shallow slope. And because stress is determined by gravity, and therefore slope, a low slope would mean low flow rates. However, they bring up the possibility that this could not be the case, citing Frauenfelder et al. (2003), who argues that slope and surface velocity have little correlation for rock glaciers.
2. Temperature could control the flow rate of rock glaciers. The modern climate (well, the climate from 1985-1999) of the mountains where the rock glacier resides might be the cause of the slow-moving flow.
3. Most of the internal structure of the Spruce Creek rock glacier could be non-deformable (debris-rich), with the deformation mainly occurring where there is more ice. However, the internal structure of the Spruce Creek rock glacier is a mystery.
4. The glacier could be stagnating as a result of ice loss due to climate change coming out of the Little Ice Age and human-caused global warming. According to Leonard et al. (2005), the glacier has been slowing down over the past century.

The data on rock glaciers, and the Spruce Creek rock glacier is lacking. So, it is hard to determine the cause. Could be one, could be more, or it could be none of the above.

I haven't found a study on the status of the Spruce Creek rock glacier today (or more recently). So, whether or not the rock glacier is active today is a mystery to me and to scientists.

References:

• Cuffey, K M, and W S B Paterson. The Physics of Glaciers. 4th ed., Amsterdam, Butterworth-Heinemann, Cop, 2010, pp. 54–55.
• Frauenfelder, R., W. Haeberli, and M. Hoelzle. "Rockglacier occurrence and related terrain parameters in a study area of the Eastern Swiss Alps." Proceedings 8th International Conference on Permafrost. Swets and Zeitlinger, Lisse. 2003.
• Leonard, Eric M., et al. “Kinematics of Spruce Creek Rock Glacier, Colorado, USA.” Journal of Glaciology, vol. 51, no. 173, 2005, pp. 259–268, https://doi.org/10.3189/172756505781829403.
• Nye, J F. “A Method of Determining the Strain-Rate Tensor at the Surface of a Glacier.” Journal of Glaciology, vol. 3, no. 25, 1 Jan. 1959, pp. 409–419, https://doi.org/10.1017/s0022143000017093.

Glacial Geology of the Mohawk Lakes (CO)

Mohawk Lakes via the Spruce Creek trail topo map.

There is glacial geology all over the Rocky Mountains and if you hike a lot up there, you have likely seen it. Sometimes it's hard to see, like the old (very old) glacial moraines under the Independence Chair of Breckenridge (but I think that is another blog post). Other times, the glacial geology of the Rockies are staring you right in the face ready to be looked at and admired. These places are in the broad U-shaped valleys and cirques nestled under the high peaks of the area.

I completed a hike recently that was full of this stare-you-in-the-face glacial geology. So in-your-face about it, in fact, that a glacier is still there -- though much smaller than it once was. This in-your-face glacial geology (pyramid-shaped peaks, U-shaped valleys, cirques, moraines, lakes, etc.) were left behind by glaciers that once dominated this mountain range during the last ice age.

This hike was along the Spruce Creek trail going up to the Mohawk Lakes just outside of Breckenridge. After leaving the forest on the first couple miles of the hike, you enter a beautiful landscape sculpted by a long-retreated glacier. A steep climb parallel to a waterfall is where you get the first hints of ancient glaciation: exposed bedrock. Rather than jagged and rough, the bedrock that the trail meanders through are smooth and polished by the glacier that once filled the valley you are ascending. But it's not all smooth! The bedrock is decorated with lines like a freshly groomed ski run.

Striations on the exposed bedrock next to the trail. Sorry I forgot to put something on the ground as a size reference, but hopefully the plants in the shot will give some clarity of scale.

These lines are called striations, which results from glaciers scraping jagged rocks and debris along the glacier-bed interface. The lines are typically parallel to the direction of ancient ice flow. And if you like geology beyond glaciers, you are looking at biotite gneiss from the early Protozoic.

The Mohawk Lakes are surrounded by this bedrock and make for one heck of a scenic area. But my favorite part is further up the trail. After passing Mohawk Lake (the one above the Lower) you climb just a little more to a tiny lake at the tree line. Here, you can see up the cirque all the way to Pacific Peak and the debris-covered glacier below it -- now much smaller than it was when it covered the bedrock you're walking through. You can see the multiple lakes ahead of you, which consist of kettles (possibly) and tarns.

With the bedrock now behind you there are wildflowers and out-of-place boulders scattered all the way up the valley. You are walking over the till of the Pinedale glaciation from the late Pleistocene sprinkled with erratic boulders. Till is simply glacial sediment deposits and features rocks and grains of all sizes in a poorly sorted mess. It seems uniform from the ground, but below those wildflowers are unsorted sediment deposits. The sediments here were deposited during the Pinedale glaciation. Assuming that the sediment deposits were from the glacier's retreat, likely between 21 thousand (glacial maximum) and 10 thousand years ago (end of the Pinedale glaciation).

Hiking above the till. The trail did not pass through a location with a good view of what till looks like, but trust me bro ...  it's down there.

Then there are the glacial erratics. Boulders that were left behind by the glacier on top of the till. In the picture above, you can see some small erratics. I took a picture of a much larger one closer to the bedrock area.

A larger erratic boulder.

Erratic boulders can provide valuable information about the previous extents of glaciers. Using the chemistry of the rock, scientists can know when an erratic boulder left the shade of the ice and into the outside world -- left behind as the glacier retreats upward.

Passing the lakes, you eventually arrive at a moraine -- a pile of glacial sediment deposits and erratics. Just beyond the moraine, however, is the star of the show: the debris-covered glacier.

Looking at the geologic map of the area, the debris-covered glacier is defined as "active rock glacier deposits" from the late Holocene (very recent). This was true, at least, in 2005 (Keller et al. 2005). So, why is there still a glacier here when most of the glaciers of Colorado have long disappeared?

Bird's eye view of the Spruce Creek rock glacier.

Debris-covered glaciers, also called rock glaciers, are glaciers that are covered in debris. This debris is typically provided by landslides and rockfalls from the surrounding mountains. Note that there is glacier ice beneath all this debris. How thick this debris layer above the surface of the glacier ice is tends to increase in the lower elevations of a glacier's reach -- in the ablation area where ice melts off a glacier. This detail about where the debris layer is thickest is important for the mass balance (the balance between accumulation; ice gain, and ablation; ice loss) of the glacier.

A thick layer of debris is like a layer of armor for the glacier. When the climate warms, whether through natural or anthropogenic processes, the added warmth to the surrounding environment takes time to penetrate through the layer of debris and into the ice. In other words, the debris layer insulates the glacier. The response of rock glaciers to climate changes is therefore much slower than a glacier without a debris layer protecting it.

Now, 13 thousand feet in elevation in Colorado is still pretty darn warm. So, active rock glaciers are not abundant in the Ten Mile Range of the Rockies. This particular rock glacier is a gem. Or maybe was a gem. The last study done on this glacier was in 2005, and it was found that the glacier was moving very slowly back then (Leonard et al. 2005). It is possible that the glacier could no longer be flowing. But I think that it's pretty cool that this glacier was flowing in my lifetime.

Sources and Whatnot:

1. “Cirque - an Overview | ScienceDirect Topics.” Www.sciencedirect.com, 2022, www.sciencedirect.com/topics/earth-and-planetary-sciences/cirque.
2. “Glacial till and Glacial Flour (U.S. National Park Service).” Nps.gov, 2018, www.nps.gov/articles/glacialtillandglacialflour.htm.
3. Leonard, Eric M., et al. “Kinematics of Spruce Creek Rock Glacier, Colorado, USA.” Journal of Glaciology, vol. 51, no. 173, 2005, pp. 259–268., doi:10.3189/172756505781829403.
4. “Tarns (U.S. National Park Service).” Nps.gov, 2018, www.nps.gov/articles/tarns.htm.
5. Wallace, C., Keller, J., McCalpin, J., Bartos, P., Route, E., Jones, N., Gutierrez, F., Williams, C., and Morgan, M. L., 2005, Geologic Map of the Breckenridge Quadrangle, Summit and Park Counties, Colorado: Colorado Geological Survey, Open-File Report OF-02-07, scale 1:24,000