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