By Max Cunningham
June 11, 2014
Mike and I hiked down 7,000 feet from Mount Chirripó to the Cloudbridge
Reserve early on the morning of June 10th to refuel and replenish supplies.
At this point, the Cloudbridge Reserve deserves a special mention. Tucked away in the forest above San Gerardo de Rivas, volunteers at the Cloudbridge Reserve work to transform old farmland into natural forest. After the cold ruggedness of the Mount Chirripó summit, the volunteers at Cloudbridge provided an exceptionally welcoming and engaging environment. Mike and I were extremely lucky to have such a supportive base camp.
I kept an eye out for interesting geomorphology as I walked along the trails of the Cloudbridge Reserve. The rivers here are particularly beautiful. The water is clear and blue, and channel beds are floored by bedrock and boulders (all granodioritic in composition, like many of the rocks atop Mount Chirripó). I was struck by the power of the local rivers; the erosional features carved into this hard, granodioritic rock were impressive.
After two days of rest and catching up on all we’d missed while isolated on Costa Rica’s highest peak, Mike and I headed back up to Mount Chirripó to continue sampling and to learn more about the processes shaping this landscape.
During our second journey, we hoped to extend our sampling range by venturing farther into glacial valleys and higher onto peaks. We targeted Valle de Las Morrenas, a valley that we knew well from our first sampling trip and that other researchers had discussed extensively.
Earlier, we sampled boulders from moraines adjacent to large lakes. This time, we targeted a steep drop-off (what we called a “lip) that occurs in the valley directly below the lakes. Looking at maps and satellite images, it appeared that the lower valley was actually a remnant cirque:
Our discovery of a large lateral moraine in the lower valley corroborated our hypothesis that a glacier produced the pronounced lip in Valle de Las Morrenas. The vegetative cover increased substantially as we moved lower in the valley, which made accessing the moraine a real challenge. After pushing through thick, woody bushes, we finally found ourselves on the crest of the moraine.
From the image it’s hard to tell, but this is actually a pretty big moraine, about 50-60 feet in height. Meandering rivers cut through cobbles along the moraine’s edge, analogous to what we saw in Sabana de los Leones, only here with water raging through the channel.
We quickly came to realize that the boulder selection on the crest of this lower moraine was a far cry from the beautiful, large, flat boulders we saw along moraines in the upper valley. Here, boulders seemed to be more deeply weathered, and more sparsely scattered.
While the lack of good boulders for sampling induced a bit of hand wringing (made worse by storm clouds quickly moving up the valley), the effectiveness of weathering on these boulders may add to the story of glaciation at Mount Chirripó. Deep weathering of boulders suggests that they have been sitting around, exposed to the atmosphere, for a long time. How long? Glaciologists have employed relative weathering techniques for centuries to estimate exposure age, but 10-Be dating will tell us for sure.