16:29 Wed Dec 01, 2021
What in TARNation?
Have you ever wondered why there are small lakes scattered throughout the White Mountains? Examples such as Hermit Lake, Lonesome Lakes, and the Lakes of the Clouds are actually glacier features called tarns.
Tarns are defined as lakes, ponds, or pools that from in glacially carved cirques. Cirques are erosional features that have an amphitheater-like shape, such as Tuckerman Ravine. Obviously, there are no glaciers found in the White Mountains today, but until about 12,000-14,000 years ago, a large continental ice sheet known as the Laurentide Ice Sheet (LIS) covered the region.
There is some debate today in the geological community over whether these features are solely from the LIS or from local glacier formation and recession post-LIS. However, we’ll save that discussion for another time.
Cirque glaciers erode and deepen the hollows by subglacial plucking and abrasion.
As defined above, tarns are bodies of water that form in glacially carved cirques. Cirques are formed through the gradual growth of mountainside hollows. When snow accumulates within these hollows, gradual expansion occurs through a group of erosional processes such as freeze-thaw activity and chemical weathering. Eventually, enough snow accumulates and compacts to form glacial ice, which further expands the cirques through subglacial abrasion and plucking of the cirque floor and headwall.
Once the glacier recedes and melts, it leaves those familiar bowl shaped ravines seen throughout the White Mountains and here on Mount Washington. After the glacier has melted, water collects in the deepened hollow forming a tarn. So, you can thank glaciers for those picturesque alpine lakes throughout the White Mountains and the great turns skiing down the headwall of Tuckerman Ravine.
Lakes of the Clouds (tarn) on the south side of Mt Washington
Jackie Bellefontaine, Weather Observer & Education Specialist