Tarn/tɑːrn/ Definition 1 of 1Pro Tip 1/3Glacial Origin A 'tarn' is specifically formed by glacial activity, not just any small lake. The valley's tarn bore clear marks of its glacial origins. Pro Tip 2/3Mountain Setting Tarns are found high in mountains and may require hiking to visit. After hours of climbing, we reached an isolated tarn. Pro Tip 3/3Natural Beauty Tarns often represent untouched natural beauty, ideal for scenic descriptions. The sunrise over the tarn was breathtaking.