a tarn is a lake but a lake is not necessarily a tarn- the dictionary on the web is vague as I guess one would be for a printed version.
The key seems to be-" especially one formed by glaciers."
But if snow melted inside a crevasse it could also fit that definition. It requires more thought than a dictionary...
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http://www.dictionary.com
tarn ( P ) Pronunciation Key (tärn)
n.
A small mountain lake, especially one formed by glaciers.
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[Middle English tarne, of Scandinavian origin.]
lake1 ( P ) Pronunciation Key (lk)
n.
A large inland body of fresh water or salt water.
A scenic pond, as in a park.
A large pool of liquid: a lake of spilled coffee on my desk.