Sheshnag lakes is created by melting glacier above on mountain.
Off the track--"Sheshnag" is the name of snake which God Shiva wears as a necklace.
The other things which God Shiva wears are :
Ardh Chandrama---Half Moon as a crescent
Ganga River-It is believed that when king Bhagirath meditated and requested God to send water on drying field,God sent Ganga River from heavens but the current of water was devastating and the danger of earth getting drowned increased, So God Shiva wrapped Ganga River in his long hair and allowed river to reach earth slowly.
But that slowness is also too fast for a common man ....U can see the current of Ganga river in places like Haridwar www.flickr.com/photos/jigisha/141606726/ and Rishikesh where one cannot even stand in water for a minute without holding chains tied to river banks.