From Tolkien Gateway

Irtinsa was a lake in Valinor, according to the early version of the legendarium in The Book of Lost Tales.

It was built by the Valar following the creation of the Sun and Moon, and was located just south of Valmar. It was surrounded by dark yews set as a maze, and its walls were made of "silver and white marbles". It was also where the Vala Lórien hoarded the dew of the Rose of Silpion.[1]