Letter from Lane End 1868 on a Sunday Evening at 4.00 p.m. (General)
The Welsh for lake is Llyn as in Llyn Idwal and Llyn Ogwen in Snowdonia. However, lakes often occur in glacial valleys blocked by terminal morrains. The Welsh word for valley is Cwm and thus the valley in which Llyn Idwal lies is Cwm Idwal. Hence the confusion in English useage.
Chris Morgan.