Croes Ddwyafan Quarry
Seems mostly to be called "Croes y Ddwy Afon" (perhaps rename?)
This quarry (OSGR SH754424) lies a good couple of miles to the SE. of Manod Mawr. It was an underground working, worked sporadically for about 50 years after about 1870, the slate being carted out.
It was re-opened in the late 1980s (as an open pit) and most of the remains seen today relate to this. However, the original dam and leat system (from the Afon Gam) can still be seen.
It can be found by taking the Yspyty Ifan road from Llan Ffestiniog, not far from the junction with the Bala road. Llyn y Morwynion is also nearby.