Nant Teyrn is a small upland stream on Yr Wyddfa/Snowdon NNR. This ECN freshwater site is co-located with the Yr Wyddfa/Snowdon terrestrial site which has been in existence since 1997. The site is on the out flow from Llyn Teyrn. The geology of the catchment is nearly all highly acidic rhyolite and dolerite with a small amount of bedded pyroclastics.