The main period for which there is evidence of the glaciation of Snowdonia is the Devensian (the name for the last glacial period in Britain). Dozens of glacial cirques were excavated amongst which are those holding the lakes of Llyn Cau and Llyn y Gadair on Cadair Idris, Glaslyn and Llyn du'r Arddu on Snowdon, Cwm Idwal within the Glyder range and those holding Dulyn and Melynllyn lakes in the Carneddau. Major U-shaped valleys or 'glacial troughs' developed at this time include the Nant Ffrancon, Llanberis Pass and Nant Gwynant. Some still host lakes formed within glacially excavated bedrock hollows such as those of Llyn Peris and Llyn Gwynant. Till is widespread though patchy; the largest extents are around Dyffryn Ardudwy and the margins of Carneddau.