The area here described lies immediately north of the Snowdon ‘massif’, and forms a part of the mountainous tract between the Passes of Llanberis and Nant Ffrancon. Its south-eastern boundary runs from the Ogwen Falls, through Llyn Idwal and Twll Du (Devil9s Kitchen) to the upland tarn of Llyn y Cŵn and thence down the Afon Las to the Pass of Llanberis. For the greater part of its length this line approximately coincides with the axis of the Idwal syncline. The northern limit of the area is defined by the top of the Cambrian ‘slate-belt’, between the upper end of Llyn Peris and the northern shoulder of Bronllwyd, near Bethesda. The area is about 8 square miles in extent, and is represented on the Ordnance Survey 6-inch map, Carnarvon 17 (four quarter-sheets), and on the Geological Survey 1-inch map, 78 S.E. (Old Series). Owing to the general strike of the stratified rocks, the main ridges and valleys have a north-east to south-west trend. In succession from north to south are the long strike-ridges of Elidir Fach, Elidir Fawr and Carnedd y Filiast, Foel Goch, and Y Garn, all of which rise to over 2500 feet. A long ridge, running northwards from Twll Du to Carnedd y Filiast, crosses the general trend of the area almost at right angles, and forms the watershed between Nant Ffrancon and the Pass of Llanberis. On the east of this divide is a series of eight magnificent hanging cwms which formerly contained tributary glaciers flowing