age sedimentary rocks in the south and northeast and of Cenozoic era strata on the Cardigan Bay coast though the latter are concealed by more recent deposits. Low grade metamorphism of Cambrian and Ordovician mudstones has resulted in the slates, the extraction of which once formed the mainstay of the area's economy. Geography Mountains The principal ranges of the traditional Snowdonia are the Snowdon massif itself, the Glyderau, the Carneddau, the Moelwynion and the Moel Hebog range. All of Wales' 3000ft mountains are to be found within the first three of these massifs and are most popular with visitors. To their south within the wider national park are the Rhinogydd and the Cadair Idris and Aran Fawddwy ranges. Besides these well-defined areas are a host of mountains which are less readily grouped though various guidebook writers have assigned them into groups such as the 'Arenigs', the 'Tarrens' and the 'Dyfi hills'. Snowdon's summit at 1085 metres (3560') is the highest in Wales and the highest in Britain south of the Scottish Highlands. At 905 metres (2970') is the highest in Wales outside of northern Snowdonia and at 893 metres (2930'), next in line. Rivers and lakes Rivers draining the area empty directly into Cardigan Bay are typically short and steep. From north to south they include the and which share a common estuary, the and its tributaries the Wnion and the , the smaller and on the park's southern margin the Dovey. A series of rivers drain to the north coast. Largest of these is the Conwy on the park's eastern margin which along with the drains into Conwy Bay. Further west the and empty into the western end of the Menai Strait. A part of the east of the national park is within the upper Dee (Dyfrydwy) catchment and includes Llyn Tegid, the largest natural waterbody in Wales. A fuller list of the rivers and tributaries within the area is found at List of rivers of Wales. There are few natural waterbodies of any size in Wales; Snowdonia is home to most. Besides Llyn Tegid, a few lakes occupy glacial troughs including and at and south of . , and to the south and west of Snowdon feature in this category as do and on the margins of the . There are numerous small lakes occupying glacial cirques owing to the former intensity of glacial action in Snowdonia. Known