The (lit. 'the cairns'; is a Welsh plural form, and is sometimes anglicised to Carnedds) are a group of mountains in Snowdonia, Wales. They include the largest contiguous areas of high ground (over or high) in Wales and England (although larger areas over 2,000 feet are found in Northern England), as well as six or seven of the highest peaks in the country—the Fifteen Peaks. The range also encloses a number of lakes such as Llyn Cowlyd and Llyn Eigiau, and the Aber Falls waterfall. It is delimited by the Irish Sea to the north, the Conwy valley to the east, and by the A5 road from Betws-y-Coed to Bethesda to the south and west. The area covers nearly , about 10% of the area of Snowdonia. The area is bordered by three main roads—the A55 (to the north), the A5 to the south and the A470 to the east. Geology In common with much of Snowdonia, the rocks forming the Carneddau originated largely during the Ordovician period between 485 and 444 Ma (million years ago). Principal among these are the mudstones and sandstones of the Arenig (478-470 Ma) to Caradoc age (459-449 Ma) Nant Ffrancon Subgroup and the tuffs etc. of the Caradoc age Llewelyn Volcanic Group. These are overlain by the sandstones, siltstones, tuffs and tuffites of the Cwm Eigiau Formation. Within the Nant Ffrancon sequence are a variety of igneous intrusions and the lavas and tuffs of the Foel Fras Volcanic Complex. The complex geology results from the plate tectonic processes taking place during the Caledonian orogeny involving continental landmasses on either side of the Iapetus Ocean moving together and colliding over a protracted period. Over time, these mountains have been eroded by the weather and scoured by advancing and retreating ice sheets. The Carneddau were formed in this way and consist of volcanic and sedimentary rock. The last ice sheet retreated about 10,000 years ago. It left behind a landscape of smooth summits above erratic boulders and scree at the foot of cliffs on the eastern side of the mountains, and moraines that created shallow lakes in the cwms. History This area was first colonised in Neolithic times, when Stone Age farmers started clearing the native forests of oak and birch that covered all but the uppermost ridges and summits. They were followed by Bronze Age people who cleared more forests and erected standing stones across