Often referred to as the Central Lowlands, this is a rift valley mainly comprising Palaeozoic formations. Many of these sediments have economic significance for it is here that the coal and iron bearing rocks that fuelled Scotland's industrial revolution are to be found. Although relatively low-lying, hills such as the Pentland Hills, Ochils and Campsie Fells are rarely far from view. This area has also experienced intense volcanism, Arthur's Seat in Edinburgh being the remnant of a once much larger volcano active in the Carboniferous period about 340 million years ago. As a result of ice age glaciers, drumlins were formed, and many hills have a crag and tail landform.