Only a pleasant walk away, the great chasms of Ebbor and Cheddar have rent the rocks asunder, forming two of the loveliest ravines in the kingdom. Northward across the intervening syncline of Mountain Limestone, pitted with swallets marking the entrances to many an unknown subterranean labyrinth, are seen the Old Red Sandstone summits of North Hill, crowned with its seventeen Neolithic barrows, and of Blackdown beyond, from whose bare top is seen the broad estuary of the Severn spreading out across the view, giving a glimpse of the coast of South Wales in the far distance, its busy factories showing their pencil-like chimneys against the dark hills behind. In the Channel the little islands of Steepholm and Flatholm mark the line of the original continuation of the great Mendip range into South Wales. The limestone shores of the former rise sheer from the sea, forming an impregnable fortress. Here, far below the level of the salt water around, a supply of pure water is obtained from the Limestone, brought, doubtless, from the Limestone area of Mendip by way of some hidden fissure. Hard by, at Clevedon, is the grave of that great friend of Tennyson, who sat here and listened to "The moaning of the homeless sea, The sound of streams that, swift or slow, Draw down æonian hills, and sow The dust of continents to be." Very truly and accurately his words describe the action that is going on, by which the swallet streams are undermining and honeycombing these hills and bearing their component rocks away to the sea. Standing on Pen Hill and looking northward, a great east and west depression is seen forming a broad low valley in the tableland of Mendip. Into this valley numerous springs and a liberal rainfall are for ever pouring their waters. Yet nowhere is there a surface channel which can carry this water away; and nowhere, save in the small hollows of the Old Red Sandstone and Shales, does water accumulate. The reason is not far to seek. The Carboniferous Limestone, evenly stratified everywhere, has been split by vertical joints into a series of gigantic cubes. Between them, the surface waters, laden with carbonic acid obtained from the atmosphere and from vegetation, have for ages made their way, enlarging them by both chemical and mechanical action, till they have become fissures capable of giving passage to an enormous quantity of water. So from one joint to another, from one bedding plane to another, the water percolates downwards until it meets with some impermeable rock beneath, or until it finds an outlet at the level of the Secondary rocks forming the valley below. Such impermeable beds are found in the Lower Limestone Shales, and the resulting outlets are well known in the great risings of St. Andrew's Well in the gardens of the Bishop's Palace at Wells, in the source of the Axe at Wookey Hole, in the Cheddar Water and other large springs, of all of which more hereafter.