The Island rivers cut out their channels when the land stood at a higher level than at present. The old channels of the lower parts of the rivers are now filled with alluvium, partly brought down by the rivers and partly marine. The channels are cut down considerably below sea level; and by the sinking of the land the sea has flowed in, and the last parts of the river courses are now tidal estuaries. The sea does not cut out estuaries. They are the submerged ends of river valleys. Some idea may be formed of the antiquity of our Island rivers by observing the depth of the clefts they have cut through the downs at Brading, Newport, and Freshwater. But to this we must add the depth at which the old channels lie below the alluvium. It would be interesting to know the thickness of the alluvium. But it is not often that borings come to be made in river alluvia. However, in the old Spithead forts artesian wells are sunk; and these pass through 70 to 90 feet of recent deposits before entering Eocene strata. Under St. Helen's Fort, at the mouth of Brading Harbour, are 80 feet of recent deposits. The old channel of the Yar, at its mouth, must lie at least at this depth. Before it passes through the gap in the Chalk downs the Yar has meandered about, and formed the alluvial flat called Morton marshes. These marshes stretch out into the flat known as Sandown Level, which occupies the shore of the bay between Sandown and the Granite Fort. What is the meaning of this extension of the alluvium away from the course of the river out to the sea at Sandown? A glance at it as pictured on a geological map will suggest the answer. We see clearly the alluvia of two streams converging from right and left, and uniting to pass to the sea through Brading Harbour. But the stream to the right has been cut off by the sea encroaching on Sandown Bay: only the last mile of alluvium is left to tell of a river passed away. We must reconstruct the past. We see the Bay covered by land sloping up to east and south east, the lines of downs extending eastward from Dunnose and the Culvers, and an old river flowing northward, and cutting through the chalk at Brading after being joined by a branch from the west. This old river must have been the main stream. For it was a transverse stream, flowing nearly at right angles to the ridge of the anticline; while the Yar comes in as a tributary in the direction of the strike. Of other tributary streams, all from the right are gone by the destruction of the old land. On the left streams would flow in from the combes at Shanklin and Luccombe--streams which have now cut out Shanklin and Luccombe chines.