Margaret Gelling says that the main Anglo Saxon settlement was not where the village of Cookham now sits, which is on fairly low ground, but up to 823ad was one or two miles to the west, on higher ground (therefore once again archaeology seems to confirm Steve Mitchell's rising water table in the immediate post Roman era – see various articles in SIS journals). This is some distance from Sashes Island, to the east of Cookham village (and Cookham bridge and weir) and may account for the quite different name of Shaftsey (Shaftsey developed into Shasays and Sashes, but the ey element = island). In the 9th century the Thames became the frontier between Wessex and Mercia, the latter conquered by the Danes. In the process the Vikings probably destroyed the extensive abbey at Cookham,otherwise at an unknown locality, once controlled by the widow of Offa of Mercia.
With a wide stream on every side Sashes island is a good defensive position and a Roman road from St Albans to Silchester probably crossed the river near by – but a little upstream. Sashes island was a place that created a problem for boats as the river widened and became shallow as it came up against the chalk escarpment on which Cliveden and Taplow are located, the river forced to switch to a southwards track for a few miles. Hence, it was a place where the Vikings would have had to beach their boats and manhandle them around the bend in the river (at low water). The river also forms several streams at this point which were probably boggy and not suitable for boatage. More recently a new channel was cut and a lock installed – which is now the main channel for boats. On the other side of Sashes island there is Cock Marsh which would also have presented problems for Vikings in boats, on the edge of which the modern village is located.