Geoscience Reference
In-Depth Information
Figure 305 shows the flow patterns that create such ripples and lead to their mi-
gration. If water speed is increasing over a flat bed, turbulent eddies will come and go,
always varying in size, speed and direction, but tending to form at regular intervals in a
mass of flowing water. Eddies can also be initiated as water flows over an irregular sur-
face and, once set up, they often scour the bed, having enough energy to pick up loose
material. In this way, sand grains can be picked up by the flow of water and carried
along the bed into ridges or ripples, separated by hollows scoured out by the eddies.
Since pockets of turbulence tend to occur at regular intervals, regularly spaced ridges
soon form in the sandy bed. These ridges take on the shape of ripples, with gentle up-
stream and steep downstream faces, and they start to migrate downstream. Eddies now
become amplified in the troughs between ridges, excavating sand which is then pushed
up the upstream side of the next ridge, before avalanching down the downstream face.
This transfer of material, from the upstream side of the ripple to the downstream side,
allows the ripple to be continuously rebuilt in a downstream direction whilst maintain-
ing its size and shape.
FIG
302.
Looking west over the eastern stem of Blakeney Spit (Fig. 294,
c4
). (Copyright
Norfolk Museums and Archaeology Service & Derek A. Edwards)
FIG
303.
Large ripples (5-15 m from crest to crest) on a sand bar in the tidal channel be-
hind Blakeney Spit, near the village of Morston (Fig. 294,
c5
). (Copyright London Aerial
Photo Library)
The inland parts of Landscape C have a surface blanket of river and ice-laid ma-
terial, often producing a gently undulating plateau surface. Much of this material is
Anglian in age, so the open and gentle valleys that have been eroded in the cover of
