SAND DUNES AND SALT MARSHES 



often show long ridges of sand extending in 

 the wind's axis to leeward of them, and these 

 longitudinal dunes are sometimes found by 

 themselves, and constitute a distinct type, 

 although not often developed to a great size 

 at Ipswich. They are prone to form near the 

 beach and appear to be indicative of unusu- 

 ally strong winds. 



Every now and then in the amphitheatre 

 waves there are cross valleys with steep wind- 

 swept walls. In the cuttings and on the sharp 

 northward faces the stratifications in the sand 

 are often marked, and the firmly packed lay- 

 ers stand out prominently, while the loosely 

 formed ones are cut away. The strata often 

 dip gently towards the south, for the sand is 

 left by the wind on the southern or leeward 

 slope, but they vary greatly and are irregu- 

 larly superimposed. The angle of the north- 

 ern slope of these dunes varies from thirty 

 to ninety degrees, while that of the southern 

 slope is about twelve degrees. 



The other kind of transverse dune— the 

 normal desert one— although rare at Ipswich, 

 appears to form only where the wind is un- 

 hampered by the binding grass, and is one 

 that resembles more closely a wave of the sea, 



18 



