124 15R. J. W. HESLOP HARRISON ON 



case, do they arise on broader stretches of level ground 

 exposed as the marsh collapses ; they take their origin for the 

 most part in rifts in the newly fallen Glyceria-Statice turf, 

 more or less parallel to the direction of the main tidal drive. 

 In such fissures, local breaks strike off laterally, and then 

 water lodges at the point of intersection, the deposition of algal 

 debris completing the action and enlarging the tiny pool into 

 a fully developed pan. 



Warming* has suggested that even on highly organised 

 primary marsh the inhibiting action of decaying algae so 

 weakens the sward as to lead to pan formation. This view is 

 scouted by Yappf but still, according to my evidence, obtained 

 by a careful consideration of the state of afifairs in Greatham 

 Marsh, it is nevertheless tenable. As was explained above, 

 these marshes are only completely covered by the tide at widely 

 separated periods ; when they are, owing to local conditions, 

 the tidal rush sweeps before it with irresistible might such 

 huge piles of Fnci from the slag walls as to bury small hollows 

 six inches deep in decaying algae and other rubbish. Here, 

 directly on the Statice, primary pans are immediately 

 inaugurated when the vegetation beneath dies off. 



Another very potent agent in determining pan-building is 

 the blocking of drainage channels, and this may happen in 

 many ways, to elucidate which the appended map of a small 

 tract in the south of the marsh has been prepared. In this 

 map the various stages from channel to pan are very clearly 

 seen, and such pans by their general shape very frequently 

 advertise their origin. To such pans the term "channel" pans 

 may be applied. 



Let us consider the position of affairs in one of the 

 secondary channels of which F forms a part. From many 

 indications, we soon discover that it is in a period of decline. 



* Warming, " Bitrag til Vaderns, Sandenes og Marskens Naturhistorie,'" 

 Mem. Acad. Roy. Sci. et Lettres de Danemark, Copenhagen. Septieme 

 er., Sect. d. Sci., 2, 1904. 



t Yapp, "Salt Marshes of the Dovey Estuary," Journal of Ecology, 

 Vol. v., 19 1 7, Page 87. 



