A SURVEY OF THE LOWER TKES MARSHES 1 23 



in some cases sloping gently upward to the level of the 

 embankment. 



But these are not the only places where such secondary 

 changes can be studied ; within the marsh itself the secondary 

 drainage channels, with all their huge curves, and their inter- 

 mittent and reversible current, have unique opportunities, on 

 a small scale but at numerous stations, of repeating the 

 operations of their parent. Within their limits marginal erosion 

 is constantly occurring, and their banks falling in, mudbanks 

 develop to play a minor part in the history of the marsh. 



Fan Formation. 



The precise course pursued in the evolution of the innumer- 

 able pans scattered at random over the marsh offers an 

 investigation of great intricacy. If anything is certain it is 

 that the causes of their development are multitudinous ! Still, 

 careful study reveals the origin of most of them. 



Had they been observed only in the saltmarsh, one would 

 have looked solely to agencies at work there for their 

 initiation, but such a restriction does not harmonise with 

 known facts. Pans differing in no wise from these have been 

 detected at two places on the railway side in Durham, one just 

 north of Darlington, and the second between Thornaby and 

 Stockton. The latter, in particular, with their rounded 

 contour, steep sides, and level bottoms approximate closely in 

 all essentials to the oldest saltmarsh pans. And their dis- 

 covery there gives us a key to the development of at least 

 some of the primary pans ; in both cases they have arisen on 

 flat stretches of soft, newly-made land admitting, at suitable 

 intervals, of the presence of standing water. From this it 

 seems that we have to look back to the time when the 

 primary marsh, in its initial stages of colonisation on soft 

 clayey silt, allowed a similar localisation of surface water. 

 In my opinion this explains the formation of the majority of 

 the primary pans. 



In a parallel sort of way, but with minor diff'erences, such 

 pans originate in the secondary marsh. Very rarely in tliis 



