122 DR, J. W. HESLOP HARRISON ON 



to attack at two danger points. The first of these is seen in 

 its south eastern margin, and the second occurs when the 

 creel< swerves sharply to the north ; as it does so it exposes a 

 high clayey blufifto bear the full onslaught of the tide. Nor 

 is this action on the two vulnerable points a normal one, for 

 its vigour is enormously increased by human activities lower 

 down toward Tees Bay ; there, as may be gleaned above, not 

 only have we the whole tidal wave concentrated by the main 

 slag wall, but further, when it enters Greatham Creek, its rush 

 is intensified by the funnel-like gorge of huge slag walls which 

 rise so steeply just below high water mark. 



At the first point, released from its previous restriction 

 within the slag barriers, the oncoming tide strikes the inter- 

 posed rhargin of the salt marsh frontally with accentuated 

 force. By its attack the supporting soil beneath the matted 

 Glyceria-Stafice sward is disintegrated and carried back when 

 the tide recedes. The turf itself, owing to its closely inter- 

 woven texture, offers a sturdy resistance and remains intact 

 long after the soil has settled out to form new mudbanks. 

 For a while, it forms a slope up to the primary level \ in the 

 end considerable sections are detached, with the exposure of 

 an escarpment upon which the eroding action just described 

 may be renewed. In this way a well-marked secondary 

 low level marsh appears, provided at times with a complicated 

 arrangement of low escarpments. Such a series, by the 

 number of its individual members, offers valuable data for 

 the estimation of the age of any given stretch of secondary 

 marsh. 



Matters at the other weak point follow a different pro- 

 cedure. Assaults cannot be carried out on such a wide front, 

 and the destructive energies of the tide are limited to the 

 undermining of the steep banks. Slowly but surely, huge 

 cave-like holes are fretted in the clay, succeeded sooner or later 

 by a collapse on a wholesale scale, when huge masses of clay 

 and soil are thrown into the creek to be smoothed down and 

 to attract silt. By-and-by. muddy spits rise aljove the water, 



