GULLIVER. — SHORELINE TOPOGRAPHY. 233 



S., 5984). The spit formed by the southward flowing current has crowded the 

 river as far south as possible, close to the cliffs on Coos head. 



Another good example of a river forced against a rocky headland by the sea 

 is seen in Garcia river at point Arena (C. S., 661). 



Bezirk river is lost in the sand dunes before reaching the gulf of Tunis (Tunis, 

 XXI). Current is indicated from the left. 



15. Tidal Scour. 



Action in Bays. — Tides as abradiug agents are most effective in 

 drowned valleys. The destructive effects of the bore have been much 

 discussed, and more work has been ascribed to tliis inrush of tidal waters 

 than that for which it is probably responsible. The depth to which tides 

 may scour a submarine channel is still a very problematical question, and 

 the amount of wearing of the shores by the tides is a subject needing 

 study.* 



Tides are not here taken up at any length, because the relation of 

 their products to stage of cycle is not as yet shown. The forms of shores 

 as determined by the changing ratios between tidal on- and offshore and 

 alongshore currents is the only point upon which emphasis is here laid. 

 As in the consideration of deltas, this point is dwelt upon because it 

 shows so clearly the importance of perceiving tlie varying ratios between 

 the several factors that determine shore forms. 



Runways. — On Hat coasts where there are broad surfaces covered at 

 high and bare at low tide or wide stretches of tidal marsh, there is op- 

 portunity for much tidal work. When the main body of ocean water 

 retreats during ebb tide, that portion lying upon the flats must flow off 

 down the easiest path, and thus runways are formed dissecting the sur- 

 face. Such runways may be broad or narrow, deep or shallow, short or 

 long, etc., according to the values of the varying factors which obtain in 

 any given case. The scouring action may continue below low tide level, 

 to greater depths, the greater the range of tide and volume of water 

 passing through the runways. 



The tidal scour if strong would tend to prevent the tying of islands 

 and closing of bays, which are normal features of shorelines in an adoles- 

 cent stage of development. 



Such prevention of island-tying is seen on the Schleswig coast, where 

 from the stage of development indicated by the long wings on Sylt 

 island (page 213), the development of tombolos would be expected. It 



* For the discussion of the scouring of tides in estuaries, see papers by the fol- 

 lowing authors : Bache, Branner, Dana, Ferrel, Mitchell, Shelford, and Sollas. 



