662 



PRINCIPLES OF STRATIGRAPHY 



which latter are also more subject to scour during the ebb tide. 

 The shallows thus become tide flats, which later are raised rush- 

 grown islands, thus restricting the water within narrow channels. 

 The original length of the La Plata was 325 miles, but about two- 

 thirds of this has been filled up in the manner described. Where 

 material of different sizes is brought by the rivers into the estuary 

 the coarser will be dropped when the current is first checked by the 

 rising tide, the finer following when the checking is complete. 

 During the scouring of the bottom at ebb tide much of the fine 

 material may be carried away again. (Willis-105 : ^pi*.) 



The floor of the Hudson has in many places been built up by 

 mud deposits to such an extent that extensive mud flats are laid 



JsLogoon.-sk- Open sea. 



Level of hiqhh'de. 



Fig. 132. Diagram illustrating the relationship between subsidence and the 

 growth of estuarine deposits. A, bar and lagoon (barachois) on 

 a young coast ; B, estuarine deposits covered by transgressing 

 sea on subsiding coast. The bar and lagoon rest on terrestrial 

 deposits and not on the old crystalline base. (After Barrell.) 



bare at low tide, a hundred or more miles above its mouth, while 

 the channel over much of this distance is comparatively shallow. 

 The rock bottom of the Hudson, on the other hand, is, in places, as 

 in the Highlands, more than 600 feet below tide level. It is proper 

 to note, however, that a part of the filling of this channel is proba- 

 bly glacial, only the upper hundred feet on the average being river 

 silt. 



The estuary of the Severn (Sollas-8g) may serve as another 

 example. The tidal channel of this river is notorious for its mud. 

 "At high tide it is filled with a sea of turbid water, thick and 

 opaque with tawny-colored sediment ; as the tide ebbs a broad ex- 

 panse of shining mud flats is revealed fringing the coast ; but so 

 like is the water to the mud that, seen from a distance, it is often 

 hard to tell where the sea ends and the shore begins. It is the 

 same with its tributaries, the Wye, the Usk, Ely and Rhymney on 



