RESIDUAL, GRAVITY-LAID, WATER-LAID DEPOSITS 37 



and salt marshes are divided into those above mean tide, as the 

 grass marshes and mangrove marshes, and those below mean tide, 

 mud banks and eel-grass areas (Fig. 20). The mangrove marshes 

 occur along the Florida coast and have played a very important 

 part in adding to the land area (Fig. 27). The other form of 

 marshes occur in more northern regions, the grass marshes being 



Fio. 27. Mangrove mar8h, Biscaync, Florida. This mangrove advances into the 

 water by throwing out new roots. (From Elements of Geology. Copyright 1911, by 

 Eliot Blackwelder & Harlan H. Barrows. American Book Company, Publishers.) 



sufficiently high so that they are covered only during the highest 

 tides. The eel-grass banks are always covered, while the mud bank 

 is intermediate between these. Holland is an illustration of what 

 the marsh lands may become, when drained and protected by dikes, 

 (b) Lacustrine. Lacustrine or lake-laid deposits consist of 



Fio. 28. Tx;vel floor of Lake Chicago, with the shore-line in the distance. (R. W. Dickenson.) 



(1) terraces and beaches representing old water levels and shores 

 and (2) the beds of extinct lakes. During glacial times, main- 

 lakes were formed by the obstruction of drainage and many more 



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