GEOLOGICAL IMPORTANCE OF SEDIMENTATION 327 



zone will normally be flooded by sea water once every two weeks, 

 while storms and extra high tides may flood still higher portions at 

 longer intervals. The mechanical deposits of the littoral zone are apt 

 to be extremely variable in nature, and the individual beds more 

 limited in area than is the case either upon the land or under the sea. 

 Muds rich in organic matter, and of irregular distribution and thick- 

 ness, are the common deposits of lagoons; sands and gravels will be 

 deposited in tidal channels and as off-shore bars, the strata showing 

 current marks and cross-bedding. Cleanly sorted sands, gravel, 

 shingle, and sometimes bowlders will mark the face of the outer beach. 

 The littoral zone is characterized, in common with land deposits, 

 by ripple-marks, mud-cracks, rain-prints, foot-prints, and fossils, of 

 land animals and plants; in common with neighboring marine or 

 estuarine deposits, by ripple-marks and brackish or salt-water fossils. 

 As distinctive shore marks held in common by the littoral zone and 

 the margins of lakes are wave-marks, rill-marks, and the shelving 

 nature of the beach. 



ORIGIN OF TENDENCY TO CLASSIFY LITTORAL AND 

 FLOOD-PLAIN DEPOSITS AS MARINE 



Although the littoral zone is seen to have a number of distinctive 

 marks which separate at least the upper half of its limits from the 

 marine area, it is regarded in the~ great bulk of geological literature ■ 

 as merely a border portion of the sea, and its deposits, except where 

 beach structure is shown, are commonly thought of ^.s marine deposits 

 made in shallow water, and ordinarily at no great distance from the 

 land. Mud-cracks, rain-prints, and foot-prints are frequently cited 

 as evidence of mud-flats exposed at low tide and bordering the sea. 



This close alliance of the littoral zone with the marine might be 

 unimportant, were it not that the majority of the criteria which are 

 relied upon as determining the presence of the littoral zone apply 

 equally well to flood-plain deposits made upon the land at any 

 distance from the sea. Furthermore, in periods of vigorous erosion 

 river deltas may completely fill up shallow seas and receive large 

 amounts of river sediments upon their upper surfaces; then by a 

 slackening of erosion or an increase in subsidence, may frequently be- 

 come submerged. In such a way river deposits bearing these features 



