548 STUDIES FOR STUDENTS 



surface had been one of river building it may underlie or overlie the 

 littoral and marine deposits according as to whether the delta was 

 retreating owing to subsidence or advancing owing to river building. 

 But in either case the physical conditions of accumulation are so 

 different upon the land and beneath the shallow sea that there is 

 to be expected a marked contrast in the character of the contiguous 

 continental and marine deposits. 



Not only will the littoral zone be narrow and transitional in nature 

 compared with the regions which border it on either side, but, as 

 shown in the preceding article, its deposits are liable to suffer much 

 destruction from the planing effect of the waves in the case of a 

 subsiding land and are the first to suffer from subaerial erosion upon 

 an emerging land. Only in the case of a delta building forth into 

 a sea is there a good chance for the preserval of the littoral record. 

 It has been further seen that mud-cracks are not a characteristic 

 feature of the littoral, but can only originate upon its upper fifth or 

 tenth, and in the places most favorable for ultimate preservation 

 there is a grading into the land surface of the delta of which in arid 

 climates the mud-cracks are more characteristic than of the littoral. 



From these considerations it is seen that mud-cracks of littoral 

 origin cannot be a characteristic or important feature of geological 

 formations. They are by no means excluded as of occasional occur- 

 rence, but it would seem safe, where certain formations are domi- 

 nated and widely characterized by mud-cracked shaly layers, to 

 assign them to another origin and most probably to one upon a river 

 plain receiving fluvial deposits. 



To speak more particularly of the features which will be asso- 

 ciated with the occasional mud-cracks of littoral origin may be noted 

 the presence of beach structures of associated sands and muds, 

 frequently fossils characteristic of the littoral zone, leaves and other 

 debris from the land, rain-prints and the foot-prints of such land 

 animals as frequent the shore. These will ordinarily be restricted 

 to animals which seek food native to the littoral or cast up by the 

 sea, as for instance the grubbing of swine for clams upon exposed 

 mud flats, or birds which run over the flats and beaches for annelids 

 or other small organisms. 



The mud-cracks should be most frequently developed upon 



