STRATIFICATION 269 
where streams are entering quiet bodies of water. 
Such conditions are found only in shallow water. 
In some cases, layers of rock have been cut by 
stream channels (Fig. 153), or washed by the rain while 
they were forming, which would not have been possible 
unless they had been deposited near the shore. Where 
sedimentary rocks have been carefully studied, as in cer- 
tain coal mines, actual 
river or rivulet valleys 
are sometimes revealed. 
On the surface of the 
layers one often finds 
the imprints of rain- 
drops (Fig. 154), or the 
footprints (Plate 12) of [& 
land animals, or cracks Fic. 153. 
of shrinkage caused by Cross-bedding in gravel bank. Small stream 
valley cut in the gravel in middle of the 
bank, and later filled with other gravel. 
the drying of the mud, 
exactly as mud cracks 
are formed at the present day, when after a summer 
shower, the water in the small road-pools evaporates, 
leaving the mud dry and cracked. These effects could 
be caused only where the rocks were accumulated on 
the shore, and exposed to the air at ebb of tide. 
Often there are fossil beaches and wave-cut cliffs 
preserved among the sedimentary rocks; and even 
more common than these, the surface of the layers 
