

114 STRUCTURAL AND FIELD GEOLOGY 
directly on the surface of fine accumulations. As a rapidly 
flowing river pushes its delta outwards, the water naturally 
shallows in front of the advancing alluvial cone—in other 
words, the zone of gravel encroaches upon the area over which 
sand was formerly distributed, while the sand in its turn is 
laid down upon the finer mud and silt. Conversely, when 
sedimentation takes place over a gradually subsiding area, 
finer grained deposits continue to advance shorewards and 
extend over the surface of coarser accumulations. In general, 
however, such changes are only developed gradually, so that 
the passage from one kind of sediment to another (either in 
a horizontal or a vertical direction), will not usually be abrupt. 
But in the case both of an advancing delta and a retreating 
coast-line, sudden changes in the character of the deposits 
must occasionally take place. During floods and freshets, for 
example, the coarser detritus hurried forward by a river will 
make an abnormal advance, just as tidal currents will now 
and again sweep fine-grained sediment further inshore than 
usual. Sudden changes like this are often accompanied by 
the process already described as “contemporaneous erosion.” 
Contemporaneity of Strongly-contrasted Strata.— When 
we consider that sedimentary deposits are in process of 
formation over enormous stretches of sea-floor, in shallow 
and deep water alike, it is obvious that the most diverse 
accumulations may yet be of contemporaneous origin. It is 
no more than we might expect, therefore, to find that such 
rocks as grit and sandstone have been formed on the same 
sea-floor as limestone. When a series of strata is traced 
across a wide area, we constantly see some of the beds thin- 
ning off, and their position in the sequence being occupied by 
others of a different kind. In this way a great succession of 
thick-bedded limestones may be split up, as it were, by the 
intercalation of shales and sandstones, which continuously 
increase in thickness, while the limestones at the same time 
gradually get thinner and thinner until at last they disappear, 
and the whole series of strata then comes to consist of sand- 
stones and shales alone. Similar changes are brought about 
by variations in the character of the individual beds them- 
selves. Conglomerate, as we have seen, gradually shades off 
into pebbly grit and sandstone, just as siliceous sandstones 
