
STRUCTURES DUE TO DENUDATION 183 
points usually to the following succession of changes: (1) a 
period of accumulation—either lacustrine or marine; (2) a 
crustal movement resulting in the conversion of the area of 
sedimentation into dry land; (3) a more or less prolonged 
period of erosion, during which the land surface is denuded ; 
(4) renewed subsidence and deposition of younger accumula- 
tions over the worn and irregular surface of the now drowned 
land ; (5) final re-elevation of the area. 
Overlap.—When the upper beds of a conformable series 
extend over a wider area than the lower beds of the same 
series, we have the structure known as overlap. The 
structure indicates subsidence accompanied by sedimentation 
over a gradually extending area, and overlap is therefore 
often well displayed in cases of marked unconformity. In 
the accompanying section (Fig. 62), for example, the older 

Fic. 62,.—UNCONFORMITY AND OVERLAP. 
rocks, a, have been much eroded, so that when submerged 
they formed a very irregular sea-floor. The hollows being 
cradually filled with sediment, 4, it is obvious that the upper 
must overlap the lower beds—each stratum extending over 
a wider area than its predecessors. But overlapping must 
take place in every case of the gradual subsidence of land, 
whether the surface of the sinking area be irregular or 
relatively smooth. As the land sinks, shore-deposits become 
overlapped by infra-littoral deposits, and these last by the 
accumulations of deeper water. 
Overlap is a structure which is not only interesting and 
instructive to the geologist, but it has also an obvious 
practical bearing. In questions of boring for bedded 
minerals it is often of the utmost importance, and failure 
to recognise the structure has led to disappointment and 
loss which might have been avoided. Similarly, in questions 
of water-supply, the possible occurrence of overlap and uncon- 
formity cannot be safely disregarded, 



