By these and similar actions, every district in which
limestones are extensively developed will be found to exhibit
a number of natural caves, rents, or fissures. The first element,
therefore, in the production of cave-deposits, is the existence
of a period in which limestone rocks were largely dissolved, and
caves were formed in consequence of the then existing drainage
taking the line of some fissure.
Secondly, there must have been a period in which various deposits
were accumulated in the caves thus formed. These cavern-deposits
are of very various nature, consisting of mud, loam, gravel,
or breccias of different kinds. In all cases, these materials
have been introduced into the cave at some period subsequent to,
or contemporaneous with, the formation of the cave. Sometimes
the cave communicates with the surface by a fissure through which
sand, gravel, &c., may be washed by rains or by floods from some
neighbouring river. Sometimes the cave has been the bed of an
ancient stream, and the deposits have been formed as are fluviatile
deposits at the surface. Or, again, the river has formerly flowed
at a greater elevation than it does at present, and the cave
has been filled with fluviatile deposits by the river at a time
prior to the excavation of its bed to the present depth (fig.
256). In this last case, the cave-deposits obviously bear exactly
the same relation in point of antiquity to recent deposits, as
do the low-level and high-level valley-gravels to recent
river-gravels.
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