The essential physical feature of this unconformability is that the beds of the younger formation rest upon a worn and eroded surface formed by the beds of the older series (fig.
The basement beds of the Keuper rest with a slight unconformability upon an eroded surface of the "Bunter" next to be described.
But while unconformability invariably bears testimony to a lapse of unrepresented time, the conformability of two sets of strata in contact by no means implies that the newer formation immediately succeeded the older one.
Where the widest gaps appear in the sequence of the fossil forms, as between the Permian and Triassic rocks, or between the Cretaceous and Eocene, examples of such unconformability are very frequent.
We may naturally expect the other proofs of unconformability will hereafter be detected at more than one point in so vast a succession of strata.
It would be easy to multiply instances of similar unconformability in formations of other ages; but a few more will suffice.
That the subterranean forces have visited different parts of the globe at successive periods, is inferred chiefly from the unconformability of strata belonging to groups of different ages.
That the subterranean forces have visited different parts of the globe at successive periods is inferred chiefly from the unconformability of strata belonging to groups of different ages.
An unconformability may sometimes be clearly established when the actual contact of the two formations cannot be seen, as where the new formation is a conglomerate containing fragments of the older.
In lithological characters the two may closely resemble each other, and there may be nothing to show that the gap represented by their unconformability is of an important character.
The need for this caution may sometimes be made evident by an unconformability between two sets of rocks, as has already been explained.
The lapse of time represented by this unconformability would thus be equivalent to that required for the accumulation of the three missing formations in those regions where sedimentation was more continuous.
