Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 33.—Unconformity between a lower and an upper series of beds upon the coast of California. Note how the hard layer stands in relief upon the connecting surface (after Fairbanks).

The meaning of an unconformity.—The rock beds, which are deposited one above the other during a transgression of the sea, are usually parallel and thus represent a continuous process of deposition. Such beds are said to be conformable. Where, upon the other hand, two series of deposits which are not parallel to each other are separated by a break, they are said to form unconformable series, and the break or surface of junction is an unconformity ([Fig. 33]).

Here it is evident that the sediments which compose the lower series of beds have been folded in the zone of flow, though the upper series has evidently escaped this vicissitude. Furthermore, the surface which delimits the lower series from the upper is somewhat irregular and shows a hard layer standing in relief, as it would if it had opposed greater resistance to the attacks of the atmosphere upon it.

Fig. 34.—Series of diagrams to illustrate in succession the episodes involved in the historical development of an angular unconformity. The vertical arrows indicate direction of movement of the land, and the horizontal arrows the direction of shore migration.

In reality, an unconformity between formations must be interpreted to mean that the lower series is not only older than the upper, as shown by the order of superposition, but that the time of its deposition was separated from that of the upper by a hiatus in which important changes took place in the lower series. The stages or episodes in the history of the beds represented in [Fig. 33] may be read as follows (see [Fig. 34 a-e]):—

(a) Deposition of the lower series during a transgression of the sea.

(b) Continued subsidence and burial of the lower series beneath overlying sediments, and flexuring in the zone of flow.

(c) Elevation of the combined deposits to and far above sea level and removal by erosion of vast thicknesses of the upper sediments.