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. (_d_) A new subsidence of the truncated lower series and deposition of the upper series across its eroded surface. (_e_) A new elevation of the double series to its present position above sea level. [Illustration: FIG. 35.—Types of deceptive or erosional unconformities.] From this succession of episodes it is seen that a break of this kind between two series of deposits involves a double oscillation of subsidence followed by elevation—a large depression followed by a large elevation, a smaller subsidence followed by elevation. The time interval which must have been represented by these repeated operations is so vast as at first to stagger the mind in contemplating it. When, as in this instance, the dips of the lower series of beds differ from those of the upper, we have to do with an _angular unconformity_. It may be, however, that the lower series was not so far depressed as to enter the zone of flow, and that its beds meet those of the upper series with apparent conformity. Such an unconformity is often extremely difficult to recognize, and it is described as a _deceptive_ or _erosional unconformity_. With a deceptive unconformity the clew to its real nature is usually some fact which indicates that the lower series of sediments had been raised above the level of the sea before the upper series was deposited upon it. This may be apparent either in the irregularity of the surface on which the two series are joined, in some evidence of the action of waves such as would be furnished by a basal conglomerate in the upper series, or some indication of different resistance of different rocks of the lower series to attacks of the atmosphere upon them (Figs. 33 and 35 _a-c_).