47. Contemporaneous Erosion.—Occasionally a group of strata gives proof that pauses in the deposition of sediment took place, during which running water scooped out of the sediment channels of greater or less width, which subsequently became filled up with similar or dissimilar materials. The diagram ([fig. 12]) will render this plain. At a we have beds of sandstone, which it is evident were at one time throughout as thick as they still are at × ×. Having been worn away to the extent indicated, a deposition of clay (b) succeeded; and this, in turn, became eroded at c, c, the hollows being filled up again with coarse sand and gravel. In former paragraphs, we found reason to believe that lines of bedding indicated certain pauses in the deposition of strata. Here, in the present case, we have more ample proof in the same direction.
| Fig. 13.—Unconformability. |
48. Unconformability.—But the most striking evidence of such pauses in the deposition of strata is afforded by the phenomenon called unconformability. When one set of rocks is found resting on the upturned edges of a lower set, the former are said to be unconformable to the latter. In the above section ([fig. 13]), a, a, are beds of sandstone resting on the upturned edges of beds of limestone, shale, and sandstone, l, s. [Figs. 14] [and 15] give other examples of the same appearance. It is evident that, in the case of [fig. 14], the discordant bedding chronicles the lapse of a very long period. We have to conceive first of the deposition of the underlying strata in horizontal or approximately horizontal layers; then we have to think of the time when they were crumpled up into great convolutions, and the tops of the convolutions (the anticlines) were planed away: all these changes intervened, of course, after the lower set was deposited, and before the upper series was laid down. In the case represented in [fig. 15], we have a double unconformability, implying a still more elaborate series of changes, and probably, therefore, a still longer lapse of time.
| Fig. 14.—Violent Unconformability. |
| Fig. 15.—Double Unconformability. |
49. Overlap.—When the upper beds of a conformable group of strata spread over a wider area than the lower members of the same series, they are said to overlap. The accompanying diagram shews this appearance. An overlap proves that a gradual submergence of the land was going on at the time the strata were being accumulated. As the land disappeared below the water, the sediment gradually spread over a wider area, the more recently deposited sediment being laid down in places which existed as dry land at the time when the earliest accumulations were formed. Thus, in the accompanying illustration ([fig. 16]), the stratum marked 1, resting unconformably upon older strata, is overlapped by 2, as that is by 3, and so on—all the beds in succession coming to repose upon the older strata at higher and higher levels, as the old land subsided.