Fig. 200. Unconformity between Parallel Strata
Fig. 201. Unconformity between Non-parallel Strata
Unconformity
Strata deposited one upon, another in an unbroken succession are said to be conformable. But the continuous deposition of strata is often interrupted by movements of the earth’s crust, Old sea floors are lifted to form land and are again depressed beneath the sea to receive a cover of sediments only after an interval during which they were carved by subaërial erosion. An erosion surface which thus parts older from younger strata is known as an unconformity, and the strata above it are said to be unconformable with the rocks below, or to rest unconformably upon them. An unconformity thus records movements of the crust and a consequent break in the deposition of the strata. It denotes a period of land erosion of greater or less length, which may sometimes be roughly measured by the stage in the erosion cycle which the land surface had attained before its burial. Unconformable strata may be parallel, as in [Figure 200], where the record includes the deposition of strata a, their emergence, the erosion of the land surface ss, a submergence and the deposit of the strata b, and lastly, emergence and the erosion of the present surface s´s´.
Fig. 202. Carboniferous Limestone resting unconformably on Early Silurian Slates, Yorkshire, England
Often the earth movements to which the uplift or depression was due involved tilting or folding of the earlier strata, so that the strata are now nonparallel as well as unconformable. In [Figure 201], for example, the record includes deposition, uplift, and tilting of a; erosion, depression, the deposit of b; and finally the uplift which has brought the rocks to open air and permitted the dissection by which the unconformity is revealed.