We have hitherto considered the stratified rocks as made up of beds which follow upon one another, in orderly sequence, and as being affected alike by the elevation or depression, folding or dislocation, to which they may have been subjected. Strata which have thus been laid down in uninterrupted succession, with sensibly parallel bedding-planes, and which have been similarly affected by movements, are said to be conformable, and the structure is called conformity. In many places, however, the strata exposed in a section are very obviously divisible into two groups, each made up of a series of conformable beds, but the upper group, as a whole, is not conformable with the lower, but rests upon its upturned edges, or its eroded surface. The two groups are said to be unconformable and the structure is named unconformity. The definition of unconformity here given includes certain not uncommon structures, which must be distinguished as having quite a different significance.

Fig. 199. - Unconformity with change of dip, or angular unconformity.

Unconformity is of two kinds: (1) There is a distinct difference in the dip of the two sets of strata, the upper beds lying across the upturned and truncated edges of the lower. This is the more usual kind and is shown in Figs. 199-202. The structure implies that the lower series of beds was first laid down under water, and that these beds were then upturned, tilted, or folded to form a land surface. Erosion next truncated the folds, planing the edges of the disturbed beds down to a more or less level surface. The land surface was again depressed beneath the water, and the second set of strata was deposited upon it. Finally, a renewed elevation, accompanied perhaps with folding or faulting, has brought both series of strata above the sea-level.

Fig. 200. - Angular unconformity, Grand Canon of the Colorado.

(Photograph by Sinclair).

While the older beds formed a land surface, they were eroded and no deposition took place upon them. Consequently, between the two sets of strata is a gap, unrecorded by sedimentation (at that point), the length of which represents the time that the older beds were above water. The processes involved in an unconformity are of slow operation, so that the gap usually implies a very long lapse of time. In many cases whole geological ages, of incalculable duration, have intervened between the deposition of the two groups of strata.

Fig. 201. - Angular unconformity, old gravels on hard shale; Kingston, N.J. (Photograph by Sinclair.) Note the smooth joint-faces of the shale, in contrast to the rugged fracture-surfaces.

(2) In the second kind of unconformity the two groups of strata have the same dip, the upper series resting upon the eroded surfaces of the lower. The processes involved in this kind of unconformity are nearly the same as in the first, so far, at least, as the alternation of land surface and sea-bottom, elevation and depression, are concerned. In this case, however, the first upheaval was not accompanied by any folding or fracturing of the beds. An unconformity of the second class is sometimes exceedingly difficult to detect and then is called a deceptive conformity. Such a case arises when the surface of the ground is made by cutting down strata to the upper surface of a hard bed, which is then depressed beneath the water, as a flat pavement, upon which new material of a similar kind is laid down with hardly a perceptible break. In the Rocky Mountain region remarkable instances of this deceptive conformity occur, where, in the middle of a mass of limestone apparently formed without any interruption, there is, in reality, an enormous time-gap. Long and careful search has made clear the nature of the contact and exposed the deception.

The existence of an unconformity, when none is apparent, may sometimes be detected by observing certain structural features which affect the lower and older beds, but not the upper. For example, the lower strata may be faulted, or intersected by a dyke of igneous rocks, the fault or dyke ending abruptly at a certain level and not continuing into the upper series.

Fig. 202. - Angular unconformity, west of Altoona, Pa. (U. S. G. S).

The lowest member of the upper series of strata in an unconformity is very frequently a conglomerate or coarse sandstone, and represents the beach formation of the sea advancing over the old land. These are called basal conglomerates. Such coarse beds are, however, not always present, and they may be only locally developed along a particular line.

Unconformities may be confined to relatively restricted regions, or they may extend over whole continents; they are very useful means of dividing the strata into natural chronological groups.