CHAPTER IX.

UNCONFORMITY AND OVERLAP.

HAT dry land has always existed somewhere throughout all geological time we know from the very existence of mechanically-formed rocks, which are the memorials of its erosion and destruction. "Lyell long ago showed that the amount of such [detrition and] denudation is to be exactly measured by the quantity of the mechanically-formed aqueous rocks, and as our present lands show us vast sheets of sandstones and clays hundreds and thousands of feet in thickness and hundreds and thousands of square miles in extent, and as every particle of these enormous masses of rock is the result of the erosion of previously existing rocks, it follows that the amount of denudation must have been just as great as that of deposition. Just as when we see a large building we know that a hole or quarry must have been made somewhere in the earth, equal, at least, to the cubical contents of the solid parts of that building; so, where we see a vast mass of mechanically-formed aqueous rocks, we must feel assured that a gap was made somewhere in the surface of the earth equal to the solid contents of those rocks" (Jukes). Conversely, also, when we have ascertained that a large amount of rock material has been removed from any existing, or any ancient land surface, we can be sure that an extensive series of stratified deposits must have been formed somewhere out of the materials thus removed, and that their deposition went on pari passu with the detrition of the land surface.

The oscillations of level which have continually taken place have caused frequent changes in the form of the ancient continents, depressing some parts beneath the sea and elevating other parts, so as to raise portions of the adjoining sea bottom into dry land. The tracts that formed dry land were constantly exposed to the detritive action of

those agencies which were described in Part I., Chapters VI. and VII., and the tracts that sank below the sea were subjected to the planing and levelling action of the sea waves (Chapter VIII.).

The result of both processes being to remove a great thickness of rock, and the special result of the latter process being to produce the more or less level surface which is termed a plain of marine denudation, it follows that when such surfaces are depressed far below the sea level, and newer deposits are laid down upon them, there must be a break or discontinuity between the older rocks and the newer deposits. The older series may form a regular and continuous sequence of beds, but the process of deposition having been interrupted by a period of erosion and disturbance, there must naturally be a certain amount of discordance between the older and newer series. discordance constitutes unconformity.

Such

The importance of these conclusions cannot be over-estimated, and without a knowledge of the facts and inferences to be described in the present chapter, the student could make but little progress in the study of stratigraphical geology.

Every country has its own geological sequence, comprising a great series of strata, which is divisible into many natural groups or stages, and between any two of these groups there are only two possible relations: they must be either in conformable or unconformable succession. These terms we proceed to define.

1. Conformable Succession. Two sets of beds are said to be conformable when the base of the upper set rests evenly upon the surface of the lower set, that surface being everywhere one of original deposition. The plane of separation between two such groups simply marks an interval of comparatively short duration, during which no déposition took place. Two or more conformable groups of beds will therefore form a regular sequence like that of the groups 1, 2, 3, 4, in figs. 117 and 119.

2. Unconformable Succession.-Two sets of beds are said to be unconformable when the base of the upper set rests in different places on different parts of the lower set, the upper surface of the latter being either entirely or

in part a surface of erosion. Such a plane of separation involves the lapse of an interval more or less long, during which parts of the beds previously formed were broken up and carried away, leaving a discontinuous sequence or a discordance in the stratigraphical succession, as in the lower part of fig. 117, where the lower group of beds, m, m,

have been tilted and planed off till the horizontal surface A, B, was produced, on which the overlying strata 1 to 7 have been deposited.

This definition of unconformable succession is a very general one, and is framed to include cases which are not generally called unconformities, though they are discordances. There are, therefore, two kinds of discordance, and, as the

C

Fig. 118.

distinction between them involves important considerations, it is necessary that they should both be clearly defined.

A. Contemporaneous Erosion.-This is a partial discordance caused by the erosion of channels and hollows in a lower group of beds before the deposition of the overlying group. Thus in fig. 118, a channel has been eroded out of

the beds, c, c, and the succeeding strata, s, s, have been deposited horizontally upon the uneven surface thus produced. In such cases the erosion has only been local and partial, and no permanent upheaval or denudation took place in the interval. It is obvious, moreover, that if the deposits were subsequently tilted, they would both be inclined at the same angle, and would strike in the same direction.

B. True Unconformity involves complete discordance, and may be defined as the superposition of one group of beds upon the upturned and eroded edges of another group (as in fig. 117). In such cases the interval between the

3

Fig. 119.

two groups must have been long enough for great geogra phical changes to have taken place; these must have included the elevation of the lower group, the production of a land surface, and the subsequent depression of this surface below the sea. The magnitude or strength of the unconformity depends upon the amount of discordance between the inclination of the two formations, and upon the extent to which the lower rocks were disturbed or altered before the deposition of the upper group. Sometimes the difference of dip is so slight that it only becomes apparent after a large area of ground has been surveyed. In other cases the faulting or flexure of the older beds before the deposition of the newer, as in fig. 119, or their flexure and contortion as in fig. 122, renders the fact of unconformity easy to detect. Finally, if, as in fig. 122, the group, s,

d

b

a

consisted of crumpled and metamorphosed rocks, and the group o RS, of unaltered sandstones including rolled fragments of the metamorphic rocks, the unconformity would be very marked, and we should infer that a long period of time had elapsed between the formation of the two groups.

The occurrence then of a strong unconformity with a marked discordance may in itself be taken as a proof of elevation, denudation, and subsequent depression, and as presumptive evidence that dry land existed there during the interval between the formation of the two series of strata. But a case of contemporaneous erosion only proves that the lower beds have been brought within the action of

Fig. 120. Contemporaneous Erosion near Nottingham.

currents, without being raised above the surface of the sea in which they were deposited.

Instances of Contemporaneous Erosion.-The subaqueous erosion of a bed can only be effected by currents, and a surface which displays such channels and hollows must therefore have been brought within the influence of currents which only act powerfully in shallow water. Erosion of this kind is consequently an indication of shallow water.

Fig. 120 is an instance observed and described by Mr. J. Shipman, near Nottingham.' It will be noticed that the pebble-beds, a, have been eroded into hollows and ridges

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