Railsback's Some Fundamentals of Mineralogy and Geochemistry Unconformities, and their significance regarding geologic time Uncormities are ancient surfaces of erosion. un-interrupted sequences of layers of sedi- In each case, the surface defining the uncon- They are "unconformable" in the sense that they mentary rock. There are three commonly recog- formity is highlighted in red in the uppermost represent disruptions in otherwise simple and nized kinds of unconformities, as shown here. panel.

Modern land Angular Unconformity surface Disconformity Nonconformity A nonconformity A discon- An angular is an ancient formity is unconformity, as its surface of erosion an ancient name suggests, is between overlying surface of an unconformity sedimentary rock erosion between layers that layers and under- are not parallel but between lying unlayered layers that instead are at an igneous or meta- are parallel. angle. morphic rocks.

The upper layers The upper, hori- were deposited The layers were zontal, layers atop the erosion deposited atop were deposited surface, so we the erosion atop the erosion can step back in surface, so we surface, so we time by re- can step back in can step back in moving them. time by removing time by removing them. them.

Ancient land surface Prior to the ero- Steno's principle of Prior to the sion, there has to lateral continuity erosion, there have been rock tells us that the may have been into which magma tillted layers were layers of sedi- was intruded or originally continu- ment that were beneath which ous before being eroded, and of rocks were meta- eroded. which no trace morphosed. Ero- remains. sion of all that rock would Just one unconformity demonstrates that our require much Steno's principle of Earth must be, at the very least, millions of years old. time. original hozontality Working upwards through the example at left, much time tells us that these would be required to deposit the original layers; much time would tilted layers were be required for them to solidify; much time would be required to tilt deposited as them, much time would be required to erode them, much time would be horizontal layers required to deposit the overlying layers, much time would be required for them to solidify, and much and later tilted. time has been required to erode those layers to give the modern landscape. As he looked at one such unconformity at Siccar Point in Scotland in the 1700s, James Hutton concluded with only a bit LBR 8/2010 SFMGUnconformities03 of hyperbole that geologic time had "no vestige of a beginning, and no prospect of an end".