Introduction

Abstract

Sedimentary rocks normally exist in more-or-less parallel layers or strata, different rock types being distinguished by different physical features. Lyell (see figure 1.1), in Principles of Geology (1830), quotes Strabo as writing in Geography about A.D. 20:

It is not, because the lands covered by seas were originally at different altitudes, that the waters have risen or subsided or receded from some parts and inundated others. But the reason is, that the same land is sometimes raised up and sometimes depressed, and the sea also is simultaneously raised and depressed, so that it either overflows or returns into its own place again. We must, therefore, ascribe the cause to the ground, either to that ground which is under the sea or to that which becomes flooded by it, but rather to that which lies beneath the sea, for this is more movable... [and] can be altered with greater celerity.