Overview

Just as a book can be called the fundamental unit of a library, so can rock formations be thought of as the fundamental units of historical geology. However, until the eighteenth century, few people really noticed that rocks come in discrete "packages," and nobody thought to try to use these formations to reconstruct the geologic history of a region until Johann Lehmann (1719-1776). Although Lehmann's interpretation of the rocks has since fallen out of favor, his insights have proven fruitful and, over two centuries later, the concept of rock formations continues to be used in much the same manner and for much the same purpose as originally proposed.

Background

The first geologists are lost in the depths of time. They were our most distant ancestors who noticed that flint and obsidian...