James Hutton, a Scottish physician (1726-1797) is often viewed as the founder of modern geology.  He observed the features and the landscape, and described how  if given enough time they could from from natural process that he could  observe.  This has come be known as uniformitarianism, an idea amplified by Lyell.  Hutton's 1788 book ended with the observation "The result, therefore, of our present enquiry is, that we find no vestige of a beginning,?no prospect of an end."  At that time science has no idea how old the earth was, and if geologists had speculated, they would have guessed an incredibly young age in the millions of  years instead of billions.

This figure shows Hutton's unconformity at Jedbergh, Scotland (John Clerk, 1787).  A soil horizon separates vertical sedimentary rocks below from horizontal rocks above.  The vertical rocks must have been lithified, uplifted, and eroded before the soil and horizontal rocks were deposited.  This is an example of cross-cutting or truncation.

Last revision 10/2/2015