London is one of the great walking cities of the world with centuries of history to explore. You can discover Roman history, the London of Dickens and Sherlock Holmes, and, of course, building stones. I recently came across a wonderful site from University College London that includes more than a dozen walks around the city. Each one reveals thought provoking aspects of the city and provides an easy and excellent way to learn the basics of geology.
Below are a couple of screen shots from one of the guides to graveyards. Happy exploring!