http://londonpavementgeology.co.uk/
Cool project in London... soon with a free app available for apple and android.
Others should follow suit.... I`ll definitely check it out on my next trip to London.
"There is a wealth of high quality rock samples of all types to be seen in the capital [London], mostly visible to the public from the pavement. A searchable collection is offered here for the benefit of students, amateurs and professionals alike. Please respect the fact that in many case the geological specimens are somebody else’s property, please only take photographs, after asking permission, wherever possible."