What I mean is the fact that most urban areas tend to be built on older urban areas, this is particularly noticeable in the UK, as our cities are much older. So a basic example is in London that cobbled streets were built on soil tracks and now covered in tarmac.
The progression of time, industrialisation and the world we live in now all leaves its mark on the city. Our current layer with probably be built on as the march of the city continues!
This is a picture of Woodward Avenue (mentioned in this blog) it is currently partially closed for the building of a metro style train line, probably a bafflingly concept for Henry Ford!