Well, that were a lot of complicated words in one title, but the concept is very simple. In geology different time periods are represented by different layers of sediment and rocks (simplified of course). By  examining what layers are deposited, or how they are folded, broken or eroded, it tells a story of the past. In urban geography this is can be used as a metaphor for the inertia of  historical constructs.

A lot of these constructs are no longer the most efficient or most desired, but they remain in our cityscape because reconverting them in the current society is more profitable (or desired) than leveling the existing structure and starting from scratch again. This gives us a window to look into the past of historical societies. This causes the existing structure to be a mix of historical and modern structures. 

Source 1 & 2

An example

An examples of this are the old city patterns. Depending of the time a city was founded, the pattern can vary from a medieval pattern, with a central market and radial street pattern, to a checkerboard pattern pointing back to the colonial area when they were consciously trying to design new and more efficient cities.

    Source Louvain - Belgium                            Source Hamilton - Canada

These are only two examples of how historical societies permanently shape the modern constructs, but there are many more on both smaller and larger scales. On a smaller scale for example, we see the reconversion of old industrial sites or conservation of heritage remained from wars. On a larger scale, nice examples are deserted cities due to relocation of industry.

Conclusion

This is a very important factor that defines the cityscape of every society you can imagine and it is so interesting to look at the world from the perspective of the geological metaphor. I wanted to explain this since I will be writing about common city patterns and how they came to be, and this is crucial to understand. Have you found elements like this were you live? I would love to hear about them!