Go and see the amazing natural phenomenon of the La Brea Pitch lake. As the picture shows, bitumen wells up from deep underground. Once on the surface some of the lighter hydrocarbons evaporate into the air, leaving the bitumen behind. This hardens to a crust that can be walked on and even driven on. In some places, as the picture shows the softer viscous tar beneath breaks the surface.
There is an old amerindian legend that a village chieftain killed a humming bird and his village was swamped by the pitch lake in revenge. It may even be at least partly true. A fault in the earth may have started to leak petroleum within human history. In that case the welling up of the petroleum from within the rocks could easily have overwhelmed a village in the hills. The methane gas may have asphyxiated them first and then the slowly welling pitch gone on to engulf their village. A challenge for archaeologists. If they are there they would preobably be well preserved.
This shows guide to the tar lake demonstrating how the tar wells up.
