By reusing the building materials and use the free spaces between ruins and buildings as farmland Official Nessa Jenkins Oh oh oh Merry Christmas Ugly sweater, after many years new earth was put on top to fertilize the soil. But the rise of the street level could have a more natural reason too, like at the Campus Martius where parts of the Solarium Augusti are covered under layers of mud and sedimentation by the many floods of the Tiber river. Decaying trees, leaves and also rotten wooden buildings would contribute to these layers of dirt.
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Also, sedimentation of sand and soil from hilltops by erosion during heavy rainfall could be a reason which covered up the ruins and buildings Official Nessa Jenkins Oh oh oh Merry Christmas Ugly sweater. Another option was that people used the free space as a garbage dump. Literally. By piling up the garbage, often the ruins were conserved. This too contributed to a rising street level. Keep in mind that people certainly didn’t see the Ancient Ruins as rich archaeological sites as we do nowadays.
As the ages passed, new buildings were built on top of these new layers of earth, piled up garbage, soil, fertilizer, and manure. This gives us a good example of how the earth and the spaces we are living in are in constant motion and that we, as humans, are mostly responsible for our own living areas. As others have said, not all the Sahara is covered by sand. There are rocky areas, mountains, and areas of soil. In places, and to the south, the Sahara transitions to scrub and grasslands.