Cities are a technology of the Stone Age.
The management of rainwater runoff is becoming a central concern for many cities; climate change is shortening the duration of transitional seasons, but increasing the amount of precipitation. In one sense, stone material in cities are central to this problem, since they prevent both percolation and soil hydration. Coincidentally, they are chosen, in part, for these characteristics, in effort to secure other urban value assets–infrastructure like transportation, piping, electricity, et al. But stones are also chosen because when exposed to moisture, they erode more slowly than human life or a government. If our life span wer only 24 hours, leaves would suffice as a city floor.
While homes may be made of wood or steel, cities are made of stone. Some places have dirt or sand roads, and structures made of sticks, or just a roof, but they are limited in dimension. They may be inhabited, they may be massive conglomerations of tin roofs, or mud huts, but are they cities? (Some countries have different legal taxonomy for village, town, or city, but does the legal status change the experience or activity of the place?) And does the weather, and subsequently the materials we lay in response to the environmental conditions, change our experience or activity? If we become a globe of water, and our cities are floating consortia of boats, will they be our cities?
Austria is considered a water-rich country with mountain springs, run off, rivers and lakes. It is also stone rich. The streets of Graz show evidence of the ongoing relationship to the hallowed ground on which the city sits. Different stones connote different eras of construction–spanning centuries–for different uses and different masonry technologies. Currently, we are in the asphalt era. A black, impermeable sheet shaped to the geolocation's demands: curved, flat, roughed, smoothed. (Some new forms of concrete facilitate percolation.)
In a city the pedestrian, cyclist, and motorist all require a solid surface. In Graz, these superficies are wrought in different stone. Some stones connote a bike lane. Some a path for the blind. The Belgian blocks of the entrances to many hofs, including into the center of Priesterseminar, connote an interior public space that borders on private; the stones are laid in intersecting arcs, connoting strength. The sidewalks, streets, and curbs are all different stones.
At the first Soil Symposium, I learned that almost 80% of the city of New York is covered in an impermeable material, concrete or roofing. Looking at the aerial map of Graz, I see the pattern of terra cotta bordered blocks with a green center in the old city. Outside the old city, where smaller structures become the norm, greenery blends with concrete and homes to average a gray area. In the surrounding hills, forest green dominates. I would estimate that 50% of the surface of Graz is impermeable.