Wooden structures are the weakest, of course. Houses and barns and office buildings constructed of wood will go away within a few years of the last person walking away from them. The water and the wind and the microbes will eat that stuff up, push it over, and rot it right down into the dirt so fast you'd think it was all part of someone's dream.
Stone work is not that much more resilient. Yes, it will hold up to erosion far better than wood, and things made of stone will retain their artificial angles for a long time, but eventually the Earth will bury it under the forest floor or mud or sand. Not even stonework can stand up to the forces that are constantly at work around us.
Metal will last even longer. Some of it can even resist the interaction with oxygen for quite awhile. But eventually it will deteriorate or, as with everything else, become buried.
I think this knowledge of the impending doom of everything human came from my very early travels with my father to search out the things created by the Native Americans who were here before us. We'd go to visit Indian mounds and we'd dig in the earth at likely sites and use sifters made of two by fours and hardware cloth to look for various Indian relics (as my dad called them). Spear points. Arrowheads. Pottery. Effigies. We found all of that stuff by the barrels that we'd admire and salt away in the basement. "There were people here before the Europeans," my dad would tell me. "Hundreds of thousands of them. Millions of them. We killed them all and everything they built is all torn down or covered up in forests or dirt or under our cities." Indeed, it was true.
So it will be again. Everything that modern man has built everywhere on the planet will one day be dust. I have it on good authority--that being the evidence of my own eyes and senses. Knowing that everything we have built or ever will build will someday be dirt and shit is something that makes it easy for me to sleep at night.
Someday, Son, this will all be yours:
A little graveyard I happened upon in the Great Smoky Mountains. I've tried a couple of times to relocate it, but I can't find it.
One of the last existing structures from a city once called Cataloochee. It was a major apple producing town that had hundreds of buildings, stores, roads, offices, and two post offices. Now it's all forest, gone back to wilderness in the Great Smoky Mountains National Park.
What the Hell is this? Found along the Boogerman Trail in Cataloochee.
A wall in the GSMNP. Going slowly back to forest. Eventually those trees will push it down and the forest floor will cover it all over.
Another house in Cataloochee. The only reason it still stands is because the park makes some small effort to preserve it as an artifact. If the horse-loving morons who often ride past it don't stop vandalizing it, soon it will fade into the dirt like everything else.
Yours truly standing along one of the withering walls along the Boogerman Trail. Already the trees are beginning to push this one over.
Long ago, when the park was formed, someone leaned this tractor wheel against this poplar. The poplar is slowly absorbing the metal wheel and will, eventually, eat it.
Once, this was part of a downtown area. The buildings that once stood in this present-day field are barely even recalled in withered memories.