What man-made monuments in the U.S. could last for 500 years?

Question

It’s the year 2568, exactly 500 years after Nuclear war destroyed the two superpowers that started it, the United States and the Soviet Union, along with others such as China, both Koreas, Japan, France, The UK, India, and the Middle East.

What American monuments that weren’t hit directly by the bombs could survive 500 years without any repair?

• By "weren't hit directly" I mean, assuming no monument was destroyed during the nuclear war, and given the post-apocalyptic conditions, which monuments would stand the test of time?

• I am interested in man-made landmarks such as Mount Rushmore and Hoover Dam.

Answer 1

Mount Rushmore

Albeit with some erosion. To imagine the result of 500 years of lack of maintenance, take a picture of the current state of Mount Rushmore and apply a mild Gaussian blur filter.

We are talking about four gigantic faces carved in stone on the side of a mountain in a region that is not densely populated, filled with wildlife, far from industrial complexes (e.g. acid rains are unlikely), and surrounded by woodland, prairies, and badlands.

Interesting part, behind one of the faces there should be a room carved inside the mountain and filled with documents. It could probably be used for something else too.

The rest

Dams

Dams require constant attention. In 500 years there may be but a slight memory of it from some pieces of concrete attached to the sides of the mountain. The water will break through and erode its way across. A lucky dam will crack at the bottom and remain as an archway across a river. I doubt that is likely to happen, but if it does, here is a list of the tallest dams in the US.

A note about gravity dams. As suggested by Andon in the comments, gravity dams may be less prone to failures and thus longer lasting, perhaps even for the timescale of the question.

Skyscrapers

Skyscrapers are marvels of engineering, but they too require constant maintenance. Not much will survive 500 years due to the elements, corrosion of the internal supporting structure, and pillaging of construction materials. At best, the foundations of these tall buildings may remain as a sign of their greatness.

Metal-made constructions

Bridges (Golden Gate), signs (Hollywood sign) or statues (e.g. Liberty Statue). These too require constant maintenance. Today the main threat is due to rusting and corrosion. In 500 years without a stable social structure, these constructions will be the number one source of free high-grade metals. Not much may be left, except perhaps the pedestals.

Other monuments and memorials

A quick look at the rest, taken from a subset of two wikipedia lists: National Monuments, National Memorials

1. High chance of surviving monuments, given that they already survived a long time on their own without proper maintenance (e.g. ancient precolombian dwellings); also, their being further away from bigger cities may spare them from pillaging or being used a quarries for construction materials; some could likely be buried under some meters of soil brought by rainwater, wind or partially damaged by minor land slides: Agua Fria, Aztec Ruins, Bandelier, Bear Ears, Chimney Rock, Fort Union^{(not really in working order)}, Hovenweep, Little Bighorn Battlefield Monument, Navajo, Poverty Point, Tuzigoot, Wupatki, Fort Caroline
2. Probably still existing, at least in part, but being a good standing castle, or a defensive structure, possibly with thick stone walls stone, it might be used in between as a defensive structure and suffer significant damage; also, not negligible chances of the structure being completely demolished during some skirmish, especially near bigger cities: Castillo de San Marcos, Clinton Castle, Fort Frederica, Fort Matanzas, Fort McHenry, Fort Monroe, Fort Pulaski, Fort Stanwix, Fort Sumfer, Governors Island, Montezuma Castle, Salinas Pueblo, Chamizal, Lincoln Memorial, Jefferson Memorial,
3. Probably still existing, but less likely to be accessible due to natural problems, such as earthquakes collapsing caves, or floods placing the grounds underwater; if no such events occur, then the monument should still be there as it has existed already for quite some time without maintenance: Glia Cliff Dwellings, Ocmulgee, Russell Cave
4. Very unlikely despite being robust and sound structures, mostly due to location, accessibility as quarries, and not easily defendable unless major masonry work is undertaken: Lee Memorial, Federal Hall, Grant's Tomb, Korean War Memorial, Martin Luther King Memorial, Perry's Victory Memorial, Vietnam Veterans Memorial, Washington Monument, War World II Memorial

Answer 2

I really like the other answer for this question but I just wanted to add a little something to the discussion. The odds of a monument's survivability relies on the massiveness and the maintenance requirements of the structure. Materials also come into play as buildings made out of ferro-concrete are stronger but last less long compared to those that aren't thanks to rust if even a pocket of air exists in its structure (according to this link).

I know you're focusing on the US, but consider the rest of the world as well - the Pyramids of Giza would definitely still be standing (unless our largest bombs were targeting Cairo), despite being thousands of years old already. Chichen Itza, Machu Picchu would also still be standing, as would more massive temples and ruins that were not located in big cities.

Now to more modern constructions: massive baseball fields would probably still be standing, as would important government buildings and others. I know you asked about monuments, but 'regular buildings' would also be around - not looking as nice but still standing. For example, the capitol building in the town of Pawnee, Indiana (from Parks and Rec) would still be standing as I'm assuming it didn't get hit by a nuke. Our recipe for concrete isn't as good as the Roman recipe, but it's not terrible, so let's give ourselves a bit more credit :D

Just wanted to add to the conversation as it seemed pretty 'dam'-heavy in the previous response.

Answer 3

(From the way your question is phrased, I'm assuming you mean "Leave remains that are recognizable as a man-made structure or monument" and not "Remain in working order." If you mean the latter, you way want to re-word your question.)

The answer is that a great many things would survive. Five hundred years is not a long time, really -- just think how much has survived 500 years, after all nearly every medieval structure is at least 500 years old, and many of them have not been maintained: Castles, Roman roads, forts, tunnels, pueblos, quarries, bridges, aqueducts, walls, statues, churches -- there are vast numbers of things that survive.

The two great enemies of human structures are glaciation (and 500 years is probably too short a time for the Earth to tip into the next ice age) and people. People can quickly destroy things by rebuilding right in the same spot and using the old structure's materials.