Let us assume any humans inhabiting the areas have 5th century technology at most, are sparsely populated and not really doing much that would impact the geography. They are also inhabited by whatever animals are appropriate for the resources and climate. How fast do things such as erosion and plate tectonics alter things?
Particularly, I’d like to know:
What happens to a beach in a hot, humid, rainy climate over the course of 1000 years? How do coastlines change with age?
What would happen to a large rainforest similar to the Amazon if it remained untouched for a millenium? Is the treeline going to recede, advance, will the length of the wet and dry seasons change? Are the trees going to get closer together, further apart, will the canopy go up or down, are the dominant species going to change? Will the ground under the roots change its features overall?
How long does it take a river to go from straight and fast to long and winding?
How will steep, round, snow-capped mountains change in 1000 years? Does it matter whether the land at the bottom is a marsh or a desert?
How have the stone buildings we know of in rainy climates held up against erosion? Have they still mostly held their shape or are they crumbling into the foundations?
One good storm can alter a shoreline. If it blows a passage through a barrier island, for example, it will alter the flow of currents and change the erosion patterns considerably.
The apartment I used to live in had the living room cut off of it to keep in habitable. The cliff is at least 20 feet shallower than when I lived there.
Coastlines can chance noticeably in 60 years. This thing was at least partially underground during WW2. Around 2000 the thing was sitting on it’s wooden foundation poles which were holding it up out of the ocean.
This is a map of some of the Yorkshire coast, with the present coastline and loads of now vanished little villages shown offshore. I assume they’ve all disappeared in the last 1000 years or so as it’s unlikely there were any historical records of any of them before the Domesday Book in 1086.
The links at the bottom include some pretty dramatic pictures of how erosion is still eating away at the land…
I don’t really know how climate affects things but…with coastlines, you can get certain cycles where sand builds up in certain places, then is washed again (it depends on deposition factors as well as on the rest of the coastline is i.e. how sheltered or unsheltered) - I’m thinking of spit formation. The main things affecting coastlines are the wind direction (which affects which direction gets most attention from wind and waves), the strenght of sea currents, and possibly also the hardness of the rock (if any) nearby, which affects how protected that stretch of shoreline is.
The rainforest would probably remain largely static in shape because it is limit by where the rain falls - the Hadley cells (Hadley cell - Wikipedia) see to that. The trees will probably continue being as competitive as they always been. The dominant species could change, and the balance could be upset, but probably not significantly in just a 1000 years (taken as an average ;))
Rivers can change course just overnight - but it just depends on how resistant the material it is travelling over (and on which side of the bend). The volume, speed and gradient all affect how well a river erodes through stuff though (a straight and fast river would do it better, but a slower river may just flood and spill water everywhere)
I suspect snow-capped mountains would be have a greater rate of erosion because of snow\ice travelling downhill as it melts during warmer times…the steepness would of course increase that rate.
I think roofs tend to cave in fairly quickly just because there’s so many places for it to do so. Otherwise I think such structures last a long time simply because of inertia - the stones have little inclination to slip out when they are held there by so much weight above. If you look at cairns (which are not actually buildings as such :p), you can get piles\arrangements of stones which have lasted for thousands of years (although in some cases, if they slumped you probably wouldn’t really be able to tell…)
Look at a few ancient descriptions of geography and compare them with today, Thermopylae was a narrow pass for Leonidas and Xerxes in 479 BC for the German and the Aussies in 1941AD it was many miles wide. Miletus was once the greatest and richest harbour in the Hellenic world, it is today about 10KM inland. As is the presumed site of Troy. Mighty Babylon is now much further away from the river then it was in either of its two heydays.
The biggest issue is climate - that will determine the spread of vegetation.
For coastlines, a lot factors are at issue. Some land is still rebounding due to the compression of glaciers - IIRC, some areas of Sweden are still rising by inches per century. Other areas, the most famous is Venice, the ground was subsiding due to groundwater being pumped out of aquifers - hence the famous “aqua alta” where a foot or so of water will flood the St. Marks square and other lower areas during the highest tides. (But that is attributed to massive well-water pumping in the shore city for industries - since banned and the subsiding has stopped).
The whole issue of erosion and deposit depends on a lot of factors. The more water that flows, the steeper the hills in the river hinterland, then more pronounced the freeze-thaw cycle in those watershed areas - the more erosion that happens. The faster the river flow the more the source material is pulverized, the more is carried downstream.
After Katrina there were a lot of articles and maps about how much the Mississippi has meandered and how badly the damming effort had allowed the erosion of the existing delta and was preventing its expansion and rejuvenation. Just look at a map to see how much the river has meandered. Where it loops, the outer edge is faster so the loop expands until it cuts through, and a crescent-shaped oxbow lake is created. There are a few spots where a tiny part of one state is on the other side of the Mississippi thanks to its meandering. (Google Kaskaskia on Google Maps). Herodotus, IIRC, estimated the age of the Nile delta based on the locals’ description of how many inches of mud were deposited every year during annual floods.
The only interesting item about rainforests that I recall - when the concern started about the clearing of the Amazon for farmland, one ecologist pointed out that the soil was not that rich; due to heavy rainfall, nutrients could be washed away quickly. what keeps it growing is that the nutrients are locked up in living plants and deadfall slowly rotting. clear away the fast growing plants, burn the deadfall so the soil nutrients are soluble or erosion-friendly ash, and it turns to dead plains that will take generations to regrow.
OTOH, the west coast rainforests of British Columbia down to the redwoods of California, centuries and millenia of needles have formed a very thick carpet of slowly decaying rich soil; only the steep slopes and clearcut will result in an infertile environment.
Find articles on what has happened after the massive forest fires in Yellowstone to see the cycle of life in temperate forests; where fires create open meadows, which grow shrubs, which shelter the next generation of saplings to grow the next massive forest, until enough deadfall creates the fires for the next cycle. Each step has its own ecology. Many such environments different areas are in cycles, not static.
Over 1000 years - a lot depends on long term climate change. We’ve been through two little ice ages and warming cycles. In biblical times, Lebanon was renowned for its cedar forests that supplied lumber for ships; Carthage in north Africa was a serious civilzation with a decent wheat crop, as was Mesopotamia, and so on. Some theories say the Maya civilization died from prolonged drought, some say from poor agricultural and irrigation habits. Some of the destruction may be man-made, some may also be the result of millennia-long shifts in weather patterns, as areas of once plentiful rain dry up… Or a bit of both.
Easter Island was, for example, a lush forested island; until all the trees were cut down to make rollers for those heads. (Or possibly, because the settlers brought rats that killed the local birds by eating their eggs, and ate the nuts so trees wouldn’t grow. (Jared Diamond, Collapse) See pictures of it today and it’s just grasslands; when the white sailors arrived, the locals could not even make fishing canoes.
The tropical rain forest of the Congo has grown and contracted many times over the millenia - sometimes the savanna comes south, and sometimes the forest grows north. This site gives an overview of sorts (I’ve seen a better article but can’t find it right now). The section titled ‘Biodiversity Features’ about 1/3 of the way down is probably the most relevant part.
Smallhythe in kent is quite a good example. Henry the 8th built ships there but if you look at it today it appears to be entirely land locked. The large navigable river that ran through it in the 15th century has entirely gone. The river still runs through the valley but its moved significanty in 500 years. The program Time Team actually did a dig there and found lots of evidence of ship building but in a place that no longer appears to have any water ways.
Goinmg along with this, the future Henry IV landed in 1399 at the prosperous fishing community of Ravenspur, and the temnporarily-exiled Edward IV (intentionally) duplicated the act in 1471. Here is what’s left of it.
Biggest thing is whether the climate changes. If the climate is steady, then absent human impacts or other changes in animals, then in most places on earth the vegetation will reach a pretty steady state in 50 to 100 years. Big tree forests will keep getting bigger for a while longer, but look pretty much the same.
But don’t discount the impacts of 5th century human technology. Even stone age peoples with shifting agriculture (such as Natives in Northeast America) could make a big difference in the vegetation (regular intentional fires thinned the underbrush and favored fire-resistant trees, plus sections of the forest were regularly cleared and planted for a couple years then abandoned). And bronze-age settlements were pretty good at stripping the land of trees for fuel. In particular, lots of ancient harbors no longer exist because the population of the harbor town stripped the surrounding area of trees for fuel and building materials, and there was no vegetation to hold the soil onto the hills, so it all washed into the harbor, filling it in.