Field boundaries round here are entirely dependent upon indigenous material lying in the field. If there have been enough stones (in our case, limestones) then the farmer, many centuries ago, would have gathered them up and built them into a field boundary. If the field eventually changed shape (e.g. the farmer bought or sold some of his land) then it would be only a little trouble to demolish the wall and rebuild it in a new place. Even travelling about The Yorkshire Dales you notice a change in the colour of the stone used in the walls. In some places there is also a tradition to put "throughs" every few layers. A "through" being a slab which is slightly wider than the wall so that it sticks out either side and helps to steady the wall. For there is no doubt that frost, rain and wind (not to speak of stoats and rabbit who often live in a wall) are all capable of demolishing a wall. On our farm parts of the walls come down every year and one of the jobs to do in Spring is to build them all up again.
The two photographs above were taken in 1994 when my father-in-law was still alive. In one picture you can see the wall down, which gives you an idea of how small stones are used to plug the gaps. My father-in-law is sorting the stones out to rebuild it. In the other picutre he has begun to rebuild and you can see how neat and tidy it starts out, before weather conditions begin to move and reshape the whole thing.
Where there are no available stones then we have hedges. Some of these are hundreds of years old and have many different species in them. Holly is always present - it makes a good, thick, impenetrable hedge which the birds like for nesting; hawthorn of course is usually there along with field maple, blackthorn, spindle and a host of small trees (ash, sycamore etc.) which have self seeded. Over the years all these are colonised with wild roses (rosa canina) and with wild blackberries. Here and there will be a hazel bush which throws out beautiful golden catkins in the early spring.