How Do Ancient Ruins Get Covered In So Much Earth?

Have you ever wondered that so how bog valleys get covered with so much of the earth over them?
Is the soil formation very fast?
A neglected garden path can become hidden in less than 10 years. So its not so surprising that archaeologists have to dig meters down to find very old buildings.

More earth to cover any area is created as rocks are broken down in small particles by weathering and erosion. More contribution comes from plant decay and by worms which convert organic material to soil via their digestive system. So a ruin in river valley where there re lots of plants and eroding water can, soil could cover it in few centuries.