Although the changes in physical geography which made and unmake continents are slowly acting around us all the time, it is not often that we can clearly see them taking effect. Photos 154 and 155 are therefore particularly interesting, for they show one of the processes at work. Part of a forest is in the actual course of being killed by the pond which is forming on sinking land. This pond and several smaller ones of the same kind can be studied in the neighbourhood of Delamere forest, in Cheshire. Here the under soil gets washed out in certain places, and the surface earth sinks and forms a hollow in which water collects. In fig. 154 you see one tree standing in the middle of the pond. It is dead, and has been killed by the water (you remember that ordinary plants are drowned by too much water) and in fig. 155 you see a large area entirely covered with water, and the dead trees standing up through it. This pond is spreading rapidly, and is a good illustration of the reverse condition from that seen in fig. 144, where the plants by their growth are filling up a pond. The washing out of the soil and the collecting of the water in this case was quite beyond the control of the plants themselves, but they are supremely affected by it. CHAPTER XXXV. PLANT-MAPS In the last chapter we noticed a few of the many facts which show us that a close relation exists between the plants and the nature of the land on which they grow. We may now try to express these facts in a simple way by making maps of the land according to the plants growing on it. There are maps of the whole of England, made by the Government, which show all the roads and houses, the chief rocks, hills, ponds, and so on. The geologists have taken these maps and added to them details of the kinds of rock and soil of which the land is built. If now we take fresh copies of the “ordnance” maps, as they are called, and put on them all the plants growing in different associations, we can compare the resulting “plant-maps” with the land-maps of the geologists, and I think you will be surprised to find how much the plant-maps and land-maps correspond. To do this on a large scale, however, is far too big a piece of work for one person, or a few people, to attempt. We can only do some small piece of work on one area which will show how the rest is done, and yield some interesting details. Let us suppose, for example, that the moor east of Settle is to be mapped. First get an ordnance survey map on a large scale--25 inches to the mile is the best, but the 6 inches to the mile will do. On the map are marked all the walls, streams, and even some of the bigger trees, so that it is easy to find on it the exact spot where you are standing. For working you should cut the sheet up into at least eight pieces, of regular size and shape, and use one of these at a time in the field.