eastward directed beach-drift along the Channel, for that up the Bristol Channel, and for that along the coasts of Cardigan Bay. Another factor is also important—the relation of wind-direction to the amount of open water off a particular coast. On the Cumberland coast, the direction of beach-drift is north and south from approximately St. Bee’s Head. This is in general conformity with the amount of open water off these two parts of the coast. However, the relationship is better seen on the east side of England. Along the Norfolk coast, excluding minor exceptions, the travel of beach material is on the whole westwards from Sheringham along the north coast and south-east and south from that same place along the east coast of the county. The dominant winds and waves approaching the Cromer-Sheringham coast are from the quarter between north and east: these, working in with the extent of open water offshore and with the general trend of the coast, are mainly responsible for the outward drift from that locality. The southward drift of beach material continues, apart from a few minor interruptions, as far as the Thames.
On the whole (except in the inner parts of the Firths of Tay and Forth) beach material travels southwards from north-eastern Aberdeenshire all down the east coast of Great Britain. It is, however, along the more open coast south of Flamborough Head that this is most noticeable. Along the south shore of the Moray Firth and the coast as far as Banff and even Rosehearty the general movement of beach material is to the west, and southwards from Wick it is also directed towards Dornoch Firth and Inverness.
On an indented coast of hard rocks it is difficult to generalise. Each separate bay usually has its own beach, and whatever solid stuff travels round the enclosing headlands does so below water level and cannot easily be traced. The individual coves of Cornwall, Devon, Pembrokeshire, the north coast of Scotland, and elsewhere may have their beaches temporarily removed by storms, but they will gather again in normal times. It is probably true to say that each bay has its own shingle and sand economy. On relatively deep water coasts, such as that of the west of Scotland, it is impossible to generalise about the travel of sand and silt.
The main contrasts we have made between the different parts of the coasts of Great Britain may perhaps be related to an even more general factor. Apart from the Lancashire coast, and excluding local occurrences of boulder clay, it is approximately true to say that a line joining the mouth of the Exe to that of the Tees separates a region of softer rocks and simpler structure to the south and east from a more complicated region of harder rocks to the west and north. The former is associated with long lines of open beach and sweeping curves along which beach and long-shore drifting are well exemplified. The latter is often a coast broken by inlets and hard and rocky lines of cliff, along which lateral movement is irregular.
CHAPTER 3 SOME ECOLOGICAL CONSIDERATIONS
IT IS HARDLY possible to understand how the vegetation is distributed round the coast-line without having some slight acquaintance with the principles of plant ecology. In this chapter we shall therefore consider quite briefly what ecology is about and also take the opportunity to explain some of the terms which are commonly used by ecologists. No attempt will be made to go more deeply into the subject than is necessary to follow the method used in the later chapters, which describe in detail the characteristic vegetation to be found in various typical habitats along the shore. For a fuller account of the subject the reader is referred to Professor A. G. Tansley’s fine book, The British Islands and their Vegetation. In the following short account the examples have been chosen as far as possible from seaside vegetation in the hope that the main characteristics of coastal habitats in general will become apparent.
Plant ecology is concerned with the study of plants in their natural habitats and their relations with their surroundings. It is thus primarily a field study and can be worked out only in the place where the plants are actually growing. The present popularity of both plant and animal ecology is to a certain extent a reaction from some of the more specialised lines of inquiry in biology, which have to be carried out indoors in laboratories.
One of the most fundamental differences between plants and animals is that the former are fixed in the soil, and cannot therefore move about when they are growing. They are thus, of necessity, gregarious and have to lead a communal life. Plants are, in fact, usually found in well-marked communities, whose composition depends on the nature of the habitat and a number of other factors, some of which are discussed later in this chapter. The word plant community is a general one which is used to describe any collection of plants growing together which can be said to possess a definite individuality. If there is much bare ground between the individual plants, which is available for colonisation by other species, the community is said to be open. The plants found growing on the front (seaward) range of sand-dunes in an area of blown sand form a typical open community (Pl. VII). Other obvious examples to be found amongst coastal vegetation are the communities inhabiting exposed sea-cliffs (Pl. II), and the mobile mud along the edges of salt-marshes (Pl. XIII). When the vegetation is more or less continuous, and competition for the available space becomes an important factor, the community is said to be closed. An open community generally represents an early stage in the colonisation of an area, but it may also be found in a habitat where the conditions are so harsh that plants have great difficulty in existing at all.
Although the individual members of an open community depend largely on the nature of the habitat, the amounts and nature of the species present will depend increasingly on their inter-relations in the available space. Usually one or more dominant species, which are mainly responsible for the general appearance of the community, can be recognised. They are frequently the tallest-growing plants present and may thus exercise a profound influence upon the other inhabitants of the community, particularly by competing successfully for the available space or by causing shade. As examples from coastal vegetation, we may mention rice-grass (Spartina townsendii), which is the main dominant species in the communities formed on the soft mud of salt-marshes along the south coast (Pl. XIV), and the sea-rush (Juncus maritimus), which frequently dominates a zone along the upper edges of salt-marshes elsewhere. The other species associated with these dominants are known as subordinate species. If these are found in nearly every example of a community, they are called constant species. Any other plants which turn up from time to time in the community, but are not really characteristic, are known as casuals.
Plant communities may be of very different sizes and importance, and it is customary to divide them into various classes. The largest unit of vegetation is called a plant formation and usually refers to a broad type of vegetation which remains roughly the same over a whole continent or even throughout the world. The character of a formation depends on the nature of the habitat and it reflects this in the distinctive life-forms of its principal species. Thus the Salt-marsh Formation contains a highly characteristic population of halophytes, whose specialised life-forms reflect the most important feature of the habitat, that of its periodical immersion by sea-water. Similarly the Sand-dune Formation contains another very characteristic population of plants, many of which are xerophytes and specially adapted to grow in the semi-arid conditions of blown sand. (Some ecologists restrict the use of this term to the ultimate climax vegetation which can be developed in a habitat under given climatic conditions, and would not therefore refer to either of these essentially transitional types as formations.)
The term plant association has in the past been used to refer to so many different units of vegetation that, to avoid confusion, it has not been employed in this book. It is now generally accepted that it should be used to describe a relatively large unit, usually a geographical sub-division of a formation which is characterised by a particular dominant species. As an example, we could say that the Oak-Beech Association is the typical form in the British Isles of the main European Deciduous Forest Formation. In the same way, the Marram-grass Association is typical of the Sand-dune Formation in this country, though associations with other plants