The boundary between the coast and the shore is known as the coastline it marks the seaward limit of the coast. The outline of the coast may be modified by sea waves.

According to D.W. Johnson, coastlines can be divided into the following classes:

1. Coastline of Emergence

2. Coastline of Submergence

3. Neutral coastline

4. Compound coastline

5. Fault coastline

This classification has two bases :

(a) the change of levels, i.e if sea level rises the shores are submerged and if the sea level falls, the shores emerge.

(b) the nature of shorelines, because it is necessary to know the nature of sea shore before emergence and submergence, whether the shore was upland or downland for instance.

Coastlines of Emergence: These are formed either by an uplift of the land or by the lowering of the sea level. This type of coast has bars, spits, lagoons, salt marshes, beaches, sea cliffs and arches. The east coast of India, especially its south-eastern part, appears to be a coast of emergence. The coast has however been invaded by the sea a number of times during the past. The west coast of India, on the other hand, is both emergent and submergent. The northern portion of the coast is submerged as a result of faulting and the southern portion, that is the Kerala coast, is an example of an emergent coast.

Coast lines of Submergence: A submerged coast is produced either by subsidence of land or by a rise in sea level. The important types of such a coastline are ria, fiord, Dalmatian and drowned lowlands. When a region is dissected by streams into a system of valleys and divides, submergence produces a highly irregular, embayed shoreline called ria coastline. The coast of south-west Ireland is a typical example of ria coastline. Some coastal regions have been heavily eroded by glacial action and the valley glacier troughs have been excavated below sea level. After the glaciers have disappeared, a jord coastline emerges. These coasts have long and narrow inlets with very steep sides. The fjord mouths are often dotted with small hilly islands which were once the outlying hills. The fiord coasts of Norway are a typical example. The Dalmatian coasts result by submergence of mountain ridges which run parallel to the sea coast. Therefore, there is a series of fold mountain ranges with alternating crests and troughs running parallel to the coast. The Dalmatian coast of Yugoslavia is a typical example. A drowned lowland coast is low and free from indentations, as it is formed by the submergence of a lowlying area. It is characterised by a series of bars running parallel to the coast, enclosing lagoons. The Baltic coast of eastern Germany is an example of this type of coastline.

Neutral Coastlines: These are coastlines formed as a result of new materials being built out into the water. The word "neutral" implies that there need be no relative change between the level of sea and the coastal region of the continent. Neutral coastlines include the alluvial fan shaped coastline, delta coastline, volcano coastline and the coral reef coastline.

Compound Coastlines: Such coastlines show the forms of two of the previous classes combined, for example, submergence followed by emergence or vice versa. The coastlines of Norway and Sweden are examples of compound coastlines.

Fault Coastlines: Such coastlines are unusual features and result from the Submergence of a downthrown block along a fault, such that the uplifted block has its steep side (or the faultline) standing against the sea forming a fault coastline.