Regular Coasts continue " for great distances without notable indentations and, for the most part, in gentle curves, convex toward the land, which are connected by curved lines or meet at obtuse angles. . . . The flatter the coast, the more perfectly is this type developed, and the coast-line runs for many kilometers in the same curve. With a steep slope the course is regular only in general; in detail it seems as though drawn by a trembling hand, with numerous little prominences, which project but a few hundred metres beyond the general coast-line and separated from one another by shallow, curved indentations." (Penck.) Flat coasts in most cases border coastal plains and are very generally regular, while the regular steep coasts are marked by lines of cliff, which abruptly break the slope of the land toward the sea. Certain regular coasts, like that of eastern North America, for example, are uniformly flat or steep for long distances, others are alternately flat and steep, and are then said to be adjusted.

Regular coasts have few islands.

Coastal plains are absent from desert regions and are built up by the activity of rivers; they may be coalesced deltas, or of littoral origin, or submarine with the river sediments distributed along the shore by currents; subsequent elevation has converted the sea-bottom into land. The longest known coastal plain, that of the middle and south Atlantic States, is chiefly of this submarine origin. Other coastal plains, like that of Holland, are of mixed origin, subaerial, littoral and marine deposits occurring near and upon one another. Diastrophic movements have but little effect in changing the character of a regular coast bordering a coastal plain; elevation merely makes a new coastline along what was before the flat sea-bottom, and depression causes the sea to advance over the very gently sloping plain, in either case without changing the regularity of the coast.

Along the steep, regular coasts the line of cliffs, though pursuing a very uniform course, is broken by small bays, giving a serrate coast-line. It is on such coasts that the destructive work of the waves is most advantageously seen (Figs. 74-77, pp. 168-171); sea-caves, isolated pillars and stacks, and lines of rocky ledges and shoals abound. The submarine part of the slope is usually gentler than the subaerial, and descends gradually to depths of ten to twenty fathoms. Above water the height of the sea-cliff is moderate, seldom more than 300-400 feet and, consequently, this kind of coast is most typically developed in rather low lands, mountainous coasts giving rise to other forms. The line of cliffs intersects hill and valley and interrupts the system of connected valleys, making it evident that land has been lost along that line. Such a coast is obviously the work of wave destruction. The serrations are due to differences in the hardness of the rocks, the softer rocks being cut into bays and the more resistant ones standing out as headlands.

The bays, however, remain small, because in them the power of the waves is diminished, and soon a point is reached where no further retreat of the land is possible until the headlands have been cut back.

In brief, the low-lying flat coasts which border coastal plains are areas of accumulation, where the land is still, or has lately been, gaining at the expense of the sea, while steep, rocky coasts, bordered by lines of cliff, are areas along which the sea is eating away the land.

Diastrophic movements have a much greater power in changing the character of the cliff coasts than of those which are low-lying and flat. If the land is sinking, its valleys become submerged and converted into bays, thus forming an irregular coastline, while, if the land is rising, the wave-cut platform forms a plain at the foot of the cliffs, which are now inland and beyond the reach of the surf. Only in the very rare instances of the cutting action of the sea keeping exact pace with the movement of elevation or depression, will the steep and regular character of the coast be maintained. Hence, such coasts are restricted to regions which are stationary or in extremely slow movement.

Geographical cycles are seldom so clearly distinguishable along the sea-coast as in the interior of the continents, chiefly because coast topography is determined more by diastrophic movements than by marine erosion, which works very slowly on account of its limited sphere of action. Subaerial denudation also, as will be shown in the sequel, is an extremely important factor in controlling the character of the coast. Nevertheless, indications of the cycle may not infrequently be found. A newly upheaved coast tends to be regular and straight, because the sea-bottom is nearly flat and, when elevated, the sea-level marks a straight line upon it. As is true of the subaerial agencies, the first effect of wave erosion is to produce irregularities, cutting out bays along the softer rocks and leaving the more resistant ones to stand out as headlands. But, as already explained, the depth to which these bays can invade the land is very limited, since in them the power of the waves is greatly reduced, and hence the surf is unable to produce an irregular coast in the full sense of that term. However, such a coast may be formed by the combined work of the surf and of depression upon a land of strong relief.

The submerged valleys of subaerial origin become bays and estuaries that run far into the land, like Delaware and Chesapeake Bays, and the coast-line becomes highly irregular.

In either case, whether the irregularities are relatively insignificant and formed by surf erosion, or of great prominence and due to depression, the tendency of marine action is to straighten the coast-line and remove the irregularities and thus to produce an adjusted coast. This adjustment is brought about by the combined work of erosion and deposition. The bays are places of sedimentary accumulation, especially if rivers enter them; the mouth of the bay is first partially or completely closed by a barrier deposited by the shore-current, behind which the lagoon is silted up and converted into a marsh and eventually into a plain.. Meantime the headlands are slowly worn back, until the power of the waves is insufficient to cut them further. Provided the coast remains stationary for a long period, adjustment follows, and the ancient coast becomes regular, as the youthful one was, but with a difference of structure, for the adjusted coast is made up of bold, truncated headlands, alternating with low-lying plains and marshes.

The Italian coast, with its broad, gently curving gulfs, is an example of an adjusted coast, and in the province of Tuscany these changes have largely taken place within historic times: "The bays of Piombino and Grosseto were cut off from the sea by bars, the lagoons thus formed were transformed into swamp, the dreaded maremmas, which in their turn were filled in. The former island of Monte Argentario was connected with the mainland by two bars." (Penck).