Twice daily on an open coast the edge of sea moves out, then in; the land grows slightly, then shrinks again. During an ice age, several times each 100,000 years, the silver line between land and sea travels much farther. Over the eons, the shore has rarely stood still, the area of land increasing and decreasing with the tides of time. Much geology is about this eternal movement of the coast; much of the rock record exists only because of it.

For many years geologists have contemplated this spectacle and wondered whether they saw the land rising or the level of the sea falling (or the other way around), a local problem. Now, believing that we understand the local part, we wonder why the relation between land and sea has been forever so unsteady.

Geological maps depicting ancient shorelines provide direct information; from them we may obtain the proportions of land and sea as they have varied with time. Many such estimates exist, each a little different from the others, all marred by lack of data, by the ravages of time, and by defects of the time scale. Nevertheless, on the grandest scale a coherent picture has emerged from two centuries of effort. Over the last 400 my, the continents have emerged from the flooded condition of the early Paleozoic only to submerge again in the Mesozoic, until the Cenozoic brought the deepest fall of the sea ever.