The shape of the modern coastlines is the result of a 70 meter rise in sea level that took place in the last 12,000 years after the last great ice sheet that covered much of North America and Europe had melted. It is important to note that the shoreline, though the edge of land, is not the edge of the continent. In most cases, the continent extends seaward 100's of kilometers beyond the shoreline. The actual edge of the continent is marked by the transition from the continental slope to the continental rise . The position of the shoreline is a function of both continental topography and sea level.
Several factors can affect sea-level change.
As we have seen, one of this factors in the amount of ice on the continents. At times when the continents were covered by great ice sheets, sea level was low and the continents were exposed. The last glacial maximum was 18,000 years ago. For the last 20 million years, the continents have been largely high and dry because there has been extensive mountain building in Asia and there has been significant ice on Antarctica. The oldest known glaical episode occurred in the Precambrian, approximately 2.2 billion years ago.