The relatively calm trailing edge of continents are called passive margins, while the leading edge of continents are called active margins.
Active margin is a zone where tectonic plates either converge with, or shear past, one another. The active margin is in the West Coast of the United States, which is characterized by rugged coastlines with narrow beaches and steep sea cliffs.
Passive margins are continental margins which are not tectonically active. Thus, they mark the sites where continents have rifted apart to become separated by an ocean. The Eastern Seaboard, as is the Gulf Coast is a passive margin.
Hence, the answer is given and explained above.
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