After the Storm: Matching Cliffs Across the Atlantic

A Mystery in the Sea Cliff

1Port Calder is a fishing town on the rocky Atlantic coast of North America. Last winter, strong storms battered the town's sea cliff, and big waves tore away about 2 m of rock in some places. While the town planned how to protect its harbor road, seventh grader Desmond Achterberg found fossils in a freshly exposed layer of gray slate. They were trilobites, extinct sea animals that crawled on shallow sea floors. He brought them to Ingrid Mulcahy, the curator of the town's harbor museum.

2Ms. Mulcahy showed Desmond photos from a museum in Ballinroe, a town on the coast of Europe about 3,200 km away across the ocean. The Ballinroe sea cliff has the layers shown in Table 1. Trilobites like these lived in shallow water near land and could not cross a deep ocean. She added that the low hills behind Port Calder are the worn-down roots of an old mountain range, and a range of old mountains with the same rock types and ages begins at the coast near Ballinroe.

3The museum also keeps fossils from other rock layers on both coasts (Table 3). Fish B lived only in rivers and lakes. Seed fern C was a land plant whose seeds were large and heavy, too heavy to blow or float across an ocean.

4Desmond's class used a computer map to slide the two continents together and measure the average misfit, the gap or overlap between them (Table 2). They tried two edges. One was the shoreline, which waves and storms keep reshaping and which moves as sea level rises and falls. The other was the edge of the continental shelf, the underwater edge of each continent at about 1,000 m deep.

5Today, about halfway between the two coasts, a long underwater mountain chain called the Mid-Atlantic Ridge runs down the ocean floor. Satellite measurements show the two coasts moving apart by about 2.5 cm each year.