我国首都北京和华北最大城市天津所在的华北平原,是一望无际的肥田沃土。每当夏秋之交,满地庄稼,一片碧绿。谁会想到,这里过去竟是波涛滚滚的汪洋大海。近年来在天津附近及其以东地区所发现的四道古海岸线的遗迹,生动地证明了华北平原曾经发生过“沧海桑田”的巨大地质变化。古海岸线的“见证人”——贝壳堤经过考察,在天津附近及其以东地区发现了四道古海岸线的遗迹,看来象是人工建筑的河边防洪堤。实际上它是从前由海浪在海滨冲积而成的,所以叫做滨岸堤。由于它的组成物质中有大量的海生贝 The North China Plain, where Beijing, the capital city of China, and Tianjin, the largest city in North China, is located is an endless fertile land. Whenever summer and autumn turn, crops everywhere are green. Who would have thought that in the past there was a sea full of waves. In recent years, the remains of the ancient coastline of the four ancient coastlines found in the vicinity of Tianjin and in the east of the region have vividly proved that there have been tremendous geological changes in the “sea mulberry fields” in the North China Plain. The “witness” of the ancient coastline—the shell embankment was surveyed, and the remains of the four ancient coastlines were discovered near Tianjin and in the area to the east of it. It appears to be a riverside flood protection embankment for artificial buildings. In fact, it was formerly formed by alluvial waves on the waterfront, so it was called shore bank. Because of its large number of marine shellfish