Tianjin Shell Dike

Tianjin Shell Dike

A shell dike is a geomorphological accumulation formed by marine mollusk shells and their fragments, along with layers of fine sand, silt, peat, and muddy clay. It is generally parallel to or has a very small angle with the coastline. Formed near the high-tide line, it serves as a reliable indicator of ancient coastlines. In Tianjin’s land accumulation plain, there are four naturally seaward-aligned shell dikes: I, II, III, and IV. These dikes are roughly parallel to the modern coastline, distributed discontinuously in a ridge-like formation, representing the positions of coastlines during four different periods. The Holocene transgression that occurred between 10,000 and 5,000 years ago submerged most of the Tianjin Plain. Later, as sea levels rose and rivers deposited sediments, the land gradually emerged, leaving the shell dikes as relics of this historical process. They have become a distinctive type of coastal landform in the Tianjin coastal zone and also serve as remnants of the ancient coastline in the Bohai Bay, reflecting the changes in the shoreline from land to sea. The dikes host a rich variety of shells, mostly from extant intertidal or shallow marine muddy seabed mollusks. The dikes range in height from 0.5 to 5 meters, and in width from several dozen to several hundred meters, with lengths varying from tens of meters to over a hundred meters or even extending for more than a hundred kilometers. Their cross-sections are convex at the top and thin out or terminate at the sides. The III and IV shell dikes are located in the coastal area, with the IV dike being the closest to the present coastline. It is distributed near the eastern part of the marine accumulation plain, close to the supertidal line, and its orientation is largely consistent with the modern coastline. Near Qikou in Hebei, it merges with the III shell dike. Composed mainly of shells and their fragments, the shell species reflect an intertidal environment. These formations date back 700 to 500 years, and by the late Ming and early Qing dynasties, people were already living on the dikes.