Seashell banks are made up of seashells and fragments, sand, turf and mud. There are four ancient seashell banks representing four different coastal locations during four periods. Transgression of the sea happened 10,000 to 5,000 years ago and submerged most of the plains of Tianjin. The sea level then declined and, as rivers formed, land also formed. The seashell banks, created during this historic period, vary from 0.5 to 5 meters high, tens to hundreds of meters wide, and ten to hundreds of meters long—even extending hundreds of kilometers in certain areas.
The first seashell bank is in the southwestern river flat from Shenqing Village in Dagang District to Miao Village First Line in Huanghua County, about 22 to 27 kilometers away from the modern seacoast. This ancient coastline existed 5,200 to 4,000 years ago.
The second seashell bank is in the western river flat from Zhanggui Village to Juge Village First Line. Shaped as a ridge in a field, the seashells in the bank reflect a lagoon and loch environment. This ancient coastline was formed 3,800 to 3,000 years ago. Also discovered here were culture relics from the West Zhou Dynasty and the Warring States Period.
The third seashell bank is in the eastern river flat, from Junliang City to Mapeng Kou First Line, about zero to 20 kilometers away from the modern coastline. The cultural relics discovered at the bank indicate that this ancient coastline existed during the East Han and Tang Dynasties.
The fourth seashell bank is near the modern coastline and the super high-tide mark in the eastern portion of the marine plain. Its direction is the same as the modern coast and it crosses the third seashell bank. The bank was formed 700 to 500 years ago and, based on history, people have settled on the bank since the end of the Ming Dynasty and the beginning of the Qing Dynasty.