The aim of this article is to reevaluate the author’s previous argument that the gravel strata found on the upper layers of the Neolithic remains at Hwangseongdong in Gyeongju and Sejuk in Ulsan were formed by the great flood that happened at around 3,000 BCE, which resulted in the extinction of the Early Neolithic Culture at these sites and made possible the Middle Neolithic Culture, which came to be newly established following the cultural influx from the region of Northeast China and is characterized by its comb-pattern. For this purpose, evidence from Sejuk and Hwangseong-dong, as well as from the site of Bibong-ri in Changnyeong, was examined, focusing on the sequences of strata at the sites, the artifacts excavated, and the radiocarbon dates. During the Formative Period at these sites, the sea level was on a rise phase; in the period of gravel strata formation, there was a drop in sea levels. The result of these changing conditions may have been the collapse of many slopes, sediment increases in rivers, and frequent local downpours. In the Korean Peninsula, which is characterized by many old mountains, surrounded by water on three sides, and exposed regularly to long rainy seasons and typhoons, the acceleration of river currents resulting from the drop in sea levels would have caused the frequent great floods that happened throughout the peninsula at the time. After the floods, which not only destroyed the previous settlements but were also accompanied by cold weather, the Middle Neolithic cultures of the Korean Peninsula began to be formed with the influx of the Xiaozhushan Culture, which originated in Northeast China at around 3,500 BCE.