4.Early Holocene Sea Level Rise and Tokoro Shell Mounds
Global warming began after the ice age ended approximately 11,700 years ago. The sea levels rose, and some areas which were previously land above the water were covered by the oceans. Lake Saroma is the remains of a bay which was created by this marine transgression and later transformed into the lake when it was cut off from the ocean as a result of sedimentation.
In addition, shell mounds have been discovered at points inland, but at one time, this area faced the sea as a result of marine transgression, and then became inland as a result of sedimentation. Hamaguri (Meretrix lusoria, Asian hard clams) which do not currently live in the oceans around Hokkaido have been excavated from the shell mounds, showing that the ocean was once warmer than it is today.
【Figure: Geological changes in the area around Lake Saroma】
