The Quaternary sediments in the Atsumi district can be divided into four stages, each reflecting a submergence, same as other areas in the Tôkai region.
The earliest stage is the Atsumi formation which is considered to be a deltaic deposit of the ancient River Tenryû. Its molluscan fauna indicates a large shallow bay such as present Ise Bay, and neither a small inlet nor an open sea. The fauna is quite different from the present community of the south open coast of the peninsula. It might be surmized either the Shima Peninsula was further projected to the east at that time or the like of a peninsula existed at that time to the south of the present Atsumi Peninsula, preventing the ancient large bay from free communication with the south open sea.
An up-warping movement has been taking place on the Atsumi Peninsula since the early Pleistocene. The brachy-anticline thus produced agrees with the present topography of the peninsula, and reveals close resemblance to those of Udo hill and Makinohara area in the Tôkai region.
