The Noshiro Plain is located at the northern coastal area of Akita Pref., and the greater part of the plain is occupied by many levels of marine or fluvial terraces. The period during which the terrace surfaces were formed is divided into four stages. They are Moya, Otsukino, Hataya and Mizusawa stages from older to younger. These four stages can be divided into a few sub-stages, each of which corresponds to one level of terraces.
Most terraces of the Moya stage are of marine origin, and they are supposed to be formed during the stage of transgression. The terrace of the Otsukino stage is a dissected alluvial fan which was formed during a regressional stage.
The terraces of the Hataya stage occupy the largest area. This stage is divided into three sub-stages (Hataya I, II and III) which correspond to three levels of marine terraces. At the Hataya I sub-stage most part of the Noshiro Plain was covered by the sea-water once again. At the Hataya II sub-stage the shoreline shifted to the west a little, and at the Hataya III sub-stage it shifted to the further west, but it still remained within the present Noshiro Plain. No evidence which suggests a remarkable regression between these three sub-stages has been found.
The set of three levels of marine terraces prior to the latest low sea-level stage (Wisconsin Glacial stage) is also found in the southern part of Kanto district: they are Shimosueyoshi, Obaradai and Misaki terraces from older to younger (Machida and Suzuki, 1971). Hataya I, II and III terraces are supposed to be correlated to them respectively. At the Hataya II sub-stage and the Hataya III sub-stage fluvial terraces were developed. Those which were formed by the Yoneshiro River can be divided into three or more levels, and those which were formed by the small rivers can not be divided. This difference is attributed to the difference of the gradient of the rivers, i.e. that of the Yoneshiro River is less than 0.3‰, while those of the small rivers vary from 3 to 9‰. The former is gentler than that of the continental shelf (about 7‰), and the latter is about the same.
After the Hataya III sub-stage the shoreline shifted to the west far beyond the present shoreline. Most rivers eroded the plain downward, but at the northern margin of the plain (the southwestern foot of the Shirakami Mountains) alluvial fans were developed (Mizusawa stage). This is supposed to have been caused by the steeper gradient (above 10‰) of the small rivers which flow from the Shirakami Mountains, and by the increase in debris production in the mountains.
The regional difference of terrace development after the maximum transgression at the Hataya I sub-stage is attributed in the first place to the way of response of the respective rivers to the regression or the lowering of sea-level.
