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English SummariesBulletin of Gunma

The process of making ridge and furrows for the wet-rice fields in the Kofun period
-An example of the very small grid type wet-rice fields in the late Kofun period -
SAKAGUCHI Hajime

This article aims to analyse the process of making ridges on the wet-rice fields which were covered with Mt. Haruna's volcanic ash (Hr-FA) and Mt. Haruna's volcanic pumice (Hr-FP) in the 6th century AD.
Firstly when did these eruptions occur?
There is a possibility that these tephra layers were in early summer, because foal's hoof prints probably within a few months after their birth have been discovered under these tephra layers (fig. 2).
Generally horses breed in spring and hoof prints do not survive after a few months (fig.).
Therefore these tephras fell in early summer.
Ridges form the grid of wet-rice fields.
The dimensions of one grid cell are 2.5 m long and 1.5 m wide.

This is how the ridges are made;
1) Ploughing (fig.).
Ploughing of a wet-rice field, including ridges which were made in the previous year before the volcanic eruption.
2) Making vertical ridges (fig. 7, 8, 9).
Making vertical ridges on a rugged surface by ploughing. In this case, vertical ridges were made from soil which was dug out from the side of the ridges, formed small ditches present alongside the ridges (fig.).
3) Making the horizontal ridges (fig. 10).
The making of the horizontal ridges was done after the vertical ridges.
The horizontal ridges were shaped in a similar way. Water for irrigation flowed in the direction of the vertical ridges, so water openings were made in each horizontal ridge.
4) Levelling of wet-rice field except for the ridges and completion (fig. 11).
The rugged surface and the small ditches were levelled. The completed ridges are higher than the previous year and the surface is flatter than the previous year.

The above proves:
1) Horses existed in the 6th century AD, but ploughing using horses and cattle did not exist at that time in Gunma Prefecture.
2) We can excavate the small ditches produced by the making of ridges.
These ditches prove that existing wet-rice fields existed even if there is no evidence for them (fig. 4).
3) The small grid type wet-rice fields in the 4th century AD are different from those of the 6th century AD by their ploughing, because there is little possibility that these ridges were made by any other way (fig. 5).

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