The fossil periglacial involutions that had been formed during the latter half of the last Glacial Age have been observed in the upper Kitakami river basin. The formative age of these structures are discussed on the basis of radiocarbon ages and deformation patterns of the scoria and buried soil layers. Involutions that marked by deformation of the scoria layer (T2S) generally show the festoon like features. Their maximum amplitudes attain 100cm, and the upper part of the deformations reach the buried soil layer (BS2) containing AT (24.5ka), which has downward intrusive or pocket like structures.
Such features strongly suggested that the deformations of scoria layer (T2S) and buried soil layer (BS2) occurred simultaneously when BS2 layer had formed the ground surface. Based on the stratigraphical relationship, the formative ages can be estimated between 24.5ka and 18ka, and it corresponds to the maximum cold stage of the Last Glacial Age. It is becoming clear that the solifluction lobes and steps had been formed on hill slopes of this area in same the period. Existences of these fossil periglacial phenomena imply that the area of upper Kitakami river basin had not been dominated by dense boreal forest during the maximum cold stage of the Last Glacial Age.