北海道中央高地大雪山上の平ヶ岳南方湿原(標高1,720m) において,.わが国で初めて,泥炭地に発達する永久凍土丘であるパルサを発見した.これにより,現成の永久凍土の存在が周氷河地形によって示され,大雪山での永久凍土の下限高度が従来の推定よりも約300m低いことが明らかになった.大雪山のパルサは,高さ20～100cm, 最大径80mの長円形を呈しており,泥炭に被覆きれ鉱物質の凍結核をもつ.本湿原では,異なった規模,発達段階のパルサが分布しており,個々のパルサがそれぞれに発生から消滅へ至る周期的な発達過程を辿っていることがうかがえる.一方, 1955～82年の期間に撮影された空中写真の比較、によると,湿原内のパルサ全体の面積は,この27年間に約36%減少している.その原因として夏期の高温化,多雨化が考えられる.本湿原での泥炭の堆積速度と現在の環境でパルサが形成されるために必要な泥炭層の厚さ (60cm) から推定して,パルサの形成開始は2,000～1,000y. B. P. 頃と考えられる.
In advanced countries, especially in the United States, a new population change is appearing which deviates from the tendency existing till about the late 1960s. It is a product not only of a change in demographic structure but also of a change in socioeconomic structure due to a high level of technology development. Previously, urbanization stimulated the economic growth of a region; urban growth, high density and heterogeneity of urban cores were basic features of urbanization. Theories and policies of regional development, such as growth center policy and polarization theory, have been considered in light of such a tendency. Therefore, the population turnaround in the United States was a great surprise to most social scientists. As a result since the 1970s a great many studies about the features of population change and its causes have been generated. The aim of this paper is to clarify the features of population change through the findings of previous research and to reveal the problems with these studies. The principal findings are summarized as follows: Due to the remarkable population changes which occurred in the United States since about 1970, the situation has already entered a stage of population deconcentration. In a reversal of the net out-migration phenomenon of the 1960s the population increase in the 1970s was recognized not only in adjacent but also in nonadjacent nonmetropolitan counties. Additionally, counterurbanization in the United States is characterized not only by such metropolitan-to-nonmetropolitan migration, but also by regional differentiation of population change due to the revolutionary development of transportation and communication technology. In the South and the West, the population increase occurred in large cities as well as in nonmetropolitan areas, whereas in the Northeast and the North Central the population decreased in the majority of metropolitan areas. In contrast, many West European countries still remain in the phase of decentralization and the main stream of migration continues to occur from central cities to their suburban areas. This tendency is occurring in Japan as well. Various factors for the generation of counterurbanization in the United States have been considered. Though the technical development of transportation and communication has undoubtedly played a significant role as a foundamental factor of population change, the change of life style for American people cannot be ignored. Due to the rather significant development of the service sector in the 1970s the role of industrial dispersal into nonmetropolitan areas was not given under consideration. According to the analysis of questionnaire surveys, many inmigrants into non- metropolitan areas were people of relatively high social status but it was pointed out from the analysis of statistical data that they were often retirees who require the quality of life. Recognizing that the phenomenon of counterurbanization may proceed in a similar way in other advanced countries, social scientists have been greatly interested in its trend. It is important that they confirm whether population dipersal is a temporary phenomenon or long-standing trend. Although the population growth in nonadjacent nonmetropolitan areas is a significant factor in understanding counterurbanization, the debate over whether this is a reversal or a continuance of the previous urbanization process is not yet resolved. As shown in Figure 4, metropolitan-nonmetropolitan migration and the movement of plants and service establishments should be more fully examined in detail within the national, regional and daily urban systems. Additionally, an analysis of migration between metropolitan and nonmetropolitan areas, and between large-scale and small-scale metropolitan areas of the hierarchical structure of the national urban system should be conducted.