are an aggregate of previously collapsed terrain because mass
movement occurs with high density during heavy rainfall.
Due to landslips and landslides, the slopes of mountainous
areas gradually retreat, causing a decrease in the altitude of
mountain ridges. The debris produced by the collapse of a
slope temporarily accumulates at the bottom of the slope and
the valley bottom, but it is transported to downstream areas
via debris flows due to flooding. Even in stratovolcanoes, the
central part of the volcanic body, which consists of steep
slopes, is engraved in a deep radiant valley, and the produced
rock debris flows downward, mainly as a debris flow, to form
an “alluvial fan“ at the foot of the volcano. In this way, the
material on the slopes of the mountains and volcanoes of the
Japanese Islands is always in an unstable state, and generally
the soil on the slopes is easily eroded or moved.
Additionally, past glacial landforms are recognized in the
uppermost part of the mountains and river valleys in the
central mountains of the Japanese Islands (the Hida, Kiso
and Akaishi Mountains) and the Hidaka Mountains of
Hokkaido (Fig. 2.13) (see Figs. 2.10, 2.12 and 2.13 in
Sect. 2.1.3). The age determined by tephra stratigraphic
analysis indicates that the mountain glaciers in Japan
advanced during MIS 4, 3 and 2 in the last glacial period.
However, in the central mountains (the Hida, Kiso and
Akaishi Mountains) and the Hidaka Mountains, the glacier
advance in MIS 2 was smaller than that in MIS 4 and 3. In
both mountainous areas, glacial landforms formed in MIS 6
have been reported, but glacial landforms formed before
MIS 8 have not yet been certified. Most of the glacial
landforms are small; cirques (horseshoe-shaped depressions
formed by glacial erosion) are shallow with steep floors, and
typical U-shaped valleys are scarce; the moraine (debris that
accumulated at the edge and surface of a glacier) at the end
of the glacier is small in scale, and the development of
downstream outwash terraces is rare.
Above the current tree lines in Japan, sand and gravel
slopes with sparse vegetation are under periglacial environments, where mechanical weathering and creep due to
freeze–thaw action on the ground surface, and snow, affect
soil conditions. On these slopes, various formations are
produced on the surface. During the last glacial period (MIS
4 to 2), the altitude of the tree lines declined, so these
periglacial regions also declined to lower altitudes, and
permafrost was formed in the plains in the northern and
eastern parts of Hokkaido (see Figs. 2.10, 2.12 and 2.13 in
Sect. 2.1.3). In the postglacial period (MIS 1), due to
warming and increasing rainfall, mountain slopes were again
covered by forest vegetation and surface erosion was
extremely slow. Moreover, in the postglacial period, as the
frequency of heavy rainfall increased, water streams with
increasing tractive force began to strongly undercut valley
bottoms. The transitional areas from the gentle upper slope
to the steep lower slope are gravitationally unstable and
collapse during earthquakes or heavy rainfall. In this convex
break of the slope, the collapse erodes the debris made in the
last glacial period or before; this is called the “postglacial
dissection front” (Hatano 1979).
In rivers flowing on mountains with a glaciated valley in
the upper stream, moraine and terraces have not been
observed to be directly connected. However, river terraces
and alluvial fans developing from a valley to the downstream basin/plain are known to have formed by erosion and
deposition almost simultaneously everywhere (see Fig. 2.8
in Sect. 2.1.3) and were therefore probably formed under the
influence of global climate change. Thus, as described in the
next section, it is possible to discuss changes in the supply
and transport of debris on the basis of fluvial terraces during
the glacial to interglacial period. This knowledge will also
provide useful information about the formation age and the
environments of fluvial terraces and associated soils.
(2) Formation and components of plains
The plains of the Japanese Islands are roughly divided into
the coastal plains facing the coast and the inland mountain
basin. Both plains are dominated by sedimentary plains
where sediments bury structural depressions (basins and
inner bays formed by relative subsidence). Mountains and
plains in the Japanese Islands are small in size, and the plains
are distributed between mountains in a mosaic pattern
(Fig. 2.13). In the mobile belt, under the temperate humid
climate of the Japanese Islands, and due to the large relief
and high denudation rate of the mountainous areas, the
massive amounts of sand and gravel produced by erosion are
transported by steep rivers and accumulate in upstream and
midstream rivers or mountain basins; otherwise, they are
transported downstream and accumulate on the coast facing
the inner bay and the open ocean to form alluvial plains.
Along the coast, “coastal plains”, formed mainly by erosion
and marine sedimentation processes such as waves and tidal
currents, are also distributed. Aquatic areas, such as lakes
and lagoons, are left as part of the plain in the basins and the
inland bays where the sedimentation of sand and gravel is
insufficient. The landforms of the plain are divided into
lowlands, terraces (uplands) and hills according to the altitude, relief, flatness and formation age. The boundaries of
these geomorphic units are generally clearly classified by the
break of slope angle and orientation (the concave break of
the slope and the piedmont line) (Fig. 2.17).
Lowlands are low-relief landforms, distributed along
rivers and coasts, at a low relative height above river and sea
levels. Lowlands are young landforms formed during the
Holocene (MIS 1) and are divided into fluvial lowland (alluvial plain) and marine lowland (coastal plain) by major
30
K. Tamura et al.
movement occurs with high density during heavy rainfall.
Due to landslips and landslides, the slopes of mountainous
areas gradually retreat, causing a decrease in the altitude of
mountain ridges. The debris produced by the collapse of a
slope temporarily accumulates at the bottom of the slope and
the valley bottom, but it is transported to downstream areas
via debris flows due to flooding. Even in stratovolcanoes, the
central part of the volcanic body, which consists of steep
slopes, is engraved in a deep radiant valley, and the produced
rock debris flows downward, mainly as a debris flow, to form
an “alluvial fan“ at the foot of the volcano. In this way, the
material on the slopes of the mountains and volcanoes of the
Japanese Islands is always in an unstable state, and generally
the soil on the slopes is easily eroded or moved.
Additionally, past glacial landforms are recognized in the
uppermost part of the mountains and river valleys in the
central mountains of the Japanese Islands (the Hida, Kiso
and Akaishi Mountains) and the Hidaka Mountains of
Hokkaido (Fig. 2.13) (see Figs. 2.10, 2.12 and 2.13 in
Sect. 2.1.3). The age determined by tephra stratigraphic
analysis indicates that the mountain glaciers in Japan
advanced during MIS 4, 3 and 2 in the last glacial period.
However, in the central mountains (the Hida, Kiso and
Akaishi Mountains) and the Hidaka Mountains, the glacier
advance in MIS 2 was smaller than that in MIS 4 and 3. In
both mountainous areas, glacial landforms formed in MIS 6
have been reported, but glacial landforms formed before
MIS 8 have not yet been certified. Most of the glacial
landforms are small; cirques (horseshoe-shaped depressions
formed by glacial erosion) are shallow with steep floors, and
typical U-shaped valleys are scarce; the moraine (debris that
accumulated at the edge and surface of a glacier) at the end
of the glacier is small in scale, and the development of
downstream outwash terraces is rare.
Above the current tree lines in Japan, sand and gravel
slopes with sparse vegetation are under periglacial environments, where mechanical weathering and creep due to
freeze–thaw action on the ground surface, and snow, affect
soil conditions. On these slopes, various formations are
produced on the surface. During the last glacial period (MIS
4 to 2), the altitude of the tree lines declined, so these
periglacial regions also declined to lower altitudes, and
permafrost was formed in the plains in the northern and
eastern parts of Hokkaido (see Figs. 2.10, 2.12 and 2.13 in
Sect. 2.1.3). In the postglacial period (MIS 1), due to
warming and increasing rainfall, mountain slopes were again
covered by forest vegetation and surface erosion was
extremely slow. Moreover, in the postglacial period, as the
frequency of heavy rainfall increased, water streams with
increasing tractive force began to strongly undercut valley
bottoms. The transitional areas from the gentle upper slope
to the steep lower slope are gravitationally unstable and
collapse during earthquakes or heavy rainfall. In this convex
break of the slope, the collapse erodes the debris made in the
last glacial period or before; this is called the “postglacial
dissection front” (Hatano 1979).
In rivers flowing on mountains with a glaciated valley in
the upper stream, moraine and terraces have not been
observed to be directly connected. However, river terraces
and alluvial fans developing from a valley to the downstream basin/plain are known to have formed by erosion and
deposition almost simultaneously everywhere (see Fig. 2.8
in Sect. 2.1.3) and were therefore probably formed under the
influence of global climate change. Thus, as described in the
next section, it is possible to discuss changes in the supply
and transport of debris on the basis of fluvial terraces during
the glacial to interglacial period. This knowledge will also
provide useful information about the formation age and the
environments of fluvial terraces and associated soils.
(2) Formation and components of plains
The plains of the Japanese Islands are roughly divided into
the coastal plains facing the coast and the inland mountain
basin. Both plains are dominated by sedimentary plains
where sediments bury structural depressions (basins and
inner bays formed by relative subsidence). Mountains and
plains in the Japanese Islands are small in size, and the plains
are distributed between mountains in a mosaic pattern
(Fig. 2.13). In the mobile belt, under the temperate humid
climate of the Japanese Islands, and due to the large relief
and high denudation rate of the mountainous areas, the
massive amounts of sand and gravel produced by erosion are
transported by steep rivers and accumulate in upstream and
midstream rivers or mountain basins; otherwise, they are
transported downstream and accumulate on the coast facing
the inner bay and the open ocean to form alluvial plains.
Along the coast, “coastal plains”, formed mainly by erosion
and marine sedimentation processes such as waves and tidal
currents, are also distributed. Aquatic areas, such as lakes
and lagoons, are left as part of the plain in the basins and the
inland bays where the sedimentation of sand and gravel is
insufficient. The landforms of the plain are divided into
lowlands, terraces (uplands) and hills according to the altitude, relief, flatness and formation age. The boundaries of
these geomorphic units are generally clearly classified by the
break of slope angle and orientation (the concave break of
the slope and the piedmont line) (Fig. 2.17).
Lowlands are low-relief landforms, distributed along
rivers and coasts, at a low relative height above river and sea
levels. Lowlands are young landforms formed during the
Holocene (MIS 1) and are divided into fluvial lowland (alluvial plain) and marine lowland (coastal plain) by major
30
K. Tamura et al.