Plateau - Daichi

Japanese: 台地 - だいち

A table-like or stepped plateau bordered on one or all sides by cliffs and topped by a flat area that is higher than the surrounding area. It is made up of nearly horizontal strata. The flat area is called the plateau surface, and the cliffs and steep slopes that surround it are called plateau cliffs. A narrow plateau with a terrace cliff behind it is called a terrace. Alluvial plateaus, formed by the uplift and erosion of alluvial fans, deltas, coastal plains, shallow seabeds, or volcanic foothills, resulting in cliffs around them, have developed widely throughout Japan since hundreds of thousands of years ago. A typical example is the Kanto Plain.

To understand the process of plateau formation, consider that global warming will cause glaciers on land to melt and sea levels to gradually rise. Tokyo Bay will expand, turning the lowlands of the downtown area into the sea, and the inlet will expand from Saitama Prefecture to Gunma Prefecture. Waves in the inlet will erode the surrounding plateau, developing a sea-eroded platform, and the resulting sediment will be deposited in the inlet. As sea levels rise further, the water in the inlet will become deeper and the waves will become stronger, further eroding the plateau, until the bottom of the inlet will become the bottom of a deep sea, creating Great Tokyo Bay.

Such an event actually occurred between 150,000 and 120,000 years ago. The expanded Tokyo Bay at that time is called Paleo-Tokyo Bay. As the sea gradually dropped 100,000 and 80,000 years ago, the deep inlet became shallower, and the sediment that had accumulated on the marine platform during the transgression period was relocated by strong waves in the coastal environment and emerged. In places where sediment was supplied from land by rivers, fluvial plains developed during the emergence period. For example, in Tokyo, the Tama River developed an alluvial fan, and the Kinu River developed a bird's toe-shaped delta (a delta with a planar shape like a waterfowl with its legs spread) on the Tsukuba Plateau. These fluvial plains became plateaus as the sea level dropped steadily tens of thousands of years ago, and about 20,000 years ago, they became plateaus in the process of dropping by about 100 meters from the present.

The plateau from that time still remains in the Kanto Plain, but not in the Echigo Plain. The Kanto Plain has risen several tens of meters (0.2 to 0.3 millimeters per year) over the past 100,000 years, while the Echigo Plain has sunk due to the continued accumulation of large amounts of sediment from the Shinano River and Agano River, and the postglacial sea rise once again turned it into an inlet, and the plateau was filled in by sediment.

In addition, plateaus completely different from alluvial plateaus have developed on the stable continents of the world. For example, the Colorado Plateau, which spreads across the southwestern part of the Colorado Plateau in the western United States, is a vast plateau formed when the Cenozoic and Mesozoic strata were eroded to a thickness of about 1,000 meters over a long period of 10 million years, exposing limestone layers that are resistant to erosion on the surface. It is called mesa, which means table in Spanish. Japan also has plateaus made of horizontal layers that are resistant to erosion, such as Yashima in Kagawa Prefecture and Arafuneyama in Gunma Prefecture, but they are all small in scale.

[Hiroshi Ikeda]

"Ikeda Hiroshi, 'An Eye for Topography' (2001, Kokin Shoin)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

四方ないし一方を崖(がけ)で縁どられ、周囲より一段と高い平坦(へいたん)地を頂にもつ卓状ないし階段状の高台。ほぼ水平な地層からなる。その平坦地を台地面といい、それを囲む崖や急斜面を台地崖(だいちがい)とよぶ。背後に段丘崖のある狭い台地は段丘とよばれる。数十万年前以降に扇状地、三角州、海岸平野、浅海底、あるいは火山の山麓(さんろく)などが隆起して侵食され、周囲に崖が生じることによって形成された洪積台地が日本各地に広く発達している。その代表例が関東平野である。

　台地の形成過程を理解するために、地球温暖化により陸地の氷河が融解し、海面がしだいに上昇することを考えてみるとよい。東京湾は拡大し下町の低地が海になり、入り江は埼玉県から群馬県へと拡大するであろう。入り江に立つ波は周囲の台地を侵食して海食台を発達させ、そこで生じた土砂は入り江に堆積(たいせき)するであろう。さらに海面が上昇すると、入り江の水深は増して波が強まり、台地はさらに侵食され、ついには入り江の底は深い海の底になって大東京湾が生じるであろう。

　このような事変が今から15万年前から12万年前にかけて実際に生じた。当時の拡大した東京湾は古東京湾とよばれている。この海が10万年前、8万年前としだいに低下するにつれて、深かった入り江は浅くなり、海進期に海食台上に堆積していた土砂は沿岸環境下で強い波によって再移動させられ離水した。川によって陸地から土砂が供給された場所では、離水期に河成平野が発達した。たとえば、東京では多摩川が扇状地を発達させ、鬼怒(きぬ)川は筑波(つくば)台地に鳥趾状三角州(ちょうしじょうさんかくす)（水鳥が足を広げたような平面形をもつ三角州）を発達させた。これらの河成平野は数万年前以降に海水準がどんどん下がり、2万年ほど前には今より100メートルほども下がる過程で台地化した。

　当時の台地は関東平野では現在も残されているが、越後平野にはみられない。関東平野はこの10万年間に数十メートル（年間0.2～0.3ミリメートル）隆起し続けてきたが、一方、越後平野は信濃(しなの)川や阿賀野(あがの)川などからの大量の土砂が堆積し続けることによって沈降し、後氷期の海進によってふたたび入り江になり、台地は土砂に埋め立てられてしまったためである。

　なお、世界の安定大陸には洪積台地とはまったく異なる台地が発達している。たとえば、アメリカ西部のコロラド高原の南西部に広がるコロラド台地は、新生代や中生代の地層が1000万年という長い時間をかけて厚さ1000メートルほども侵食された結果、侵食されにくい石灰岩層が地表に露出してできた広大な台地で、スペイン語でテーブルを意味するメサとよばれる。日本にも香川県の屋島や群馬県の荒船山をはじめ、侵食されにくい水平層でできた台地があるが、いずれも小規模である。

［池田　宏］

『池田宏著『地形を見る目』（2001・古今書院）』