Valley (English spelling)

Japanese: 谷 - たに（英語表記）valley

A depression between mountains. A small one is called a valley. Although it is a word for a mountain, the boundary between a mountain and a valley is not clear. From another perspective, the slope of a mountainside can also be considered a valley wall slope. The distinction between a stream and a valley is also not clear. Generally, a stream is considered to be a linear feature attached to a mountain, and a valley is considered to be something larger than a stream.

[Shigemi Takayama]

Valley Classification

Valleys can be classified in various ways based on their origin, shape, and arrangement. Valleys formed by structural causes such as faulting and folding are called structural valleys, while valleys formed mainly by the erosion of rivers and glaciers are called erosional valleys. The cross-sectional shape of a valley varies depending on its origin and stage of development. A valley with a flat bottom is called a flat-bottom valley or bed valley. A wide valley with concave slopes on both sides of the valley is called a basin valley. In contrast, a valley lacking flat land at the bottom is called a missing-bed valley or V-shaped valley. A valley with vertical or nearly vertical slopes on both sides is called a canyon. In humid regions, rivers usually flow through the valley, and the erosion and deposition of the flowing water gradually forms a flat valley bottom. Therefore, valleys can be divided into infant, adult, and old valleys based on their stage of development. According to the American geomorphologist W.M. Davis's idea of erosion cycle (topographic cycle), valleys become wider and shallower over time. Therefore, a V-shaped valley is a young valley, a bottom valley is a mature valley, and a basin valley is an old valley. In the case of glacial valleys, they form a U-shaped valley. Also, in arid regions, even if there is a valley, water often does not normally flow through it. During the occasional heavy rainfall, it flows as a river.

A valley is called a longitudinal valley when its extension direction is parallel to the direction of the mountain range. Typical examples are the valleys of the Brahmaputra River and the upper reaches of the Indus River, which flow between the Himalayas and the Trans-Himalayan Mountains. They are formed under the control of geological structures, but there are various causes for longitudinal valleys. A valley that crosses a mountain range is called a transverse valley. Since it is impossible for a river to flow across a mountain range, the course of a transverse valley is often special, such as an antecedent river or an overflowing river that flowed through the mountain range before the mountain range was formed, or a valley formed by erosion along a fault that crosses the mountain range. In the Appalachian Mountains, there are longitudinal valleys that run parallel to the folded structure and transverse valleys that cross them. In Japan, the Mogami River, Agano River, and Omono River, which flow from the basin of northeastern Japan into the Sea of Japan, form transverse valleys that cross the mountain range.

Most eroded valleys are river valleys. Valleys begin to form on the ground surface when rainwater gathers and begins to flow linearly. The channels through which surface water gathers and flows during rainfall form depressions called gully or rain gullies, which gradually grow into smaller valleys. When the bottom of such a dry valley reaches the groundwater table, water begins to flow constantly. The water gradually increases the depth and width of the valley through incision and side incision, and the channel is extended by valley head erosion. Infant valleys are dominated by incision, making the valley deep and the sides steeply sloping. The cross section is V-shaped, with waterfalls and rapids at the valley bottom, and the vertical section is discontinuous. Because the valley walls are steep and unstable, the material that makes up the slopes constantly collapses, but the material that accumulates at the valley bottom is quickly carried away by the water current, gradually widening the valley. In adult valleys, the disappearance of waterfalls and rapids makes the cross section smooth, side incisions predominate, widening the valley, and the slope of the valley walls becomes gentler. Sediments accumulate at the valley bottom, forming floodplains and causing free meandering. In old valleys, the valley only widens, and incisions almost completely stop. The valley is wide and shallow, with a gently sloping cross section.

Glacial valleys have flat valley floors and nearly vertical valley wall slopes, and are known as U-shaped valleys. At the head of a valley glacier, an amphitheater-like Kar (German for cirque) forms. Glacial erosion differs from river erosion in that solid materials collide with each other, and can leave abrasions called glacial erosion marks on the valley walls and valley floor. The ends of ridges left by valley glaciers are often cut off, forming severed spurs. This is because, unlike water currents, glaciers do not flow around ridges that intersect at an angle to the flow direction and act as obstacles.

A lowland that has collapsed due to a fault is called a fault valley. A valley that is the result of displacement due to fault movement is called a fault valley, while a valley that is the result of selective erosion along a fault line is called a fault line valley, and they are different types of erosion valleys. A collapsed lowland bounded by two or more nearly parallel faults is called a graben, and in some cases, a river runs through it, as in the East African Rift and the Rhine Rift. A valley that develops along the syncline axis of a fold is called a syncline valley, and a valley that develops along the anticline axis is called an anticline valley. In the case of a syncline valley, the depression in the syncline forms a valley as it is, but in the case of an anticline valley, the anticline part is eroded and becomes lower than the surrounding area, and then a valley is formed. Conversely, if the syncline is made of harder rock than the anticline and the erosion rate is different, a syncline ridge is formed. There are many examples of this type of inverted topography in the Appalachian Mountains of the United States. Both syncline and anticline valleys form vertical valleys. In the midstream of the Shinano River, between the Kubiki Hills and the Uonuma Hills in Niigata Prefecture, a valley that develops along the fold axis can be seen.

In arid regions, even if there is a valley, water does not usually flow, and only flows immediately after heavy rain. In the desert, even if water flows, it is often a river with no end, and is called a wadi or arroyo. Since there is no constant water flow, the river path is constantly shifting, and the shape of the valley can sometimes become unclear downstream.

After erosion smooths the longitudinal profile of the riverbed, the valley may be rejuvenated or rejuvenated by ground movement, sea-level change, climate change, river capture, or other factors.

Valleys also exist below sea level, and are called submarine canyons. There are two types: valleys that were originally on land but have now sunk below sea level, cutting shallowly into the continental shelf, and valleys that cut deeply into the continental shelf slope and reach the deep sea floor. The latter are formed independently of land, and are thought to be the result of faults, landslides, and turbidity currents. Particularly deep and narrow valleys are sometimes called submarine channels.

[Shigemi Takayama]

Classification by valley cross section
©Shogakukan ">

Classification by valley cross section

Valley Development
The valley topography (1) that begins at Amegiko (a deep valley) passes through a deep, young valley (2), then widens and becomes a mature valley (3) with gentle side (valley wall) slopes. In the old valleys (4) and (5), where incision has almost stopped, the valley width widens and the river starts to meander freely. ©Shogakukan ">

Valley Development

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

山と山との間の窪地(くぼち)。小さいものは渓(けい)という。山に対する語であるが、山と谷との境界は明瞭(めいりょう)でない。山腹斜面は見方をかえれば谷壁斜面ともいえるからである。沢と谷との区別もはっきりしない。一般的には山に付属する線状地物が沢で、谷はそれより大きいものと考えられている。

［髙山茂美］

谷の分類

谷はその成因、形状、配置の状態などによっていろいろな分類の仕方がある。谷が断層運動や褶曲(しゅうきょく)運動などの構造的原因によって生じた場合に構造谷といい、おもに川や氷河の侵食作用によってつくられた谷を侵食谷という。谷の横断面形は成因、発達の段階によってさまざまな形をしている。谷底が平らな谷は平底谷または床谷(しょうこく)という。谷の両側の谷壁斜面が凹形の斜面からなる広い谷を盆谷という。これに対して、谷底に平地を欠くような谷を欠床谷またはV字谷という。両側の谷壁斜面の傾斜が垂直またはこれに近い急傾斜のものを峡谷という。湿潤地域では、ふつうは川が流れていて、流水の侵食と堆積(たいせき)作用によってしだいに平らな谷底が形成されていく。そこで、谷の発達段階から幼年谷、壮年谷、老年谷に分けることがある。アメリカの地形学者W・M・デービスの侵食輪廻(りんね)（地形輪廻）の考え方に従えば、時間の経過につれて谷は広く、浅くなる。したがってV字谷は幼年谷、床谷は壮年谷、盆谷は老年谷の地形的特徴を表す。氷食谷の場合にはU字谷をなす。また、乾燥地域では谷があっても、普段はその中を水が流れていないことが多い。たまに降る豪雨の際に川となって流れる。

　谷の延長方向が山脈の走る方向と平行している場合に縦谷(じゅうこく)という。ヒマラヤ山脈とトランス・ヒマラヤ山脈の間を流れるブラマプトラ川、インダス川の上流部の谷が典型例で、地質構造に支配されて生ずるが、縦谷にはさまざまな成因によるものがある。山脈を横切っている谷を横谷(おうこく)という。川が山脈を横切って流れることは不可能だから、横谷の部分の流路は、山脈ができる以前からそこを流れていた先行川や表成川の場合、山地を横切る断層に沿って侵食が働き、谷を生じた場合などのように特殊なケースが多い。アパラチア山脈には褶曲構造に並走する縦谷とこれを横切る横谷とがみられる。日本では、東北日本の盆地から日本海に注ぐ、最上(もがみ)川、阿賀野(あがの)川、雄物(おもの)川などが山脈を横切って横谷をなしている。

　侵食谷の大部分は河谷である。地表面に谷ができ始めるのは、雨水が集まって線状に流れるようになってからである。降雨時に表流水が集まって流れる通路はガリgullyまたは雨溝とよばれる窪地を形成し、しだいに小さな谷へと成長していく。このような涸れ谷(かれだに)の谷底が地下水面に達すると、恒常的に水が流れるようになる。水流は下刻(かこく)と側刻とにより、しだいに谷の深さと幅を増し、谷頭侵食によって流路を延長する。幼年谷では下刻が卓越し谷は深く、側面は急傾斜をなす。その横断面はV字形で、谷底には滝や早瀬がみられ、縦断面は不連続である。谷壁斜面は急傾斜で不安定なために、斜面を構成する物質が絶えず崩れ落ちるが、谷底にたまった物質はすぐ水流に運び去られ、徐々に谷幅を増す。壮年谷では滝や早瀬が消失することにより、縦断面は平滑化し、側刻が優勢となって谷幅を拡大し、谷壁斜面の傾斜も緩やかになる。谷底には堆積物がたまり、氾濫(はんらん)原を形成したり、自由蛇行が発生する。老年谷では谷幅の拡大のみが行われ、下刻はほとんどやむ。谷は広くて浅く、縦断面は緩勾配(こうばい)となる。

　氷食谷は谷底が平坦(へいたん)で、谷壁斜面が垂直に近く、U字谷とよばれる。谷氷河の谷頭には円形劇場のようなカールKar（ドイツ語。圏谷の意）が生ずる。氷食は河食と異なり、固体物質どうしがぶつかり合うので、谷壁や谷床には氷河削痕(さっこん)とよばれる擦り傷が残ることがある。谷氷河の通った跡は尾根の先端が切られて切断山脚となっていることがある。水流と違って、氷河は尾根が流下方向に対してある角度で交わり、障害物となっていても、よけて流れないからである。

　断層によって陥没した低地は断層谷という。断層運動による変位の結果生じた谷が断層谷で、断層線に沿う選択侵食の結果生じた谷は断層線谷とよび、侵食谷の一種であるから区別する。ほぼ平行する2本以上の断層によって限られ、陥没した低地を地溝とよび、東アフリカ地溝帯、ライン地溝帯のように川がその中を流路とすることもある。褶曲の向斜軸に沿って発達する谷を向斜谷といい、背斜軸に沿って発達する谷を背斜谷という。向斜谷の場合、向斜部の凹地がそのまま谷を形成するが、背斜谷の場合には、背斜部分が侵食されて周囲よりも低くなったのちに谷を形成する。逆に、向斜部が背斜部よりも硬い岩石からできていて、侵食の速さが異なると向斜山稜(さんりょう)を生ずる。アメリカのアパラチア山地にはこのような地形の逆転した例が多くみられる。向斜谷も背斜谷も縦谷をなす。新潟県の頸城(くびき)丘陵と魚沼(うおぬま)丘陵に挟まれた信濃(しなの)川の中流部では褶曲軸に沿った方向に発達する谷がみられる。

　乾燥地域では、谷があっても普段は水が流れず、豪雨の直後だけ水流をみる。砂漠では水流が発生しても末無川(すえなしがわ)となることが多く、ワジwadiまたはアロヨarroyoとよばれている。恒常水流がないために流路は絶えず移動し、下流部では谷の形が不明瞭になることもある。

　侵食作用によって河床の縦断面が平滑化したのちに地盤運動、海面変化、気候変化、河川争奪などの原因により谷の若返りまたは回春がおこる。

　海面下にも谷は存在するが、これは海底谷とよばれる。もと陸上にあった谷が海面下に没した谷で大陸棚(だな)を浅く刻むものと、大陸棚斜面を深く刻んで深海底にまで達するものとがある。後者は陸地と無関係に生じたもので、断層、地すべり、混濁流によると考えられている。とくに深くて狭い谷を海底チャネルとよぶことがある。

［髙山茂美］