Mount Akagi

A double volcano with a truncated cone shape that rises in the northwestern part of the Kanto Plain in Gunma Prefecture. It is also called "Akagi-san." Together with Mount Haruna and Mount Myogi, it is known as one of the "Three Mountains of Jomo" and has been loved by the people of Gunma Prefecture since ancient times. The outer rim is a mountain ridge that connects Mt. Kurobi, the highest at 1,828 meters, with Mt. Choshichiro, Mt. Araya, Mt. Kuwagara, and others. Inside this lies a caldera that stretches 3 kilometers east to west and 4 kilometers north to south, and includes the bell-shaped (troide) central crater cone Mt. Jizo (1,674 meters), the crater lake Ono, the marshland Kakumanbuchi, and the crater lake Konuma. The view from Mt. Kurobi is excellent, and there are also parasitic volcanoes such as Mt. Nabewari and Mt. Suzugatake. The base diameter of the mountain is about 30 kilometers, and the bedrock is made up of Palaeozoic, Mesozoic, and Tertiary layers, but the upper part is made up of lava and agglomerates made of pyroxene andesite, tuff breccia, etc. During Typhoon Kathleen in September 1947, huge rocks and earth and sand were swept down from collapsed areas halfway up the mountain, causing many collapsed houses and swept away casualties in the coastal areas of the Arato River and Shira River on the southern slope, and the Numao River on the western slope.

The annual average temperature around Lake Onuma is 7.9°C, -3.4°C in January and 20.0°C in August. Snowfall occurs in winter and thunderstorms are common in summer. There is also an abundance of lowland to subalpine trees, including beech, maple, Japanese oak, Erman's birch, and silver birch, as well as alpine plants. The Shinsakadaira area, at an altitude of about 1,400 meters, is bustling with tourists in mid-June when large colonies of Renge Azalea, the prefectural flower, come into full bloom. In the summer, dairy cows and sheep are pastured in this area. Agriculture and livestock farming is thriving at the foot of the mountain, with rice paddies in the Radiating Valley and mulberry, wheat, vegetable, and dairy farming prominent in the Gengata area. In particular, the southern slope is home to Akagi-style houses for sericulture, and by 1957 (Showa 32), 34 areas had been developed after the Second World War, with 700 homes settled there. In 1935 (Showa 10), the area was designated a prefectural park, and it rapidly developed as a tourist destination. It took only an hour and a half by bus from Maebashi to Daido on the shores of Lake Onuma on the Akagi Road (Maebashi-Akagi Prefectural Road). In 1982, a highway was opened from Daido, running across the northern slope to Sonohara Dam in the middle reaches of the Katashina River and Oigami Onsen. In 1971, the National Akagi Youth Exchange House was built. There is also Shirakaba Ranch, a livestock research station, and a ski resort, and on the southern slope are Chuji, Nashigi, and Akagi hot springs, as well as Akagi Shrine.

[Murakami Sadao]

"Miyama Library 1 Akagi" (1961, Gunma Prefectural Library) "Murakami Sadao's "Geography of the Foothills" (1967, Maebashi, Kanhodo)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

群馬県、関東平野の北西部にそびえる截頭円錐(さいとうえんすい)型（コニーデ）の二重式火山。「あかぎさん」ともいう。榛名山(はるなさん)、妙義山(みょうぎさん)とともに「上毛三山(じょうもうさんざん)」とよばれて、古来群馬県民に親しまれている。外輪山は、最高の1828メートルを示す黒檜(くろび)山と、長七郎(ちょうしちろう)山、荒(あら)山、桑柄(くわがら)山などを結ぶ山嶺(さんれい)で、この内側に東西3キロメートル、南北4キロメートルのカルデラがあり、鐘状(しょうじょう)型（トロイデ）の中央火口丘地蔵(じぞう)岳（1674メートル）や、火口原湖の大沼(おの)、湿原の覚満淵(かくまんぶち)、火口湖の小沼(この)がある。黒檜山からの眺望はまことによく、鍋割(なべわり)山、鈴ヶ岳などの寄生火山もある。山体の基底直径は約30キロメートルで、基盤は古生層、中生層、第三紀層であるが、上部は複輝石(ふくきせき)安山岩からなる溶岩や集塊岩、凝灰角礫(ぎょうかいかくれき)岩などで構成されている。1947年（昭和22）9月のカスリーン台風では、中腹以上の崩壊箇所から巨岩や土砂を流下して、南斜面の荒砥(あらと)川や白(しら)川、西斜面の沼尾(ぬまお)川の沿岸地域では多くの倒壊流失家屋や死傷者を生じた。

　大沼付近の気温は年平均7.9℃、1月零下3.4℃、8月20.0℃で、冬季は降雪がみられ、夏季は雷を発生しやすい。また、低山帯から亜高山帯に属する樹相が豊富で、ブナ、カエデ、ミズナラ、ダケカンバ、シラカバおよび高山植物がみられる。標高約1400メートルの新坂平(しんさかだいら)一帯は、6月中旬県花のレンゲツツジの大群落が満開となり、観光客でにぎわう。この付近では夏季に乳牛やメンヨウの放牧も行われる。裾野(すその)は農牧業が盛んで、放射谷には水田、原形面には桑、小麦、野菜、酪農が目だつ。とくに南斜面には養蚕のための赤城型民家があり、第二次世界大戦後の開拓地も1957年（昭和32）にはすでに34か所に、700戸が入植していた。また1935年（昭和10）県立公園に指定され、観光地として急激に発達し、前橋から大沼湖畔の大洞(だいどう)まで赤城道路（県道前橋赤城線）でバス1時間半となり、さらに1982年大洞から北斜面を縦断して片品(かたしな)川中流の薗原(そのはら)ダム、老神(おいがみ)温泉方面に通ずる自動車道が開通した。1971年には国立赤城青年交流の家ができた。白樺牧場(しらかばぼくじょう)、畜産試験場やスキー場もあり、南側山腹には忠治、梨木(なしぎ)、赤城の温泉や赤城神社もある。

［村木定雄］

『『みやま文庫1　赤城』（1961・群馬県立図書館）』▽『村木定雄著『裾野の地理』（1967・前橋・煥乎堂）』

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