Useful Rocks: Picking up Stones in Tanzawa V

 

Outcrop in Tonodake Subgroup.
Greenish strata are typical for Tanzawa Group.

As Tanzawa area is near to Tokyo, the place has been utilized in many ways for human activities for centuries. One of such service the mountains provided was, surprise, surprise (at least for me), mining. The place once had several veins commercially operated till about 60 years ago. They were very thin lodes. In addition, the quality of minerals harvested from there was not high. They died down eventually and disappeared. Still, for example in Tokawa Park, the remnants of such activities can be found in pebbles. Before starting, let’s remind Tanzawa’s geological map we referred to these several weeks.

We’re still remaining Mizunashi River of Tokawa Park at the point of black arrow. The Park is in Hadano Basin 秦野盆地 next to “blue” Oyama Subgroup 大山亜層群. Right next to this Oyama Subgroup, there is Tonodake Subgroup 塔ノ岳亜層群, colored pale green in the map. The pebbles of past mining activities come down to Tokawa Park from Tonodake Subgroup. Here is the first stone for today found in Tokawa Park.

It’s a natural whetstone from around the end of Tokawa Forest Road that is the most upstream area of Mizunashi River 水無川 in Tonodake Subgroup. It is considered that the stone has been well-known since B.C. People excavated the stones from several ruins of that time in south Hadano Basin, together with early iron tools for rice cultivation. The public record of the early 19th century, 新編相模国風土記稿, mentioned the stone “Tokawa Whetstone” as popular product among local commoners who needed whetstone for daily-use farm and kitchen tools. As a whetstone, straight volcanic rocks from cooled magma deep underground are too hard. Instead, when magma with lots of silicon dioxide cooled rapidly near or on the surface ground, it becomes rhyolite with very small uniform particles of quarts and feldspar. When such rhyolite receives hot spring bath from volcanos, feldspar turned soft sericite and kaolin. The hard particles of quarts and feldspar are mixed with soft sericite and kaolin, and Voilà, whetstone is born. We can somehow observe this orchestra of stone particles from this microscopic photo of the above whetstone.

We can see powdery particles of quarts and sericite/kaolin dotted with yellowish pyrite and greenish epidote. The smoky green that come from pyrite and epidote is a trademark for Tokawa Whetstone. According to the way whetstones made naturally, it’s obvious the right heat was needed from magma at several stages of formation. Thanks to the Plate Tectonics, Tonodake Subgroup received it millions of years ago. And our sharp-eyed ancestors found it useful thousands of years ago. Hm. The next one is also useful for keen-eyed humans. It’s a rough piedmontite, found in Tokawa Park.

The headwaters for Mizunashi River are in south folds of Mt. Toh 塔ノ岳 (ASL 1491m). There especially the area of ASL 1,065m to 1,341m around Mt. Shindainichi 新大日 (ASL 1340m) there are thin lodes of manganese. When Japan started total war in the early 1930s, development of manganese mine was very fashionable everywhere. (Manganese can fortify iron and thus improve the quality of guns of that period.) The mine here in Tanzawa was one of such mines. A prospecting here begun in 1933. Then almost all the cities in Japan turned into piles of rubbles thanks to American carpet bombings, and Japan lost the war in 1945. Tanzawa’s mining for manganese of inferior quality died down around 1949. But surely, there remain lodes and the rocks and pebbles come down the water of Mizunashi River. The stone I found in Tokawa Park is one of them. Its microscopic views show us many slightly different surfaces from point to point.

Pinkish

Black millefeuille-like feel is there.

Tanzawa’s rocks containing manganese were originally at the bottom of ocean on Philippines Plate. They were ashes, aka chert, from underwater volcanoes piled up at the bottom. It contained quarts and manganese. Crustal shortening due to the crush of Philippines and Eurasian Plates pushed up the deep-sea sediments. Next there occurred intrusion of plutonic magma (more to it next week) deep down the shortened crust. The magma emitted high heat and pressure to the chert and metamorphosed it into piedmontite, which is a mixture of manganese, quarts, and other minerals. The stone here is a part of this story. Have you noticed something in common for the first whetstone and the piedmontite? Both were metamorphosed tuff by uber strong magma. This is the difference between Oyama Subgroup and Tonodake Subgroup. I’ll talk about it next week. Stay tuned. 😉 Oh, I should not forget about it. These mining sites on the slope of Mt. Toh are located at the points very difficult for casual hikers to approach. The roads once there were abandoned more than 50 years ago. In 2026, you need professional guides to find the way. Moreover, as I’ll tell you eventually the reason, the soil there is easy to collapse, i.e. VERY VERY dangerous to go. Please do not risk your life!

Hmmm. Looks pinkish ...
 in Tonodake Subgroup area.

At the entrance of Tokawa Park, there is Tanzawa Visitor Centre. The contact address is

Hadano Visitor Center for Tanzawa-Oyama Quasi-national Park
丹沢大山国定公園・自然公園秦野ビジターセンター

1513 Horiyamashita, Hadano, Kanagawa Prefecture, 259-1304
〒259-1304 秦野市堀山下1513

Phone: 0463-87-9300
https://www.kanagawa-park.or.jp/tanzawavc/