久井・矢野岩海の形成過程

Block field is an accumulation of subrounded to subangular blocks without fine sizes over solid or weathered bedrock. The block field at Kui and Yano in the Hiroshima Prefecture has been designated as a natural monument by the Government. The formation process of it is described from a standpoint of weathering in this paper. The blocks at Kui and Yano are composed of granodiorite and are rounded with diameter of 1 meter to several meters. The block fi eld occurs at the mountain slope surrounded by the low-relief surface called the Sera surface. Weathering profi le developed in this area including the Sera surface is divided into 4 zones with depth: (I) grus; (II) small corestone in grus matrix; (III) large corestone with some intervening grus; (IV) fresh or slightly weathered granite with joints. Depth of I and II-zones is more than 30 meters. The blocks at Kui and Yano are inferred to be originally associated with corestones of the III-zone: the friable grus of the III-zone was readily washed away, whereas corestones were left in situ after exposing of the III-zone. Therefore, the formation of block fi eld was related to weathering process and formation of residual hill surrounded by the low-relief surface.