In 1961, an Eschrichtius akishimaensis fossil was discovered at Akishima City, Tokyo, in the Tama Riverbed beneath a railway bridge on the JR Hachiko Line. The fossil was excavated from the Komiya Formation (equivalent to the Hirayama Formation) of the Kazusa Group, suggesting that at least two lineages of the genus Eschrichtius survived in the Early Pleistocene. Downstream from the excavated site, many island-like landforms of about 2 m in height developed parallel to the river flow. The grooves between the islands are long and narrow downstream. When the water level rises, the islands look like a swimming herd of cattle, which is why Koizumi (1996) called them “herd-like landforms (gyugun tikei).” Gravel was actively extracted from the bed of the Tama River from around the Meiji era until 1968. There were as many as 104 gravel pits (as of the end of fiscal year 1963) between Ome City and Komae City. Gravel extraction triggered exposure of the Komiya Formation, which consists mainly of semi-consolidated sand, and the water flow gradually carved longitudinal channels, forming the landforms shown in the photo, which are rarely seen elsewhere in the world. Unlike fossils, the herd-like landforms do not remain and are gradually lost, showing the framework of the Anthropocene.
(Photography & Explanation: Tetsuya WARAGAI)
