The upriver district of Ishikari River where TSUKIGATA conducted a survey was the land called 'Shibetsubuto (also referred to as 'Kami Shibetsu Atari')' and it is said to have been just like a virgin forest with thick trees.
月形が調査を行った石狩川沿上流は「シベツブト（上シベツ辺とも）」と呼ばれる土地で、樹木が生い茂り正に原生林そのものであったという。