As far as we know, there are no Ainu remains in the area. The first Japanese came to pan for gold in the Horoka Tomamu River in the early 1900s. According to a newspaper report, gold was discovered in the Horoka Tomamu River in 1909 (Meiji 42) by one Kano Chogoro. There probably were earlier less publicised discoveries. More gold was found in the Taisho period (1912-26).
In 1932 (Showa 7) Ikukomagumi, a Sapporo company, sold the rights to gold panning in the river to another group, run by Kojima Yunosuke from Kyoto. Kojima's gold operation lasted from 1933 (Showa 8) until 1939 (Showa 14), employing about 20 people. They worked mainly at Ichinosawa ('first ford') and Ninosawa ('second ford'). Kojima also looked for gold in other places in Tomamu, but could only find it in the Horoka Tomamu River. After 1939 a few individuals continued to look for gold, but activity finally stopped by the late 1940s or 1950s.
In 1946 (Showa 21), the Japanese government designated Shimo Tomamu (including Horoka, the hamlet at Ichinosawa) as an emergency resettlement area for war-displaced Japanese, and the new settlers began to arrive in the area in the following year. In Horoka there were at least two families from Sakhalin and two from northeast China. The area was still known at that time for its big trees, some or many of which may have been cut down by the new inhabitants. The Shimo Tomamu Primary School (Horoka branch) was founded in 1955 (Showa 30) and eventually closed in 1970 (Showa 45).
The area is now completely uninhabited. As far as we know, no remains still exist above ground of any buildings. The structures would probably have been made entirely of wood.
Sources: Shimukappu Mura Shi (1963) and the Shimukappu Mura Hyakunen Shi (2006)