谷沢川は武蔵野台地の南西部に位置し，付近の地形は数段の段丘地形で，大蔵台・荏原台面・武蔵野2面・武蔵野3 面・沖積低地に区分される。谷沢川は武蔵野2 面を流れる，延長約6.3 kmの小河川である。上流の約5.5 kmは北西から南東方へ流れているが，等々力駅近くで流路を南方へ急に変え，延長約1kmの渓谷を横切り，多摩川に合流する。渓谷を形成する以前の旧谷沢川が形成した沖積低地は，等々力駅近くから東方へ向い，呑川低地に合流している。本報では，最初に，谷沢川の流路が位置する武蔵野2 面を構成する武蔵野礫層と関東ローム層の堆積状態を明らかにした。次に，等々力渓谷付近の武蔵野礫層・関東ローム層，さらに不圧地下水の性状を精査した。これらの成果を基に，等々力渓谷の形成史として図11を作成した。

The canyon, located in Setagaya Ward of Tokyo, is a rare attraction in the heart of the city, drawing visitors year-round. The surrounding terrain consists of several river terraces. Through geological investigations, it has been revealed that the unconfined groundwater flowing through the area played a major role in the formation of the canyon. Todoroki Canyon was formed in the sequence shown in Figure 11. 1. Musashino 2nd Terrace was a riverbank of Tamagawa River approximately 70 to 80 thousand years ago. 2. Musashino 3rd Terrace was the same riverbank approximately 50 to 60 thousand years ago. Due to fluvial erosion by Tamagawa River, Musashino 2nd Terrace became an upper terrace, with many channel heads formed by the unconfined groundwater at the terrace scarp. The old Yazawa River flowed on the newly-formed upper terrace, forming an alluvial plain on both sides of the stream. 3. Musashino 3rd Terrace became the lower terrace approximately 40 to 50 thousand years ago. The channel heads at the terrace scarp were enlarged due to unconfined groundwater, with the Hudoudou Valley channel head being the largest. 4. The current landscape is the result of the channel head of the Hudoudou Valley connecting to the old Yazawa River. The head of Hudoudou Valley progressed upstream and connected to the old Yazawa River, resulting in the flow of water from the upstream of Yazawa River to Tamagawa through Hudoudou Valley, completing the formation of Todoroki Canyon on Musashino plateau in Tokyo. The downstream of the old Yazawa River was renamed as Kuhonbutsu River.