영어 질문좀 할게요. 답 부탁드립니다.

[토플러]

In order to build the aqueduct, engineers and builders first constructed an elevated causeway across the Lake, consisting of rocks, dirt and sand that were held together by wooden stakes. After construction of the foundation of the aqueduct was completed, a second, raised portion was built from stones and mud on top of the bottom one and also held in place by wooden stakes.

여기에서 one 이 가리키는 것이 무엇일까요??

미리 감사드려요.~!!^^

[Heavenly1]

수로를 만들기 위해서 먼저 높이가 있는 causeway (둑)을 만들고 이 둑위에 다른 더 높은 둑 (나무말뚝으로 지탱된) 을 만든다