This upper section of the canal has best survived periodic flooding intact. The canal bed is not always straight but curves in several places to follow the contour of a natural hill. By cutting into the hill along a constant level and transferring the earth to the outer bank, builders minimized excavation and earth moving. The extreme upper part of the canal was cut straight across level land, where bedrock was the greatest difficulty. This rock was blasted out, and some of it was used in canal construction. The rock remains visible in places along the upper part of the canal bed, often showing holes drilled by hand for blasting. In building the canal, all trees and plant growth were removed, and excavations were carried out by hand or with mules and drag lines. Stumps were removed with a variety of machines common at the time. Each bank was given a core of clay resistant to leaking, and archaeologists have found this clay layer still intact in this area.