The project will focus on the cutoff wall and other structures on the surface and below ground in the region around the Upper St. Anthony Falls. There are many important landmarks in this area, many of which are contributing elements to the St. Anthony Falls Historic District.

The cutoff wall is located under a 15-20 ft thick Platville Limestone layer, which makes up St. Anthony Falls. Underneath the limestone layer is a thick layer of St. Peter Sandstone, which is soft a relatively erodible sandstone. During construction, workers created a vertical shaft through the limestone and into the sandstone layer. They then slowly excavated a narrow slot from west bank to east bank, filling with concrete as they went. The work was slow and dangerous but ultimately resulted in the creation of the 6-ft wide by 40-ft tall concrete cutoff wall. The cutoff wall is represented in yellow in the figure below.

On September 7, 1868, workman had begun excavating the Eastman Tunnel, under Hennepin Island and under the Mississippi River. The tunnel was to be the tailrace for a flour mill William Eastman and his partners planned to erect on Nicollet Island. By October 4, 1869, workers had reached the foot of Nicollet Island. That morning, they arrived to find water running through the tunnel. Within hours, the sandstone walls began collapsing and cavities grew under the limestone. By nightfall, a section of the limestone tunnel roof at the downstream end collapsed. This began an eight-year battle to save St. Anthony Falls.

The Eastman Tunnel is represented by the red in the image below. The yellow area are locations of where we believe subsequent failures in the sandstone occurred prior to constrution of the cutoff wall.