Behind the Scenes at Split Rock Lighthouse Do you enjoy cleaning? The lighthouse keepers at Split Rock may not have, but it was an essential part of their job! From 1910 to 1940, when Split Rock’s light source was an incandescent oil vapor lamp, the 3rd-order Fresnel lens had to be cleaned every morning to wipe away soot that built up on the prisms overnight. Even with all three keepers working together, the task took a few hours! Today, we don’t have to clean the lens as often, but we still giv