M. Weinberg
The rectangular altar on Mt. Ebal was once filled with alternating layers of earth, ash and fieldstones. Here, inside the altar’s exterior wall, bordering the edges of the photo, we see the excavation of the fill in progress.
When archaeologists removed that fill, they discovered an interior dividing wall, center, extending part of the way across the altar. As the archaeologists continued to excavate, they came to an ash layer (to the left of the dividing wall), which they preserved for a time. To the right of the dividing wall, they continued excavating and reached bedrock. The walls of the altar had been built directly on bedrock.
Sometime after this photo was taken, the area in front of the dividing wall was excavated, and a circular structure on bedrock was revealed.