Part 3 What We Found Below the Walls of Sacsayhuaman Three pits were excavated along the western portion of the lower walls of Sacsayhuaman. Two on the lowest level, and one on the next level up. After a few days of digging it became clear that this was not going to be as simple as I originally thought. Large boulders were encountered during the dig. They sat directly in front of the blocks we wanted to date. Luckily some were able to be removed, and we could just barely work around the others. This brings us to an interesting observation and leads to several questions. What was the purpose of these large stones? Were they intentionally placed in front of the walls to help secure and stabilize them? Were they part of the original wall and fell off at some point? Were they destined for being part of the wall, but never made it because of halted construction? The boulders were surrounded by a mix of small angular gravel chips, mostly composed of limestone along with a mix of finer sediment, mostly clay and silt (smaller than sand which is what we’re after). Depending on the pit and the distance from the surface there are sections where the gravel chips are more abundant (what a geologist would consider clast supported), and others, less abundant (what we would call matrix supported). This material is at least 1-2m thick for the pits in the lower walls, and likely thicker in the upper wall. I should also note that there are some igneous gravel clasts, what is locally referred to as green diorite, similar in composition to the Inca Slide. Though it also outcrops on the hill the walls are built around. Some clasts are larger than gravel, but smaller than the bigger boulders. Questions to consider based on these observations are: How were these angular limestone gravel chips made? Where did they come from? The massive wall blocks are made of local limestone. Are these chips from their carving? Or were they made specifically as earthworks sediment fill and blended with local soil? Where did the fine sediment come from? Was it sourced locally from the citadel hill or in creation of the adjacent plain? You may also be asking what was below this layer of gravely fill? Below this fill is heavy red clay. We interpret it as the Maras Formation. A Cretaceous sediment that appears to have filled in around the Cretaceous limestone and karst of the Ayabacas Formation. Though at the base of the pits it is mostly red clay, this formation can have layers of bleached sediment where it turns white or yellow, layers of gypsum, and layers of fine sand (this is the kind of sand we are hoping is right below the blocks). Some have asked why we stopped digging. It is because we hit this Cretaceous Formation. It is the equivalent of hitting bedrock. There are no more stones below it. So what was directly below the basal wall blocks of Sacsayhuaman? First I should say that in all three pits there was only one additional row of wall blocks below the row visible at ground level. Below that basal row of blocks we found 6 to 8 inches (15-20cm) of what we interpret as a human created foundation layer, composed of angular gravel chips and fine sediment. The difference between this layer and the fill that makes up the pit walls is that the foundation layer is partially altered and bleached of color. We assume this is because of movement of ground water through this layer. As water drains off the hill and from behind the walls it travels down through the more porous sediment and then hits the more impermeable layer of clay in the Maras Formation and then travels along this contact where the water causes some alteration of the sediment.