Sandstone, mortar and wooden beams were the three primary construction materials. The Ancestral Puebloans shaped each sandstone block using harder stones collected from nearby river beds. The mortar between the blocks is a mixture of local soil, water and ash. Fitted in the mortar are tiny pieces of stone called "chinking". Chinking stones fill in the gaps within the mortar and added structural stability to the walls. Over the surface of many walls, the people placed a thin coating of paint, called plaster, the first things to erode with time.