The earliest projects had a layer of streambed stones arranged as a foundation, upon which the adobes were placed. Later, stone and masonry were used for foundation courses, which greatly added to the bearing capacity of the brickwork. Aside from superficial leveling, no other ground preparation was done before construction started. There is some evidence to indicate that the initial structures at some of the outposts were produced by setting wooden posts close together and filling the interstitial spaces with clay. At completion, the building would be covered with a thatched roof and wall surfaces would be coated with whitewash to keep the clay exterior from eroding. This type of construction is known as "wattle and daub" ("jacal" to the natives) and eventually gave way to the use of adobe, stone, or "ladrillos". Even though many of the adobe structures were ultimately replaced with ones of piedra or brick, adobe was still employed extensively and was the principal material used in building the missions as there was an almost universal lack of readily-available stone. The adobes were laid in courses and cemented together with wet clay. Due to the low bearing strength of adobe and the lack of skilled brick masons ("albañils"), walls made of mud bricks had to be fairly thick. The width of a wall depended mostly on its height: low walls were commonly two feet thick, while the highest (up to thirty-five feet) required as much as six feet of material to support them.