View of disintegrating mud brick walls at Old Testament Jericho..
This picture was taken forty years after the excavation of this area and it is clear how over this period of time the mud brick walls have disintegrated—and Jericho only receives 6 to 8 inches [15-20 cm.] of rain each year. These walls were kept in tact by plastering their surfaces which kept the rain from penetrating to the mud brick core.
This image courtesy of Reinhold Buxbaum.