The source of inspiration for the design of the chambered long barrows remains unclear.[24] Suggestions that have proved popular among archaeologists is that they were inspired either by natural rock formations or by the shape of wooden houses.[44] It has been suggested that their design was based on the wooden long houses found in central Europe during the Early Neolithic, however there is a gap of seven centuries between the last known long houses and the first known chambered long barrows.[32]