30,000 BC were excavated at the Seokjang-ri site in South Chungcheong Province. The earliest examples of pit-house architecture are from the Jeulmun Pottery Period. Early pit-houses contained basic features such as hearths, storage pits, and space for working and sleeping. Log houses were built by laying logs horizontally one on top of one another. The interstices between the logs were filled with clay to keep the wind out. Similar houses are still found in mountainous areas as like Gangwon-do province. Elevated houses, which probably originated in the southern regions, are believed to have first been built as storage houses to store grains out of the reach of animals and to keep them cool. This style still survives in the two-story pavilions and lookout stands erected in melon patches and orchards around the countryside. In the Mumun period buildings were pit dwellings with walls of wattle-and-daub and thatched roofs. Raised-floor architecture first appeared in the Korean peninsula in the Middle Mumun, c. 850–550 BC. Megaliths, sometimes called dolmens, are the burials of important and prestigious persons of the Mumun Pottery Period (1500–300 BC). They have been found in great numbers and along with stone-cist burials, megaliths and are the main examples of mortuary architecture in the Mumun. There are three types of megaliths: (1) the southern type, which is low and often a simple slab with supporting stones, (2) the northern type, which is larger and shaped much like a table, and (3) the capstone type, which has a capstone with no supporting stones. The distribution of the dolmens would imply some relation to other global megalithic cultures. Proto–Three Kingdoms period (c. 1st-2nd century BC to 2nd-3rd century AD) It is a subdivision of what is traditionally called Korea's Three Kingdoms Period and covers the first three centuries of the Common Era. Archaeological evidence of ondol (Hangul: 온돌), the Korean floor panel heating system, was found in the architectural remains of early Protohistoric period. According to Chinese text Sanguo Zhi, it recorded the existence of three types of prehistoric dwellings in Korea: pit houses, log houses and elevated houses. Only the remains of pit houses have been identified, however. Pit houses consisted of a 20–150 cm deep pit and a superstructure of grass and clay supported by a tripod-like frame made of timber to provide protection from the wind and rain. Pit houses of the Neolithic period had circular or oval