Until now, there is still no any detail research on khmer prehistoric stone yet but there was existing document about Vertically sacred stone and stone coffin in Kampuchea Krom as below.
Vertically sacred stone
Until now, there is still no any thorough research on vertical stone or Menhir stone which was put on standing for religious causes in prehistory yet. This stone, in France, called Menhir is in the state of universal. The recent finding of these typical stones in Traninh of Laos and in Thailand revealed that the Menhir culture is one of stone architectures certainly built by Khmer ancestors. Off Memot, in the middle of a rounded village a vertical stonewasdiscovered but in Buddhist style because its lotus-shaped parts were embedded below.So what we see here is that when Indian religion propagated this area those sacred Menhir stones changed their forms and meanings and finally became border posts for pagodas. Please be informed that factors that can be main assistance to the view is that Buddhist-venerated sites is located in Kalasin province, Thailand there were numerous vertical stones found in prehistory in combination of borders in Indian style. In this context, we regard Khmer nationals to be really experienced with the vertical stone culture; however, under influences of Brahmanism and Buddhism those Menhir columns have become border posts or Buddhist boundaries (Sima) particularly in Chenla era. Mostly of them contain the carved biography of Buddha during last existence or Jataka.On the other hand, besides belief in vertical stone in the center of or near any district, village, or town which today called delimited stone (Thmor lak), our Khmer people also believe in grown stone that have similiar meanings to Menhir column that presumably called spirit stone of ancestral guardian of the soil. (Thmor Nakta Chas Srok).Truly, the belief in natural stones is a Neolithic culture that is universal because stone stands for solidity or immortality of nature in general and in the context of life, vertical stone is a symbol of organ or genitalia for reproduction.Until today, according to the will of people this typical column also stands for spirits or for stone of ancestral spirits and it has a power that stations and play a sacred role in protecting and looking after offspring in communities.