The assumption that Trethevy had a cairn/mound is one based on archaeological evidence from other portal tombs and what is to be seen at the site today . Prior to the placement of the capstone the portal ,back and doorstones would have been freestanding but the cairn would likely have supplied support for the orthostats and provided a ramp for the erection of the capstone . Putative astronomical alignments are not a basis for deciding on the possible architecture of a monument or has been suggested in other cases , dating .
George
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