Cambrian Itinerary of Welsh Tourists by Thomas Evans 1801.

At a little distance from the house is one of the largest and most entire cromlechs [Plas Newydd Burial Chamber [Map]] in the whole principality. It is double, and consists of a greater and a less: the greater is twelve feet long by twelve broad, six high, and the upper edge two feet thick. This cromlech rested originally on five stones, hut one being detached or thrown down, four only bear its weight at present, leaving a space between, of five feet high, and six square. The smaller adjoins closely to the narrow end of this, and is six feet square, resting on three si ones, the fourth having fallen down. Not far from hence, is a large carnedd, part of which, being removed, discovered a cell, seven feet long and three wide, covered with two flat stones, and lined with others. On the top of one of these stones were two semicircular holes, large enough to take in the human neck, which evidently prove it to have been the place of interment of some great personage.