Immediately to the right of the Stables a gap in the trees gives an incomparable view across the Strait to the mountains of Snowdonia, while to the left can be seen the cromlech. One of the best preserved of Anglesey's many neolithic monuments, this consists of four or five large boulders which once enclosed the burial chamber of one of the seafaring invaders who colonised the island long before the Roman invasion.
To the antiquarians of the eighteenth century this and other cromlechs were concrete evidence of a continuity which linked the Welsh Bards with the ancient Druids (whose last-ditch defence against the Romans is said to have taken place on the shores of the Menai) and beyond, to Hebrew scripture. Cromlechs were thought to have been built as sacrificial altars, and the priest of the local parish, Henry Rowlands, whose Mona Antiqua Restaurata of 1723 put this case at length and with great conviction, cited Exodus xx (`Thou shalt not build an altar of hewn stones') to explain why the upper slabs were formed of boulders and not dressed stones.
The Rev. William Bingley,writing in 1800, thought it unlikely 'that these erections should have been intended as altars for druidic sacrifices ... The upper stones are, in general, too small, and much too high for a fire to be kindled upon them, sufficient to consume the victim, without burning the officiating priest'.
Originally the stones of the Plas Newydd cromlech would have been covered with an overburden of earth and without it they have been subject to both weather and vandalism: in 1799 Humphry Repton proposed the insertion of a `wedge of marble' (not 'a common wedge', which `might mislead future antiquaries'); it was to be inscribed 'To preserve A Druidical Monument which is of a date before the Christian Era (Tho' lately endangered by wanton mischief) ...
Also within the park are the remains of a megalithic chamfered cairn or carnedd, and this too suffered damage in the eighteenth century, when `Sir Henry Bayly, supposing this mound a mere heap of rubbish, began to level it, but meeting with human bones the workmen were ordered to desist.'