St. Cornely was Pope at Rome, from whence he was hunted by Pagan soldiers who pursued him. He fled before them, accompanied by a yoke of oxen, which bore his baggage and on which he mounted when weary. One evening he arrived on the outskirts of a village called Le Moustoir where he wished to stop; having, however, heard a young girl insulting her mother he continued on his way and arrived shortly at the foot of a mountain where there was another small village. He then saw the sea in front of him and immediately behind him soldiers in battle array. He stopped and transformed the whole army into stones. As a souvenir of this great miracle the inhabitants of the surrounding country erected on the spot where he stopped a church dedicated to St. Cornely. That is the reason why these long lines of stones standing to the north of the village of Carnac are seen, and why so often at night ghosts are observed walking in the alleys called 'Soudardet sans Cornely' or 'Soldats de St. Cornely'. Pilgrims from all countries flocked to the place to implore St. Cornely to cure their diseased cattle. He cured them all in remembrance of the great services rendered to him by his yoke of oxen during his flight.
The pilgrims, coming to the 'Pardon of St. Cornely', passed among the stone soldiers. The men were supposed to bring stones, the women earth, and to drop them on an elevation near to Carnac, where in time they formed the mount of St. Michel.
Le Rouzic then goes on to hint that perhaps the worship of St. Cornely actually replaced the original worship of the ox here. Hmm who knows.
From 'The Megalithic Monuments of Carnac and Locmariaquer' by Z Le Rouzic (trans. W. M. Tapp), 1908, which you can see in full on the Internet Archive.
In "Excavations at Carnac" by James Miln (1877) he describes some mounds (the 'bossenno' or Caesar's Camp) to the east of Carnac, which seem to be the ruins of Roman houses. Interestingly, from page 16...
It happened one day when I was absent during the dinner hour of my workmen, that an English lady and her son came to see the diggings. The latter amused himself in working with a pick about that part of the construction in the room No. 1 which resembled a chimney, where he discovered a polished stone celt of a white colour, which he showed to his mother: neither of them, however, was aware of its value, and it was flung aside amongst the debris to be carted away. It was not until the following day, when I happened to show them the polished stone celts in the museum in Carnac, that they informed me of their discovery, and regretted that they had not known better. Exertions were made to recover the lost axe, but without success.
The discovery of a stone axe in what appeared to be a chimney was all the more interesting from its crrelation with a custom still observed at Carnac, that of building into the chimney of the dwelling-house a stone celt which is supposed to preserve the house from being struck by lightning. It is to be noted also that the name of the stone axe or celt in the Breton language is Mein-Gurunn, that is to say, the Thunder Stone.
The legend of Carnac which explains these avenues of monoliths bears a resemblance to the Cornish story of 'the Hurlers,' who were turned into stone for playing at hurling on the Lord's Day, or to that other English example from Cumberland of 'Long Meg' and her daughters.
St Cornely, we are told, pursued by an army of pagans, fled toward the sea. Finding no boat at hand, and on the point of being taken, he transformed his pursuers into stones, the present monoliths.
The Saint had made his flight to the cost in a bullock-cart, and perhaps for this reason he is now regarded as the patron saint of cattle.
I have been informed by a priest, but I know not how far it may be correct, that Carnac signifies literally, in the Breton language, a field of flesh. If this be the meaning of the word, it would lead one to conjecture that these stories were placed in memory of some great battle, or as memorials in a common cemetery of the dead.
The people here have a singular custom, whenever any of their cattle are diseased, of coming among these stones to pray to St. Cornelius for their recovery. Such a practice may be a remnant of pagan superstition continued in Christian times; but I must remark that St. Cornelius is the patron saint of the neighbouring church.
I cannot learn that the peasantry of this country have any traditions about Carnac; and I must here observe than no relations or accounts given either by the poor or more enlightened people of Brittany can be depended upon.
.. Tradition has given to the site of these stones the name of Caesar's Camp, but tradition in such a question is an insufficient guide. M. Cambry, led by another tradition, reported to him by an old sailor, that a stone was added every year, conjectures, though with hesitation, that the monument has some connexion with the astronomy of a remote age.
We pursued this [rough track] until the extreme ruggedness of the plain rendered further advance almost impossible.. I was [pleased] that my drive was at an end, and was not less pleased to find that no garrulous guides pounced on me when I alighted from the carriage.. I was happily alone; for Carnac is one of those places where solitude becomes a luxury, and consequently where guides would be more than usually vexatious and troublesome;
for what could they tell the visitor respecting the mysterious ranks of obelisks, the purposes of which have baffled speculative investigations and learned inquiries?
Nothing beyond the whimsical legend current among Bretons, that the stones of Carnac are the soldiers of a mighty army petrified by St. Cornely, who, being hard pressed by them, took the effectual method of frustrating their murderous purposes by turning them into stone.
The skeletons of the soldiers, adds the legend, may be seen on certain occasions at midnight, in the churchyard at Carnac, performing penance for the sins committed in the flesh against the saint, and listening reverently to sermons preached by Death himself.
If you are curious to know more, you will be shown the pulpit of the grim preacher, a dilapidated stone Calvary, and, if you have sufficient courage, you may even hear the sermon; though, if accounts be true, the penalty of intrusion, on being detected by the ghastly congregation, is far more severe than that with which Tam o' Shanter* was threatened.
p246 of Charles Richard Weld's "A vacation in Brittany' (1856) - now digitised at Google Books.