Since the Great Circle crosses Guernsey, there is a requirement for two ‘sea-marks’: one at the western end of Lihou Island and one at the southernmost part of the Guernsey mainland, where the Great Circle crosses. These will provide both necessary functions: to alert mariners going eastwards that they are approaching the Guernsey coast and then when to pick up the Great Circle again south of Guernsey; to alert mariners going west that they are approaching the Guernsey coast and then when to pick up the Great Circle west of Guernsey.
Are there two identical Megalithic survivals at these very specific places? Yes. They even have the same name: Venus Pool. Here is the one at the very western end of Lihou Island
It is a tidal rockpool of considerable size. Unfortunately there are no available photos of the one at Corbiere as it is unused due to rock slides but it can be taken as similar. Of course neither of these are regarded as Megalithic, even the name is held to be a Victorian coinage. They are not even considered to be man-made, merely the end products of the way the constant battering of the tidal seas act on weaknesses in rock strata. In other words this is just another example of nature doing something at a remarkably convenient place. Two remarkably convenient places.
It may be asked how common such “swimmable rockpools” are and the answer would be “In nature, somewhat rarer than tidal islands.” There are for instance none
on the south coast of mainland Britain. While tidal pools are extremely common on any rocky foreshore, the chances of any given concatenation of rocks being watertight on a scale more than a few feet across and a few inches deep are vanishingly small. Children play on every British beach but there has yet to be a sign posted: “Beware the rockpool”. As usual Guernsey has two of these incredibly rare beasts and both are precisely, precisely
, where they are required.
The Great Circle does not actually cross Jersey, so one Venus Pool is all that is required here, at the extreme southwestern corner (also, it will be recalled, named Corbiere) in order to keep both eastbound and westbound mariners clear of the land. Here it is though somewhat dilapidated since it is not used by swimmers:
The modern Corbiere lighthouse in the background indicates that not much changes, ancient or modern. So that would be three remarkably convenient places. And of course Burgh Island itself will have to have a 'Venus Pool' to alert incoming ships they have reached landfall. Here is the Mermaid Pool on Burgh Island.
Four convenient places.