Doing a little research on 1287, England and what a surprise to find the following on Wikipedia... (edited)
Apparently the Call of Cthulhu writers did their research.
February – South England flood, affecting the Cinque Ports of England: A storm surge destroys the town of Old Winchelsea on Romney Marsh and nearby Broomhill. The course of the nearby River Rother is diverted away from New Romney...
A cliff collapses at Hastings, ending its role as a trade harbour, and demolishing part of Hastings Castle. New Winchelsea is established on higher ground.
December 14 – A huge storm and associated storm tide in the North Sea and English Channel, known as St. Lucia's flood in the Netherlands, kills thousands and reshapes the coastline of the Netherlands and England.
In the Netherlands, a fringing barrier between the North Sea and a shallow lake collapses, causing the fifth largest flood in recorded history, which creates the Zuider Zee inlet and kills over 50,000 people; it also gives sea access to Amsterdam, allowing its development as an important port city.
In England, parts of Norfolk are flooded; the port of Dunwich in Suffolk is further devastated; and in The Fens through the vehemence of the wind and the violence of the sea, the monastery of Spalding and many churches are overthrown and destroyed:
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Caleb: I’ll share the investigation with you, but this was a real and devastating disaster that reshaped Easy Anglia. The writers definitely did their research!