Hi Dave43, Wheal here.
Know this area pretty well and I am sure that you are correct to think this is a crop growing in a field where the housing estate at Trevenner Lane now is. The postcard seems to give an optical illusion that a launder is supported on a row of posts and I am sure this is all it is. Tolvadden Mine is slightly to the east of the spot which appears to be roughly where the Mount Haven Hotel now stands. Just below here below Chymorvah Hotel is the sight of Mounts Bay Consols according to Hamilton Jenkin complete with a shaft on the cliff edge (now lost to coastal erosion).
Also it is difficult to see where the water would have come from as it looks like it comes from the high ground around Trevenner Square.
Hope this is useful.
Wheal.
Photograph:
🔗121581[linkphoto]121581[/linkphoto][/link]
poke around long enough and you'll find something..