A water catchment of a sort you wouldn't want to drink from says either salt or sewerage, and as it doesn't seem to be straight lines, I will say salt.
'Sort of' a war memorial says it is a commemoration of something colonial, so probably a first landing or something similar. The other option is a ship-wreck, with the story on the big rock, and the memorial list of victims on the front one.
Where about and which one is now someone else's guess.
