Before there were dikes there were terpen.
I was disgracefully ignorant of terpen until I read Robert Van de Noort's North Sea Archaeologies: A Maritime Biography, 10,000 BC to AD 1500 (Oxford, 2011). Van de Noort devotes many pages to terpen, which are artificial islands constructed between 500 BC and 1000 AD in parts of the Netherlands, Denmark, and Germany. The earliest terpen were simple mounds on which a house was built; later several or more would be joined together to form small elevated villages protected against high tides and flooding. They might be as much as forty feet high. With the coming of dikes (c. 1000-1200 AD), terpen ceased to be constructed.
Here's a terp, viewed from above.
And here's another, photographed from ground level.
I've been to the lowlands and might have seen these peculiar topographical features with my own eyes, but I certainly didn't register them. Even if I had, I might have confused them with the grave barrows or tumuli that are common in Scythian Bulgaria, or even with drumlins, which have a similar look to the uneducated eye -- although drumlins are natural rather than man-made.
I love it that "terpen" retains its ancient plural, along with words such as oxen and brethren. There aren't many such words in the language (children is a reduplicated plural, the original "-er" plural supplelmented by an -en plural -- and some day to be superseded, I predict, by "childrens").
Van de Noort's book is fascinating reading for those who like terpen and bronze-age sewn plank boats and such. It's rich in detail but might have been even richer if the author hadn't felt the need to waste space by calling the seashore a "place of liminality" and shipboard "a heterotopia sailing upon the Deleuzian ocean" -- and also indulge himself with many unconvincing speculations dependent on such peculiar, modish, transitory vocabulary.
When it comes to odd words, give me the old ones every time. Here are some satisfying lovely words that were as new to me as terpen: saltern (an area in fenlands set aside for salt manufacture); cofferdam (a temporary dam), thwart (a seat in a rowboat), leister (a two-pronged spear for fishing), dromond (a medieval galley) and broch (a dry-stone tower).