What is the cause of these
The pic was taken at about FL130 and probably about -20C.
Could be a roll cloud, like ‘morning glory’ or a wave formation. Were there gaps between the wave crests?
Was it a coastal location?
Alternatively they could be medieval english ‘ridge and furrow’ clouds 
Probably a bit of mountain wave (which you can get off quite small mountains, the Isle of Man’s hills will generate wave. And you don’t have to go to Minden to experience wave soaring up to 30,000 feet – the Scottish mountains which are much smaller will generate wave that high).
The location was over the English Channel, coming back from Paderborn. No terrain at all anywhere. The waves were closely spaced – at a guess 500m.
A different phenomenon that I’ve seen a couple of times now, is a dark (grey) line in the otherwise completely white stratus cloud.
I’ll take a photo when I see it next time.
Someone once told me that it is caused by a river which is running below the overcast.
Here is something possibly similar
The wind was blowing towards me.
How about this one?
