Feral horses, belonging to my grandfather, ran wild over thick banks of water hemlock, and there were more than a few rattlesnakes.
Educated: A Memoir by Tara Westover
I was told it is like "thick forest", so thick does not modify banks even it is "thick banks of".
He was standing at the head of the table; the rest of us perched on benches, studying the thick planks of red oak.
Here, I think thick is for planks
So, my question would be in the form of "the thick A of B", which one does thick modify, A or B?