ぬかるむ has the definition of "to be muddy" in my dictionary, but it is a verb. Furthermore, all the examples use it before a noun, as opposed to saying 丘はぬかるむ. Is it correct to say this? Why is ぬかるむ a verb, and not an adjective, or rather can someone explain the possible mentality that went into ascribing the characteristic of being muddy to being a verb as opposed to an adjective?
When you try to walk on a muddy road, the mud makes it difficult to walk across. In other words, the muddy road is bothering you. ぬかるむ is a word implicitly meaning "directly bothering".
Because hills do not directly bother you (maybe the road is what it's directly bothering), 丘はぬかるむ is grammatically correct, but is a strange expression.