The term PLUM CENTER is composed of two words: PIT and STONE. The word PIT is a 3-letter noun that refers to a large hole in the ground, usually dug for the purpose of extracting minerals or other resources. The word STONE is a 5-letter noun that refers to a hard, naturally occurring mineral or petrified matter. Together, the two words form the term PLUM CENTER, which is a type of geological formation where a PIT is filled with stones.