Black dirt also comes from the mainland and is basically fertile top soil. The hand filled bags have less gravel in it than the machine loads. The Mayans used it for balast way back on their trips to the island and probably also used it for farming here. Large mounds of it can be found all about the island. The little park that is for sale is an example of this (and has soil stolen from it). Mixed with horse manure, the stuff is awesome for growing all "kind" of good stuff.

Much of the black dirt coming from northern Belize cane country was stripped off of Maya mounds hence is full of broken pottery shards and interesting little figurines, etc.

I wonder how much archeology has been lost in the process?

Ah! Yet another rhetorical question...have you ever done anything to find an answer?

Not rhetorical, just a muse. But certainly a question to which probably no-one has the answer. Have you not heard how much has been learned in other places from studying the contents of dumps?