Dirt

I was having a conversation with my dad recently about the dirt on a baseball field. For those unaware, the largest patch of dirt in the infield extends out from the 1st - 2nd and 2nd - 3rd basepaths. See the picture on this page.

We came to the consensus that the original fields would have had no dirt at all, just grass. Then certain areas would have gotten worn out. Worn grass leads to bad hops and its difficult to maintain, so gradually they replaced those areas with dirt (or stopped reseeding the areas as grass died). Our rationale was that there is no reason for the dirt to go back towards the outfield as much as at does, except that the infielders spend the most time there, which would kill the grass.

Also, this dirt theory explains why the path between 1st and home and the path between 3rd and home is dirt (and in the old days the path between home and the mound).

I'd be interested to hear other theories.