I believe it is the Delaware River. I seem to remember the term "Delaware mud."
I saw a post which claimed the Baseball mud comes from Baltimore, as the dirt there is 'extra fine'. Although many years ago, as a resident of the Philadelphia area I was told the apparently widely accepted story of how the mud comes from only one secret place in New Jersey, there may be some truth to the Baltimore story as I vividly recall the infield dirt at a funky softball field where we played bar league games, was the finest I had ever seen. The outfield was trash, but that infield was the best. I saw it, and it was major league quality, if not even better.
check out http://baseballrubbingmud.com/main.htm for some more answers about baseball mud. I heard the Mississippi River. Mississippi mud.
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For the last 70 years, every single ball used in major league play has been initially scuffed up, prior to use, with a special blend of mud that is made only by a company in New Jersey, using mud that is found only in that state. The location of their mud hole, and the changes (if any) they make in this mud, have remained a closely guarded trade secret for all these decades.
to remove the shine and finish from the ball.
Unless and until this shine is removed, the ball is difficult to properly grasp.
The mud used to remove the shine also does not scrape the ball, a combination that is unique to this mixture. It has been used in all major and minor league games for decades (since 1938 in the AL), and its composition has been a trade secret of one company that entire time.
This mud is called Lena Blackburne Baseball Rubbing Mud that comes from a "secret location" in New Jersey.