The red clay was there long before the battle. I have the the book where I secured the information to answer your question.
Regards
David
Source
New Jersey Troops in the Gettysburg Campaign from June 5 to July 31, 1863 by Samuel Toombs
Page 103
New Jersey Troops in the Gettysburg Campaign: From June 5 to July 31, 1863 (Classic Reprint) [Toombs, Samuel] on Amazon.com. *FREE* shipping on qualifying offers. New Jersey Troops in the Gettysburg Campaign: From June 5 to July 31, 1863 (Classic Reprint)