By nature of the coin's design, a somewhat subtle die clash.
Clashed in several spots, though, so I imagine it must've hit pretty hard!
Didn't notice until I uploaded the pics, but there may be visible clash marks on the reverse as well. To the east of "WA" (in Iowa) touching the mound, and directly north of that on the N side of that mound. But I need to find another Iowa quarter to compare.
ADDED I just looked at another Iowa quarter, the reverse looks the same on both, so the reverse is not a clash mark





