Dad said he found this rock in Lake Lewisville after the water receded. Anybody know what causes this sick swirl pattern?

He said he found it close to the shore, and that it was uncovered when the water had receded. I know(?) iron oxidation causes the yellowing of some rocks, but does anybody know what could’ve caused the swirling pattern? He said he didn’t find any other similar looking rocks; don’t know if he was actively looking for other similar looking rocks, though.