rust on grass

has anyone ever seen rust on grass before? i was up north in southern minnesota for the weekend at my aunts house and i was walking throu her grass and i looked down and there was like a rusty residue all over my shoes. she asked me what it was and i have no clue, i asked her if she watered the grass at all and she said no so its not coming from water and it hasnt rained in over a month so any help would be great