Digging old outhouse

[Oldiron]

Hello,

I'm trying to locate an old outhouse on my grandparents property. My question is, if I think that I'm in the right area to dig, how far do I have to dig before I might find remants? I found an area approximately 62ft from the back of the house which my steel probe sunk rather easy in the ground.

I dug to a depth of 4ft and didn't see any color change within the sand. At that depth, should I expect to see a change in color with the soil if I'm digging in the right spot?

All the best,
Michael