[Farmall] Gas tank residue

[genscan tds.net]

This may be a rather stupid question to you veteran mechanics, but I have
encountered a gas tank on a Farmall B I just bought that has something like
an inch of some gunk in it, that I am puzzled as to what it is.  I first
thought it was just a horrible amount of rust, but when I scrap the bottom
of the tank, the metal looks pretty good--not much visible corrosion.  I
would imagine there is a little rust in it, but it almost looks like someone
poured about three  boxes of Cream of Wheat cereal into the tank and let it
rot.  The stuff sometimes comes out in actual chucks over 3/4 inch thick.
It is dark brown to black in color.  Any ideas as to what it is, and any
suggestions as to the best way to clean it out?