What is this stuff in my block?

Well the 4th flush (with the auto dishwashing detergent) came out still somewhat rusty. Looking into the head through the t-stat opening, the amount of soft gunk on the walls came down considerably. So the repeated flushes are making progress. It looks like most of what it left is harder deposits. I may go back to the commerical flush for a 5th fluch cycle. I have taken many dozens of engines apart and never saw anything like this one.

Good points on the heater and rad; I was planning to do the rad upside down out of the car; just gotta hope that the heater core is not too plugged by now. I think I will go out now and reverse flush it.

Yep it is probbly just old coolant residue, rust, maybe some Bar's Leak or similar.

BTW car is new to me for the past 2 weeks; water would have been from Luray VA. Not sure how hard it is/isn't. (Water here is almost 100% pure, from a mountain well; no dissolved minerals.)