What is this stuff in my block?

Soooo, as part of gogin throuhg the '62 Dart, engine flushign was on yesterdays agenda. Went to drain all the coolant and when I pulled the block pug in the rear, nothing coesm out....???? Poked my finger I the hole and hit something semi-solid. Kept poking and then a small stream starts, and finally I bust through something and the ususal torrent comes out.

Looking in the catch pan, I find some chunks of 'stuff'; it has the consistency of half dried plasticene, and has colors varied from a slightly rust tinged tan slate rock, to a greenish/brown. I am speculating that this is old antifreeze combined with rust but maybe some Bar's Leak was in there....?? Just wanting to see if others have even seen anything link this or have opinions.

In looking into the thermostat opening into the head, there is a nice slimy layer of pure rusty stuff coating the inside. The water pump comes off next! Am on the 4th flush (1 commerical flush, 2 with vinegar, and the 4th with auto dishwasher detrergent). It is getting cleaner but what a mess! This car has been driven a total of 3k miles in the last 42 years, with a few bits of maintenance.....sooo low miles can be a blessing and a curse.