WHAT IS THIS??

It has been 20 some years since I worked in machining cast metals but we still found sand in them. The tool coolant spray would often bring it out. In this case, lower back of an engine block, it's not going to travel. There is no turbulence. Only circulation. The small stuff might lift and travel a little but will return to the same place again when circulation stops. Get a clear jug of sanding creek water and see for yourself. Swirl the filled jug to cause circulation the set it down unlevel. Come back tomorrow to find clear to lowest point.
Having said all that, I couldn't resist breaking some of the slag from left rear of my 273 while the starter and knock out were removed.