Miranthis said

After a bit of searching on various forums, this seems to be the consensus for the hard, gritty, clay-like stuff I found and had to chip out of the recesses to get a socket on the head bolts:

"This used to be common back in the 50's, 60's and 70's with oils with high paraffin content, leaded fuel, not the greatest of filters and if it sat for extended periods of time between runs. It's a pale gray looking clay/putty and once it settles to the bottom of the pan the only way to get it out is remove the pan and literally chip and scrape it out. You only see it now on old original engines from that time period as most have been rebuilt or scrapped since the days of that wonderful old oil... Oil would wick it's way up the main cap bolts and the deposits would settle out but it was like a layer of partially dried cement on the bottom of the pan."

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