Mystery Goop

Can anybody tell me what this stuff is?

Today I found some chunks of this white waxy stuff buried in the dirt underneath some concrete. It's snowy white and about the consistency of Crisco. I'm amazed that it's still soft and sticky after all these years. I don't know when this particular slab of concrete was poured, but it's probably at least 50 years old. This white gunk has been underground all that time.

These samples have a dimple pattern on them because the underside of the concrete included some broken-up pieces of pottery or something that was patterned. That may be a clue. Was there some reason this gunk was stuck in this particular spot? Is it some sort of modeling compound? Preservation material? Lubricant? Sealant?

At any rate, I've set the stuff aside until I figure out what it is. It's very soft and soapy and tends to come off on my hands, so I'll just rinse it with water and put it out of the way. Any suggestions are welcome.