I was just thinking it could just be sand and the slopes of the sides are related to the angle of repose.
A recent show on the History channel had a guy who is an engineer that was called in to help fix detioriration of one of the pyramids (step?)
His theory was that the quarrying of the stone near the pyramids would have left huge amounts of rubble—and where did it go? His idea is that the pyramids were faced on the outside and the interior filled with sand and rubble from the nearby quarrys. It makes sense from an engineering POV compared to hauling in millions of stones weighing over a ton and placing them at the rate of one every 20 seconds to get the job done in the time alloted. He says the Egyptians were very smart and this is the way they would have done it.
Not saying that’s the way it was done, but it is an intriguing theory. I know some tunnels have been dug into pyramids and found rock, but how far, how deep? Enough to make this theory a “no-go” from the git-go?