Antlers were indeed laid down and left as offerings. In fact some of the Stonehenge ones were hundreds of years old, antiques in prehistory, when they were finally laid to rest.

But there was a reason for them to do that, whatever it may have been. There is no real reason for a time capsule inside the hill. Nothing can justify such a daft decision except for some idiot in a suit coming up with an idea that appealed to other idiots in suits.

"Oh Tarquin, another vote-winner in the marginals!!"