Nobody actually knows what it would have looked like, but what you are seeing is a concrete overhang to protect the carvings from the rain.

The reason no one knows what it would have really looked like is because someone built a hillfort on its top. When the did so they dug an encircling ditch into the mound itself behind the kerb. This actually cut the outer ends off the two passages.

There is a spillage of quartz stones by the entrances, so there probably was a small white facade around each entrance (something I think was far more common than we now have evidence for.)