It's not impossible that a glacier could create a hemispherical depression and that, later, a small rock could get caught in it and blown round and round in an act of a o liam erosion.

My long barrow in Bolton is fifteen units high - that's either metres or yards - both are within the error limits. I was disappointed it isn't the highest L.B. in the country but it seems 'as high as' the tallest Long Barrow. Still no formal response though. It's fragile and wide open to destruction.