No, Nigel, Moth was right. The principle of ray-tracing can be used to prove it. If you were to stand at the point where the top of the shadow strikes the ground and look back up at the hill (wait for a cloud so you don't hurt your eyes), you would not be able to see the flat top. What you would see is the curved edge around the summit and it is this that is creating the top part of the shadow, elongated into a semi-ellipse because of the angle of the sun. If you can't be bothered to travel to Silbury to try this, then hold a coin horizontally at arms length in front of you a few inches above eye level and look up at it. Can you see the top surface or do you just see the curved edge?