Far be it from me to get in the way of Einstein, who seems to know what s/he is talking about here, but I was told that the distance to the horizon is about six miles.
This was a question on my GCSE maths exam (giving info away about my age there!) the answer is less than mikewith suggests at about 5000 metres, there's a good explanation as to how it's calculated here http://www-istp.gsfc.nasa.gov/stargaze/Shorizon.ht m
But "Einstein" don't you think that maybe the reason you can see the cliffs is that they rise up pretty high in the sky?!?... although it is just a passing thought!!
Farrukh what do you mean Earth curvature in 40km? and Einstein if you change the parameters (i.e. introduce cliffs)in the equation posted by Obo then of course the distance will change
i know....i tried to keep the question as simple as i could...without added things like cliffs etc. Hence the line in my original question about having your toes in the water.