Raised beaches on Jura

The raised beaches on Jura and Islay were particularly impressive, Through binoculars, I could see huge pebble beaches further up the coast of Jura which are now on the top of a small cliff. I have been struggling to work out how you end up with two beaches. My issue has been - wouldn't the first beach be worn down. I suppose I'd be expecting a lower beach and the upper level would be what used to be behind the beach. I know that there is isostatic rebound going on. I found one site that put a figure to it: 10cm over the next 100 years. That's 1mm a year. https://www.telegraph.co.uk/news/earth/earthnews/6226537/England-is-sinking-while-Scotland-rises-above-sea-levels-according-to-new-study.html It may not always have been fixed at that level. I wonder what the rate of erosion is of waves cutting into the beach. It is probably the same. Maybe the pebbles on the raised beach represent storm beach conditions so the regular erosion didn't reach it.