Raised beach at Kintra on Islay

I found a lovely example of a raised beach at Oban recently and reminded of these that I found on Islay last year. The idea is that the weight of the ice over the land mass during the Ice Age pushed the continental crust down. When the ice melted, it has been rebounding ever since. I have read about the ice being high enough in places so that only the peaks of the mountains can peak out. That would suggest say a 1000m deep layer of ice. Ice has a density of 917 kg per cubic metre. That would mean a pressure of 917,000 Pascals underneath. That's almost 10 times atmospheric pressure. What bothers me is that there was time for the waves to cut a flat beach platform but that has been preserved instead of there being a gradual slope as the land moves up a few cm each year.