Why are there gravel beaches? In thousands of years of waves crashing on the beach shouldn’t all the gravel have become sand?
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One thing to consider is the changing sea level. Just 20 thousand years ago, at the top of the last Ice Age, the average sea level was 130 meters (400 feet) below what it is today. The ice melted and the sea level rose by about 10 thousand years ago, so all the modern beaches only became beaches that long ago. In terms of geology, not that much time has passed to break down all the gravel everywhere.
Another thing to consider is that beaches aren’t permanent, they are being constantly replenished, for example through rivers. That is, the sand on a beach is being constantly destroyed and washed away by the sea, but a nearby river is bringing a constant supply of fresh sand as replacement. In fact, damming up a river can cause a beach to deteriorate and disappear in as a short time as a few decades.
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