@Faintdreams My immediate reaction was "this is more geology than geography" because whether a beach is sand or pebble depends on the slope of the land as it meets the sea, which is itself a function of the geological history of the landmass in question, and especially of how long it has been adjacent to the water. So I did a little poking around. I'd be shocked if there were no pebble or cobblestone beaches in equatorial regions, but you are correct in saying that pebble beaches are more common in northern/southern climes. This is because of the previous ice age, which had both land and ocean locked up under glaciers until about twelve thousand years ago. So the coastlines of previously glaciated areas have had less time for erosion to work on the materials at water's edge. Basically, the older the landmass, the flatter it is likely to be. And the flatter the landmass, the more likely it is to have sand deposits along the coast, rather than pebble or cobblestone deposits.