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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Water can wash sand to the shore, it can also wash the sand away from the sure. It depends on the nature and geography of the shoreline, currents, climate, etc..
soft sandy tourist beaches have sand dredged from off the coast to make it more accessible for recreation.
places like clearwater beach area good example, it has some of the softest sand you can walk on and they spend good time and money dredging it into a long strip along the water for beachgoers and tourists.
I am going to go for a top-level comment here and simply mention that, yes, tens of thousands of years of erosion will break gravel down into sand. Most places where there is gravel though, is because there has not been tens of thousands of years of erosion at that spot. The earth experienced a huge change in sea level some 10,000 years ago or so (not an instantaneous process but rose several hundred feet/100 meters in a few thousand years about that time frame). Most shorelines today are young (roughly 6000 years or thereabouts).
In many places, the regions were glaciated and the sand was all pushed away so there weren’t beach sands nearby which could migrate along with the rising sea level. The ground was down to bedrock, so beaches had to start from scratch. Elsewhere, the sand was deposited by the glaciers and entire regions are massive sand zones (think Long Island or Cape Cod).
On top of this specific problem, there is an energy issue which matters. Even in places where erosion is happening and cobble-gravel should be (is) breaking down into sand, the wave interaction energy is relatively high and sand (and smaller) material gets carried away. The general result is deposition of that sand up the coast (down-direction of the prevailing current) and as bars in slightly deeper and thus calmer water. You end up with a steady-state condition in the gravelly zones where new cobble/gravel rock is being created from nearby exposed bedrock, and whatever cobble/gravel is breaking down is being transported away and sand is slowly accumulating elsewhere.
But the main reason for lack of sand in gravelly regions is one of time. As in, there has not been enough to make the loads of sand needed for sandy beaches. It is not always glaciated regions that have a lack of sand either, it is often regions where uplift or land rejuvenation occurs for tectonic or volcanic reasons. The land isn’t stable so the land doesn’t sit in place long enough to develop sand deposits.
The sand doesnt stay on the beach forever. It came from somewhere. Its going to somewhere. As it shuffles along on this path, from inland rocky outcropping to ocean deep. We get a snapshot of how fine it was at that moment on the beach.
Also consider the geography of the beach in relation to hills and mountains. Rockfall from cliffs and mountains, as well as rain washing sediments down from the hills can place “newer” more coarse rock, or fine “older” silts in places that may seem odd.
This is something I’ve always wondered. Perfect example is the beach in Cobourg Ontario. West of the harbour, or “upstream” the beach consists of large pebbles; and East of the harbour, is a natural sand beach. So I can only conclude that the natural current, although very slow. has something to do with it. It’s like the harbour acts like a snow fence and allows the lighter sand to curl around and settle in the lee, while the heavier pebbles settle on the upstream side.
*Thousands* of years?
Back in school I worked in the oceanography department with people who studied this very thing.
Every beach has a thing called “beach kurtosis”. Basically, this is the statistical distribution of sand grain sizes. Every beach has its own “signature”. If you try to truck in sand to replenish a beach, and the kurtosis of the sand you bring doesn’t match the natural kurtosis of the beach, the sand will wash away.
In short, your gravel beaches have gravel because it’s natural for them.
Kurtosis is determined by the patterns of the waves that hit the beach. This depends a great deal on where in the world the beach is located *and* the topography of the ocean bottom, which effects the way the waves are funneled up to the beach. Two relatively near-by beaches can have different kurtosis due to having different underwater landscapes.
Our oceanography department was working on predicting the kurtosis of a beach by doing computer analysis of the underwater topography in the area.
(We had one grad student who did his master’s thesis on the kurtosis of the beaches of southern California. Smart boy.)
Given that beaches are forming all the time, some are very young and simply haven’t had time to get there.
Also, some are in areas that are less severely weathered so will take longer than others to get there.
Fresh material is constantly being delivered to beaches – shells from various animals dying, rocks from falling cliffs, or sediment from rivers. All of these take time to break down.
Erosion will also expose rougher layers from tide, wave, current and weather effects – some beaches are rocky in the winter with big storm waves stripping away sand, then sandy in the summer with smaller waves depositing sand.
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