# Waves on sandy beach separating differently-sized rocks?

I recently went to a beach, and I noticed that the sand seemed to have a few distinct regions. I'm wondering what would cause this, and how it works.

One region is the part of the beach which is kept wet by the waves, but is far enough away that most waves don't seem to reach all the way to the end of it. This part consisted of small-grained, "soft" sand.

This region is followed by a region with a lot of small rocks (mostly the size of, say, someone's smallest toe). This region was located a bit past (i.e.: further into the water) the lowest point the water tended to reach. This size of the grains of sand of this region blends smoothly into the previous region, so they get progressively larger as you walk further out to the water.

The last region is another one of fine-grain sand, and it also had ripples in it (I'm assuming that this is just because of the waves, because I noticed that the waves picked up sand from the peaks of the ripples and pulled it back out to sea).

The first two distinct regions seem to be the result of some type of sorting mechanism induced by the wave. If so, how does this work?

And why was there a second region of fine-grain sand after the coarse-grain region?

• You have to factor in bathymetry, the rock source location, direction of along-shore currents, etc. I feel like someone could give a general answer about how coarse and fine grain sand are separated and deposited (e.g. fine grain sand gets caught in currents and are transported further). But, without some diagram or the location of the beach... I don't know if you will get a satisfying answer. Can you say what beach? You might check out tulane.edu/~geol113/COASTAL-PROCESSES-1a.htm
– f.thorpe
Commented Jun 5, 2016 at 4:50
• @farrenthorpe Thanks, that link helped a bit. And here's a satellite view of roughly the location, although you can't see the sand, of course: goo.gl/maps/MHsSLzyMqCw Commented Jun 5, 2016 at 7:00
• Ah, Lake Michigan. I might have given a try at a black sands beach answer, but I don't have experience with lake beaches.
– f.thorpe
Commented Jun 5, 2016 at 13:59
• When I've seen the type of separations you mention, the predominant area were the wave front "crashes" is the original source of rocks and coarse sand because there is active erosion (e.g. a cliff or rock protrusion). However, reefs and underwater features can affect this greatly. The finer sands can stay suspended longer times before being deposited... so they travel longer distances. But then, if you have another rock face nearby, much of it will stay contained within the surf zone of that beach. Sorry, I'm not sure if that is getting at what you want.
– f.thorpe
Commented Jun 6, 2016 at 2:54
• Really basic stuff I'm offering here so apologies if this is something you are aware of. Grain size in sediments has almost everything to do with the amount of energy that is provided by the transporting agent. So the higher the energy, the larger the grain size. Of course, along a shoreline, as already mentioned, the slope of the shoreline plays a large role too. Very generally speaking, the steeper the shoreline, the rockier the beach is likely to be. Poking around, I found this link: geography-fieldwork.org/coast/coastal-landforms.aspx
– Ton
Commented Jul 28, 2016 at 17:05