# 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
Jun 5 '16 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 Jun 5 '16 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
Jun 5 '16 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
Jun 6 '16 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
Jul 28 '16 at 17:05

## 2 Answers

There are two ways in which waves can sort particles on a beach, and both depend on surface area to weight ratio. The larger, toe-sized particles you describe have less surface area to volume (weight) ratio than the sand-sized particles, so were first to drop out of the water flow. The lighter particles had more surface area per unit of weight for the waves to act on, so were slower to drop out and were carried further up the beach. Why the lowest level sand grains were precipitated in deeper water, before the toe-sized particles, is less easy to understand, but one possible reason is that they contained some heavy mineral such as tin, which altered the surface area to weight ratio and caused them to drop out first. This principle is well seen in gold panning machines, where the heavy gold particles drop out of the flowing water first,while the lighter particles are carried further on, even when they are larger than the flecks of gold.

After looking at the photo of the region I would say there are a couple of processes involved. First this being a lake the most energetic water level would be at the wave zones. Essentially the boat wakes and wind waves all strike the shore in a relatively narrow band. This would uplift and allow the fines, sand, to be transported either upwards on the shore, soft sand region, or down lower into the firm sand zone. Once in the water there is another sorting process. The wave action on the shore also generates another erosional force and that would be the longshore current. The water washing onto the beach generates a current that flows along the beach. This current is where the ripples are and what you are seeing is sand being moved along this zone. Finally, there are human factors. The beach outlined is surrounded by groins, these are barriers placed into the water perpendicular to the beach to prevent the sand being washed away by the longshore current, see https://www.nap.edu/read/11764/chapter/5#59. Finally being that the beach is a near a major metropolitan area it is very possible that the beach also gets sand added to it from time to time by the municipality. Overall, you have the waves and the longshore current moving sand out of the wave zone and transporting it along the shore but you also have groins and possible human sand transport keeping some sand in place at the beach.