What is Pebble Beach like?

Why are are rods, disks, spheres and rollers (Types of pebbles) distributed on a pebble beach like they are?

• Also why are these pebbles shaped the way they are? I take it their disribution on a beach has something to do with their shape? like to hear from geologists and geographers out there thanks.

• Answer:

  First we should address the energy that will sort these pebbles and deposit or move them. Wave energy tends to have more force in the beachward direction as some of the energy dissipates as it moves up the beach. Obviously rising and falling tides will change this dynamic slightly. Also waves often do not strike a beach parallel to the beach, so this oblique wave direction develops a longshore movement of the sediment and the water. Seasonal variation in wave height also strips some beaches of sediment in winter and replaces them in spring and summer. Here is an illustration of a beach profile: http://geoserv.geology.wmich.edu/kominz/C12beach.gif The size of the pebbles will determine how much wave energy is required to move them. As the water deepens away from the beach, the energy effect of the waves on the bottom decreases, till at the lowest parts of the beach only the largest storm waves will disturb the bottom. Size will tend to decrease as the energy of the environment decreases, meaning the largest pebbles are more likely to be found higher on the beach in the highest energy zone. The density of the rock will also have a slight effect. Gold prospectors have been taking advantage of that physical property for centuries. These various shapes of pebbles will each respond to being moved by water in slightly different ways, creating a sorting process by the wash action of waves. Either of the forms that is longer in one dimension will be harder to move if it falls into an orientation where the longer axis is along the path of the waves moving to and from shore. Disks will be more stable than rods, because they are harder to move. Spheres will be the easiest to move. I'm not sure how rollers are different than rods. In any case, the grain shapes will tend to be sorted out by the energy of the beach. The shape is often related to the type of rock. Sand grains are often spherical because they weather from igneous rocks where they were formed as consistently sized crystals of quartz. Round pebbles are often composed of rock that has no dominant cleavage. Quartz would be one of those. Shale or slate which is highly laminated is likely to form disks as the laminations break apart. Other rock that has cleavage in two directions at right angles, and would tend to form rods. An example would be feldspar. Here are some cleavage examples: http://www.geotech.org/survey/geotech/cleavage.html One question you didn't ask is how can this information be useful? Besides the obvious importance of understanding beach erosion and deposition, especially where human intervention has altered the beach environment, geologists may want to be able to recognize the patterns of a beach in the deep subsurface. Understanding how these clasts are arranged and sorted by a beach will allow interpretation of a core, an outcrop, or even a well log to obtain information about the deposit. One significant part of the story could be understanding which direction the beach was oriented, and which direction the deep water was located in a past age. Beaches happen to make excellent petroleum and gas reservoirs, and one of the largest gas field discoveries in Canada was made partially because a geologist was able to predict which direction the beach trended. Longshore current and beach drift: http://www.earthscape.org/t1/vip01/vip01c.html Beach profile: http://www.seafriends.org.nz/oceano/beach.htm More beach morphology: http://www.gpc.edu/%7Epgore/geology/geo101/coastal.htm