Note in the picture that the closer beach is comprised of pebbles, cobbles and boulders, while the beach to the north is comprised of sand. Why is this so?

The answer lies in the rocks contained in the cliffs behind the beaches. The bedrock in the cliffs behind the rocky beach are comprised of the Monterey Formation, Altamira Shale– upper part, and is comprised of siliceous & phosphatic shales, limestones, and siltstones. In short, rocks that are well indurated (hard). The cliffs behind the sandy beach are made of sandstones and mudstones of the Malaga Mudstone formation and the Valmonte Diatomite, all of which weather into finer particles. Also note how the cliffs gradually disappear to the north. Just beyond that point is where the Palos Verdes Fault goes offshore. The Palos Verdes fault is uplifting the Palos Verdes Peninsula, exposing the siltstone, sandstones and comglomerates that comprise the Monterey formation in the coastal cliffs at Malaga Cove. Rain, waves and mass wasting break down these rocks and deposit the resulting sediment onto the beach below. Most of the beaches along the Palos Verdes Peninsula are composed of cobbles and boulders and not sand as is expected.

The reason for this is wave energy. It takes energy to move sediment. Typical waves are not strong enough to move sand-sized particles; this is why most beaches are comprised of sand. However, the incoming waves are more concentrated along headland areas such as the Palos Verdes Peninsula. The concentration of this energy is strong enough to move sand-sized particles, but not the heavier pebbles, cobbles and boulders.

Recall that it takes energy to move particles.  Some minerals have a greater specific gravity (heavier) than others, like magnetite, pyrite, galena, and garnets.  Two minerals can be the same size yet have different densities. 

• Larger particles    = more energy
• Heavier particles = more energy

In areas with high wave energy, grains that are small and/or light will be preferentially washed away, leaving behind grains that are either too large or too heavy to move.  Areas that have a lot of these "heavy minerals" in the rocks will have them incorporated into the beach sand as well. This is what is happening on the sandy part of the beach.   Most of the time, black patches can be seen in the sand. These are fine grained peices of a mineral called magnetite, weathered out from volcanic and plutonic rocks to the north.

Exposed in the cliffs is the Valmonte Diatomite. Diatomite is the rock form of silica oozes, which formed in deep ocean basins 13.5 - 7.5 million years ago.

Plankton such as diatoms and radiolaria belong to a group called microplankton - organisms too small to see with the naked eye so a microscope is needed. These plankton typically accumulate very slowly and become incorporated with the muds on the seafloor. Organisms with silica shells dissolve fairly slowly in seawater; organisms with calcium carbonate shells dissolve rapidly in seawater, especially below the Calcium Carbonate Compensation Depth (CCCD). Below this depth in the ocean, anything made of calcium carbonate dissolves very rapidly. Yet we can find vast deposits of each type of organism on the seafloor. How does this happen?

Some areas of the ocean's surface are high productivity areas. In these areas, there is a larger than normal concentration of these silica or calcium carbonate shelled organisms. Since there are more of them in these areas as opposed to other areas of the ocean, they accumulate in large deposits on the seafloor. when they die. Therefore, more of them get preserved than dissolved and thus become a part of the rock record.

Diatoms have a hard outer shell with most of the soft tissues inside. Thus, when they die, what's left is a hard shell with a hollow interior. This is what makes the diatomite so very light weight.