The Outer Beach is an artificial beach - tourism in the late 1920's and early 1930 brought the need to make the beach look more like, well, a beach. So sand was imported and placed on both what would become the outer and inner beaches of Cabrillo. Coastal processes such as longshore transport are constantly stripping off more sand than is naturally replaced. So every seven years or so the beach sand must be replenished.
Located outside of the breakwater, the Outer Beach is exposed to the open ocean and thus stronger waves. The sand here is coarser than that in the inner beach, making the beach slightly steeper than the one inside the harbor. The sand is composed of mineral fragments = quartz, potassium feldspar, plagioclase feldspar, biotite and occasionally hornblende) derived from the mechanical weathering of rocks, most likely from the San Gabriel Mountains.
The Outer Beach at Cabrillo during low tide. Note the high tide shoreline.
Storms bring with them high winds and waves. During periods of stormy weather, these high-energy waves reach higher up onto the beach and strip smaller and lighter grains off of it, exposing larger particles like pebbles, cobbles and boulders that were buried beneath it. Sometimes two particles can be the same size, but have different densities. Minerals that have a higher density than others are called "heavy minerals", and include magnetite and garnet. The sand can often have a dusting of black magnetite during the winter as the lighter particles can be washed away while the heavier magnetite is left behind.
The storms do not even have to make landfall to affect the beach. Storms simply passing off of our coast can kick up high waves and swells, which will head toward the coast and do damage. Below is a photo of the Outer Beach looking east towards the breakwater, taken on Saturday, February 20, 2016 around 12 noon. Note how all of the sand visible in the photo above has been stripped away, exposing pebbles and cobbles.
The 2016-17 winter season brought lots of storms and rain to the Southern Californi Coast. Again, the lighter sediments were stripped off of the Outer Beach, leaving behind a lot of cobbles and boulders. Storm waves reached far enough inland to create a berm just south of the Cabrillo Beach House. Below is a photo of the berm and wave erosion observed on Saturday, April 8, 2017.