Younger Lagoon is part of a system of marine terraces along the California coastline that were formed approximately 100,000 years ago as a result of the uplifting of land caused by plate tectonics, as well as climatic changes facilitating a lowering of sea level.
Over time the waves breaking on the beach have pushed sand up into the channel, forming a barrier between the fresh water of the lagoon and the salt water of the ocean for much of the year. This barrier is known as a berm, which consists of an underlying layer of bedrock and a sand layer on top. The rock shelf usually inhibits the exchange of salt and fresh water, however there may be some seepage of seawater through the sandy layer.
Marine terraces are ancient shorelines consisting of a relatively flat wave-cut platform (rocks in the surf zone that have been eroded by waves). This photo was taken at the cliffs near Long Marine Lab.
Over one night in February, 1992, a storm collapsed the rock arches on the beach at Younger Lagoon and washed away the remains without a trace.
Sea arches and caves are characteristic features of Santa Cruz County coastal geology.