The Palos Verdes Peninsula is an uplifted exposure of the Miocene Monterey Formation, a rock formation composed of sediments that were deposited between 17.5 and 6 million years ago in the shallow ocean off the coast of California. The sediments are distinctly diatomaceous and rich in organic matter. Diatoms are microscopic organisms that build shells of silica, and the shells dissolve and precipitate as silica cementafter burial to deeper and hotter depths below the surface of the Earth. In this way, soft diatomaceous sediments transform to much harder porcelanite and chert over time. The brittle cherts can deform and shatter under tectonic deformation and form excellent petroleum reservoirs. This formation is very important because it provides a detailed record of past oceanographic change and is the source rock for much of the oil and gas in California. As you wander around coastal California, you can often see outcrops of the Monterey Formation. The Monterey Formation is usually a distinctly light color with well-developed bedding, but at the beach the rocks may be varied and colorful.
Most of the material stretching northwards from the bottom of the path that winds down to Malaga Cove is Valmonte diatomite or Malaga mudstone. Geologists have determined that during the Miocene, when the Malaga mudstone was laid down, the oceans were about sixteen degrees warmer than they are today.
The Valmonte diatomite here is exceptional in that in several places you can find extremely clean boulder and rubble piles of it. Diatomite is an extremely light rock, very easy to lift. This beach is a great place to grab a camera and take pictures of yourself looking strong as Superman!
Towards the northern end of Malaga Cove lies the Repetto Siltstone and the San Pedro Sand, the youngest of the rock units in the Palos Verdes area. When viewed from a distance, the sea cliff is made up of a series of breathtaking anticlines and synclines. Anticlines are folds of rock humped up like an "A", while synclines are rocks folded in the shape of "U"'s. Unfortunately, beach erosion and vegetation are covering up much of the cliff, obscuring the large-scale geologic structure.
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. There, interbedded in the Repetto Siltstone, is a conglomerate bed located in the cliffs directly behind the beach with the larger clasts, and it is missing from the cliffs just a bit to the north. 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 conglomerates 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. There are two reasons for this.
- When a conglomerate bed weathers, the sand breaks down and erodes away first, leaving behind the more resistant cobbles and pebbles. These cobbles and pebbles are predominantly what are deposited on the beach below.
- 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.
On Malaga beach itself lie diffuse bands of black sand, which is made of the minerals magnetite and ilmenite. Use a magnet and push it into the sand, and it will come back up more whiskered with thick "hairs". These magnetic minerals basically have been washed out of the weathered volcanic and plutonic rocks found in the mountains to the north.
From the overlook at Malaga Cove we can look north toward Santa Monica Bay. Redondo and Hermosa Beaches are just to the north. Get out and walk to the pavilion and overlook. Enjoy the view, and then proceed down the road to the beach.
Leave Malaga Cove and continue on Paseo del Mar back to Palos Verdes Drive. Turn left on Hawthorne Blvd and drive up the hill to Crest Road. Turn right on Crest Road and continue to Crenshaw Blvd. Turn right on Crenshaw Blvd and follow Crenshaw beyond the stop sign at Crest Rd. Turn Right on Park Place and park across from Del Cerro Park.