Geology (cont. 7)


Sea level rose and fell often during this period in geologic history, and more mud was deposited again, similar to but younger than the Del Mar Formation. Known as the Ardath Formation, it can be seen south and north of Beacon’s Beach in Leucadia. About 42 million years ago, sea level rose again and more sand was deposited. Compressed and cemented, it makes the rock known as the Scripps Formation. (Some geologists are calling this the Santiago Formation, but let’s stick with the better-known name, Scripps.) This rock holds up the sea bluffs north of Moonlight Beach. Because the mudstone and siltstone of the Ardath Formation are softer and weaker layers, waves undermine the stronger sandstone above it, as happens with the Del Mar and Torrey Formations. Dangerous rockfalls are the result, so stay away from the bluffs on the beach. There is a small outcrop of the Scripps sandstone near the western part of the trail at the lagoon. It is highly weathered and quite soft here. It erodes very easily, resulting in small rockfalls.


Around 40 million years ago, chunks of the Sonoran volcanic rock, the purplish rhyolite, along with pieces of the andesite and granitic rocks were being carried to the west in quickly-moving streams coming out of the high mountains in the east. The streams tumbled the rocks with their very high velocity, and the rocks became smooth cobbles and pebbles. They were deposited, when the rivers lost their velocity on flat ground, in a very large alluvial fan along the coast, on top of the Torrey and Scripps sandstones. They were cemented together by minerals in the water together with sands and muds of the shore. This is called the Poway or Stadium conglomerate.


Page 7

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