Lesson adapted with full permission from fastflyer's Crystal Cove - Geology -> Biology.
What’s A Geologic Column?
A geologic column is a diagram that shows the subdivisions of geological time in a particular area. For this EarthCache, we are concerned with the geologic column for Southern California, and with the Vaqueros Formation and the San Onofre Breccia within this column, in particular.
Do you see the abrupt vegetation changes on the hillsides around you at this location? Just a little ways away in the Angeles National Forest, the ground is covered in native sage scrub. Yet here, is looks much more barren and smooth. The brown grass here makes the hills look like velvet. In many places you see boundaries between coastal sage scrub (commonly on the Vaqueros Formation) and brown grass (commonly on San Onofre Breccia). Since rocks differ in their mineral content, porosity, and resistance to erosion, biological species with varying needs live on different rocks.
The Vaqueros Formation
Beginning 25 million years ago, sediments (sand, silt, clay, and skeletons) slowly accumulated in shallow Pacific Ocean waters and formed the geologic layer known today as the Vaqueros Formation, the oldest exposed rock unit in the park.
The Vaqueros Formation is a sedimentary geologic unit, which is widespread on the California coast and coastal ranges in approximately the southern half of the state. It is predominantly a medium-grained sandstone unit. The sandstone unit consists of well-sorted grains, averaging medium-size, typically quartz and feldspar with some black flecks, and in form it ranges from cross-bedded to massive and thick-bedded. Occasionally it contains pebbles, especially near its base where it sits on the red non-marine Sespe Formation. Some fossils – including mollusks and barnacles – can be found in the Vaqueros, also near the base of the unit where the depositional environment was nearest shore.
Being resistant to erosion, it forms dramatic outcrops in the coastal mountains. Its color ranges from grayish-green to light gray when freshly broken, and it weathers to a light brown or buff color as it becomes a clayey soil which supports chaparral.
The San Onofre Breccia
Between 15 and 12 million years ago, sediments forming the San Onofre Breccia were deposited from islands offshore as California was torn apart by tectonic forces. Magma (molten rock) worked its way up through joints and faults, forming andesite and diabase dikes (igneous intrusions cutting across the rock fabric) and flows.
Breccia (pronounced brech-ee-uh) is a sedimentary rock composed of a mixture of broken angular fragments (clasts) of mineral or rock embedded in a fine-grained matrix (the “glue” that binds the clasts together). The fragments may range in size from fractions of an inch to bus-sized blocks. They may consist of a single rock type or of many types, as is the case with the San Onofre Breccia formation.
Logging this Earthcache:
Send a note with the following:
(1) The name of this earthcache
(2) The number of people in your group
(3) Look at the hills right above you (north). Is the land in your field of view predominantly part of The Vaqueros Formation or San Onofre Breccia? Why do you think that?
Enjoy the Earthcache!
Sources:
--http://www.geocaching.com/geocache/GC543MZ_crystal-cove-geology-biology?guid=aba0581f-513c-4b47-9122-ad5b1a968898
--http://www.crystalcovestatepark.com/geologyguide112309small.pdf