Remember there is no container to find here! Think of this Earthcache as more of a geology field trip. To claim credit for this cache you will observe some rocks near the posted coordinates and then answer some questions. Observing the rocks will enable you to answer the questions listed in the “Logging Requirements” section below.
Cautions:
- Some of the cliffs are cordoned off for safety and to prevent accelerated erosion caused by foot traffic. Do not cross into the areas which are cordoned off.
- Some of the conglomerate is undercut and weakly adhered. There is no need to walk on the conglomerates. For safety you should stay away from steep portions of the conglomerate layers.
- Some portions of the sandstone are steep. The sandstone is decomposing which means loose sand and poor footing. For safety you should stay away from the steep sandstone faces.
- Some combinations of high tides or high surf may create hazardous conditions at or near the Earthcache coordinates. Use good judgment and do not proceed if there is any danger of being caught by a wave!
Point Lobos State Natural Reserve:
- All features are protected – do not collect rocks, plants, sea life, etc.
- Stay on designated trails
- Bicycles allowed on paved roadways only
- Do not disturb wildlife
- No dogs allowed – not even if confined to your motor vehicle
- Fee area - $10 per motor vehicle, senior discount for age 62 and up - $9 per motor vehicle.
Logging Requirements:
- Find the sandstone layers where embedded “concretions” have weathered out. The photo above shows a portion of the weathered sandstone and should help you locate the right area. Once you find the weathered sandstone then estimate the thickness (from the conglomerate layer on one side to the conglomerate layer on the other side). Also describe the inclination of the sandstone layers where they dip below the upper layer of conglomerate as well as the compass direction of the downward inclination (which direction are you facing if looking down the sandstone face?).
- Use the “Send Message” feature in my profile to send your findings. Be sure to mention that the message is about “GC4NJ55 – The Carmelo Formation Earthcache.”
- Go ahead and post your found log online – please do not post your answers to the logging requirement in your online log. I will contact you only if your answers are not clear.
- Feel free to post any snapshots of your activities at the Earthcache site, so long as the posted snapshots do not reveal the answers to the logging requirement questions.
The Big Picture
A general understanding of “plate tectonics” is helpful in understanding the geology observed at Point Lobos State Natural Reserve. The Earth’s crust is comprised of a number of sections which are called “plates.” The crust below oceans is generally denser – more weight per unit of volume – than the crust carrying the continents. So where oceanic crust and continental crust are pushed together the continental crust will tend to ride up over the oceanic crust. This bends the oceanic crust downward under the relatively lightweight continental crust. This circumstance is seen all along the western coast of North America where the crust underlying the eastern Pacific dives below the continent. When the oceanic crust is pushed deep enough it melts. Some of the melted material rises to form volcanos. Mount Shasta in northern California is an example of this type of volcano. Melted material which stops before reaching the surface may cool forming a “pluton.” The oldest formation visible at Point Lobos is such a pluton. The cooled magma formed a type of rock called “granodiorite.”
Now we must consider a complication to this simple model. The oceanic crust does not get pushed perpendicular to the west coast of the North American continent. The motion of the oceanic plate has a northward component compared to the continent. The result is a bunch of slivers and crumbly bits squeezed northward along the margin of the continent. Perhaps the best known evidence of this is the San Andreas fault. The “Cretaceous Granodiorite” underlying much of Point Lobos can be thought of as one of the crumbly bits. Not only has it been moving northward all this time, but has also migrated to the surface where we can see it exposed around Point Lobos. The Carmelo Formation occupies a former submarine canyon which cut into the “Cretaceous Granodiorite.” The formation of these submarine canyons is not well understood, but similar submarine canyons are present nearby. The image below shows a rendering of the Monterey Canyon in Monterey Bay. It is over a mile in vertical extent and is about two hundred and ninety miles long. This rivals the dimensions of the Grand Canyon in Arizona, but is all below the ocean’s surface.
Sandstone and Friends:
There are two types of rock which predominate near the Earthcache coordinates. Both are the result of material deposited in the ancient canyon. One is sandstone and here includes some small-scale geological curiosities known as “concretions” – more on those in another Earthcache! Sandstone is a type of sedimentary rock. It is formed from sand – just as one might guess from the name. Over time the individual grains of sand are gradually bound to one another as water dissolves minerals and then precipitates those minerals among the grains of sand.
The other type of rock formed from deposits in the ancient canyon is conglomerate. The conglomerates practically explain themselves. They are formed of many hundreds of thousands of pebbles and cobbles (big pebbles) in a sandstone matrix. The pebbles and cobbles are more properly referred to as “clasts” which means bits of pre-existing rocks incorporated into a new composite rock. Because these clasts are rounded then may surmise transport from some distance away. The clasts in the conglomerate are primarily of volcanic origin.