While geocaching in the area, I noticed something peculiar of the rocks just below the waves in Ocean Beach. I noticed they were well compacted, unlike what lies below the waves on the north side of the Ocean Beach Pier (loose sand). On the south side of the pier, the rocks begin to transition from Quaternary in age to Cretaceous. Quaternary rocks were formed in the last 2.5 million years and are some of the youngest rocks on earth. These quaternary age sediments refers to the beach sand which is loose and mostly uncompacted. As you go south towards Point Loma, there's a transition between the loose sand and the compacted sandstone. The sandstone which makes up the cliffs of Point Loma transitions at roughly the Ocean Beach pier and are of Cretaceous in age (146-66 million years ago), meaning they were forming around near the last days of the dinosaurs.
The sandstone rock found here is part of the Point Loma Formation, which formed around 76 to 72 million years ago. Since then, the rock has uplifted due to tectonic forces along the coast of California and has formed immense cliffs. Because of this uplift, the rock has fractured in some areas and has formed what we call joints. Joints are fractures in rock due to some sort of stress. Stress can occur in 3 different orientations (X, Y, and Z). The largest value of stress in a given orientation (X, Y or Z) is denoted as Sigma-1. Sigma-1 always has the most amount of influence on a rock, as it is always expressing the highest value of stress. Wherever the Sigma-1 direction is, the rock (or material) will tend to fracture perpendicular to this direction.
The sandstone here is fracturing due to the uplift caused by local faults, which has been uplifting Point Loma for millions of years. As the rigid rock is uplifted, it begins to crack (fracture) in specific angles based on the orientation of Sigma-1 which it itself is based on the regional tectonic forces uplifting the land. These cracks tend to be microscopic as the rock is uplifting from being burried, however, once the rock is exposed at the surface these cracks accumilate and grow due to weathering. This process makes these microscopic cracks into noticable jonts at the surface, which are orientated based on the principle stresses as outlined above.
Works Cited:
https://www.nps.gov/cabr/learn/nature/geology.htm
https://structuralgeo.wordpress.com/2014/06/18/jointing-and-mechanical-layering/
http://rogermarjoribanks.info/sense-movement-structures-part-1-theory/
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Ocean Beach Joints" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with the joints in the background. The jonts are located along the beach and they are best viewed from above along the pier (suggested waypoint), although they can be viewed from anywhere near the posted coordinates. Note, they may be slighly below the waterline depending on the tides.
3. Describe the colors, textures, and grain compaction of the sandstone. Is it poorly compacted, well compacted, or somewhere in the middle? Note, I am not asking about the beach sand!
4. Describe the regular spacing between joints AND give an estimate of the average distance between joints.
5. Estimate the compass direction (0-360 degrees) that these joints are orientated by.
6. Assume sigma-1 is orientated horizontally and in the same plane as the joints. Based on your answer to question 5, estimate the sigma-1 compass direction (0-360 degrees). Hint: Read the second to last paragraph