Caldecott “Orinda Formation”Weathering Earthcac
In California, United States
Size:  (not chosen)
How Geocaching Works
Use of geocaching.com services is subject to the terms and conditions in our disclaimer
As an earthcache, there is no “box” or “container” to discover. Rather, with this cache, you discover something about the geology of the area. For more info, consult www.earthcache.org.
This is handicap accessible (able to be done from the comfort of your car!)
Send the answers to #1-#3 to me through my geocaching profile. DO NOT post the answers to any logging requirements on this site.
1. List the name “GC24Y5N Caldecott “Orinda Formation” Weathering Earthcache” in the first line of your email. Also, list the number of people in your group.
2. Identify 3 things/forces that are eroding the cliff in front of you.
3. Identify whether A) primarily mechanical B) primarily chemical C) a good mixture of both
4. Post a picture of yourself and/or your GPS with your log that shows the cliff behind you OR if you’re really against a picture, tell me which cardinal point direction the rock exposure is FACING and its PRIMARY COLOR.
I will only respond if you have incomplete logging requirements. Go ahead and log your cache
This is one of the numerous Earthcaches available on the roads above/around the Caldecott Tunnel. There is a wide pullout on Skyline Blvd., big enough for several cars across the road from the cliff in question.
In this earthcache, you are examining the rock, trying to determine the REASONS for weathering that is actively shaping the cliff in front of you.
Weathering is the decomposion of Earth’s rocks, soils, and minerals through direct contact with the atmosphere. Weather occurs without movement, meaning that such things as erosion are NOT considered to be weathering, though erosion may cause the environment for weathering (see below).
There are two types of weathering, MECHANICAL(PHYSICAL) and CHEMICAL.
Mechanical weathering – involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice, and pressure. Mechanical weathering is the cause of the disintergration of rocks. The primary process in mechanical weathering is abrasion – where the rock size is broken down into smaller particles. The rock components themselves are not altered.
Chemical weathering – involves the direct effect of atmospheric chemicals or biologically produced chemicals in the breakdown of rocks, soils, and minerals. The composition of the rock components changes in chemical weathering.
The materials left over after the rock breaks down combined with organic material creats soil. The mineral content of soil is determined by its parent soil sources. Thus, a soil derived from a single rock type can often be d efficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types often makes more fertile soil.
Types of Mechanical Weathering:
Thermal expansion –
This type of mechanical weathering happens when there is a great CHANGE in temperature on a cyclical basis. Examples include the desert where the rocks are baked in the daytime sun, then evening temperatures plummet. The stress of the temperature change causes the peeling off of the outer layers of rocks in thin sheets. Thermal expansion is enhanced by moisture.
Frost disintegration –
This process can also be called frost shattering, frost-wedging or freeze-thaw weathering. It often happens in environments with a lot of moisture and where temperatures often fluctuate above and below freezing temperature. When the water freezes, it crystallizes and expands, producing stress on the rock. Rocks that are porous are especially susceptible to this type of weathering.
Hydraulic action --
This is when water (generally from powerful waves) rushes into cracks in the rockface rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rockface and widens the crack itself.
Salt-crystal growth --
Causes disintegration of rocks when saline (see salinity) solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock. It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts.
Biological Weathering --
Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration.
Types of Chemical Weathering:
Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In the case of acid rain, the acidicness of rain is 2-3x normal, with resulting ph levels being as low as 3.0 (a moderate level acid) compared to normal rainfall at ph 5.6.
Carbonation – This takes place when rain combines with carbon dioxide. It has an effect on rocks such as limestone or chalk. This takes place because the rainfall produced is a weak carbonic acid which reacts with calcium carbonate.
Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.
Biological-Chemical Weathering --
A number of plants and animals may create chemical weathering through release of acidic compounds, i.e moss on roofs is classed as weathering.
The most common form of biological weathering is the release of chelating compounds, i.e acids, by plants so as to break down aluminium and iron containing compounds in the soils beneath them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.
Alt & Hyndman, Roadside Geology of Northern andCentral California. Mountain Press Publishing. Missoula: Montana. 2000.
(No hints available.)
Last Updated: on 2/25/2014 11:17:10 AM (UTC-08:00) Pacific Time (US & Canada) (7:17 PM GMT)
Coordinates are in the WGS84 datum