Welcome to Sandeel Bay, a cove located at the East coast of Hook Peninsula.
At this location, you can observe perfect examples of rocks in different layers, grain sizes, colors and beddings.
Access to the cove is via a small road off the R734 - watch for the brown fishing sign. Parking coordinates are placed just above the cove, where the road broadens and the road nearly ends.
Note - Access may be restricted at high tide - check tide times beforehand.
Please send answers in English, German or Spanish language.
Many different sizes, colors, shapes and textures of rocks exist. But all rock types can be sorted into three mayor categories, based on how they are created: igneous, sedimentary and metamorphic.
Igneous rocks form when melted rock cools down, underground from magma, above ground from lava.
Metamorphic rocks are formed by alteration under great heat and pressure from pre-existing rocks of any of the three types.
Sedimentary rocks are composed of pieces of other rocks. When any rock is exposed to the elements (e.g. wind, water, sunlight) it is eventually broken down into smaller particles. These are moved around the surface of the earth and settle somewhere, are glued together and become a sedimentary rock.
At this beach you will mainly find sedimentary rocks, therefore I will explain this kind of rock more detailed. You will learn how to identify, which kind of sedimentary rock you look at.
Classification of sedimentary rock:
Clastic sedimentary rocks
are subdivided according to the particle size. Grain size varies from
Very Fine (less than 1/256mm): clay in shales and claystones
Fine (1/16 to 1/256mm): silt in siltstones
Medium (1/16 to 2mm): sand in sandstones
Coarse (over 2mm): gravel, cobble, to boulder sized fragments in conglomerates (rounded fragments) and breccias (angular fragments).
Biochemical sedimentary rocks
are created when organism use material dissolved in air or water to build their tissue. E.g. most types of limestone are formed from corals or mollusks, coal formed from plants that have removed carbon from the atmosphere and combined it with other elements.
Chemical sedimentary rocks
form when mineral constituents in solutions become supersaturated and inorganically precipitate, e.g. oolitic limestone or rock salt (halite).
“Other” sedimentary rocks
include rocks formed by pyroclastic flowed, impact or volcanic breccias.
Main colors of sedimentary rocks:
If the rock contains iron(II) oxide, which only forms under low oxygen circumstances, the rock will get a grey or greenish color.
Iron(III) oxide in a richer iron environment is often found in the mineral hematite, which gives the rock a red or brown color.
In arid continental climate the rocks are in direct contact to the atmosphere. Oxidation gives the rock a red or orange color.
Organic material can color the rock black or grey, because it is formed from dead organism, mostly plants.
Fossils:
Almost all fossils one can find in sedimentary rock, because the temperature is low enough not to destroy them.
Sedimentary structures:
Sedimentary rocks are laid down in layers called beds. A bed is a layer of rocks with a uniform lithology and texture. Beds form by deposition of layers of sediments on top of each other. Single beds can be a couple of centimeters to several meters thick. Beds normally form in horizontally, but sometimes multiple sets of layers with different orientations exist in the same rock. This structure is called cross-bedding.
Graded bedding is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts.
Your tasks:
The cave
You will find a really small cave at the given coordinates. Please climb into the cave (WATCH YOUR HEAD!) and use your flashlight. When inside the cave, observe your surrounding from ground to ceiling, from left to right. Please describe the main sedimentary rock, that you find in this cave (Type of rock, grain size, color of rock, fossils, etc.). Which kind of rock from the list above do you find inside the cave?
The wall
About 30m to the right of the coordinates, you find a natural wall. The wall consists of multiple layers of sedimentary rocks. Please observe the layers and describe each layer by grain size and color. Do you find the rock from the cave here, too? Which of the layers is it?
OPTIONAL:
Take a picture of the rock with you and/or your GPS and post it with your log. Pictures are not required but are appreciated.
Send your answers through my profile at geocaching.com (through e-mail or message center) and NOT in a log. You don't have to wait for me to approve your answers. Please also add any cacher with cachername, who was in your group into your email. That makes life easier for me.
Once you send your answers feel free to log this EarthCache as found. I will contact you, if there are problems with your answers.
Sources:
- Geology for Dummies, Alecia M. Spooner, Geology for Dummies, Hoboken: Wiley Publishing, Inc., 2011
- Wikipedia.org
- Misc. webpages about geology