BOLTS of SNOT--Cape Town EarthCache

This earthcache in Cape Town will require you to make a field geologist's sketch of a prominent geological feature. You will need a pen/pencil, a piece of paper, and a cellphone with a camera or access to a scanner in order to submit the answers.

This is one of two BOLTS of SNOT earthcaches I have placed in South Africa--the other is BOLTS of SNOT--Hermanus. While the BOLTS of SNOT are common to each, the type of geology—and therefore the Terminology and Geology sections—differs in each case. If you complete both of the BOLTS of SNOT earthcaches you will have learned about two very different geological features and will have used your field geologist's skills to record your knowledge. The difficult for this is set to 4 because of the work involved in completing the BOLTS of SNOT portion of the task.

The coordinates for the geocache will require you to park nearby and walk along the beach and onto the rocks. Please be aware of the tides and plan your trip accordingly. The waves here can be treacherous, and the rocks can be slippery. At GZ you should be standing on a large rock roughly three feet thick and six feet by six feet square. It is a prominent formation in comparison to the low and well-eroded sandstone around it. All the information you need to gather is beneath your feet. At stage two you should be standing on a smaller criss-crossed rock. Again, all the information you need to gather is beneath your feet.

There are three sections to this earthcache, each of which you will needs to review in order to submit your answers. While you can easily do this on-site, you will likely be better-served by reading through the sections before you arrive at GZ. This will help you complete the earthcache a bit more quickly. Once on-site you will need to have the list of terms in Section II handy. Finally, before setting out you should skip down to the "Requirements for Logging This Cache" section to get a better understanding of the final tasks.

The sections:

I. Explanation of BOLTS of SNOT
II. Terminology
III. Geology of Cape Town and the Cape Town Peninsula

Equipment You will need the following equipment in order to complete this Earthcache:

• GPS unit;
• Pencil or pen;
• Paper;
• Camera-equipped cellphone, or a scanner.

Section I—BOLTS of SNOT

Earth science is a field of study that often requires scientists to record observations in the field that they can then take back to the laboratory. For hundreds of years field scientists used field sketches and diagrams to record their observations for later study. Nowadays, of course, people can use cameras to record images with some precision. However, a field sketch is still a handy tool, especially for students and Earthcachers.

In the earth sciences, these field sketches are detail renditions of an area of study. Usually a simple line drawing of a geological feature made with pencil or pen on paper, the sketch provides as much detail as possible. So the sketcher does not need to be an artist, simply thorough.

From Learning Space:

Drawing a sketch is one of the best ways to start [studying a feature], as it forces you to notice many aspects of the exposure. It also helps you to build up a picture of which aspects are significant and which are incidental or even irrelevant to a geological study. The aim of a field sketch is that it provides a record of your observations [. . .]

• Spend a minute or so looking carefully at the rock face to begin to recognise some general details.
• Now it is time to start committing some of your observations to paper! Draw in the skyline and the base of the exposure. This marks out the area that you have to concentrate on.
• Sketch in the main features using simple lines wherever possible. If necessary, rub out and redraw particular areas until you're happy with them. Part of the skill of making successful geological observations of complex exposures is to follow any key feature (the top or bottom of a sedimentary bed, for instance), as far as you can with your eye until it disappears from view.
• Similarly, the skill of making an effective geological sketch is to draw a discrete, continuous line (where the feature is continuous), rather than to sketch a vague series of unconnected lines. Ignore shadows from sunlight and features such as loose boulders, fallen branches, etc. You can indicate any patches of vegetation using your own simple symbols for grass, trees, etc.
• Label the features you have drawn.

There is also a pretty good PDF here that details how to draw a field sketch.

A sketch is not enough. The details of the feature need to be categorized and labeled for later study. The technique for labeling goes by different names, the two most common of which are BOLTS and SNOT, and they are an important part of life in the field for earth scientists. The two acronyms stand for:

• Border: This is the frame that surrounds a diagram or map
• Orientation: A marker that shows the cardinal direction of North
• Legend: A key, usually in the corner of the diagram, showing what the symbols in the diagram mean
• Title: The title of the diagram, as well as notes naming each of the features on the diagram
• Scale: A map or a diagram is of course smaller than what it represents. A notation on the map might show the ratio of inches to feet, or there might be a drawing of a person, showing him/her in relation to the features in the sketch
  
  and
• 
  
• Scale: As above
• Notes: Labels on each of the features
• Orientation: As above
• Title: As above

As you can see, they are similar, with BOLTS asking for a border and legend, and SNOT asking for notes. It should be noted that a BOLTS/SNOT diagram contains these elements, but is not limited to these elements. That is, a field diagram should be a faithful, detailed, and accurate rendering of the object of study. It should contain as much detail as is needed for someone to understand the area being studied. So it is probably accurate to think of a field sketch as a drawing or sketch of an area which is then labeled using the BOLTS/SNOT convention.

For the logging requirements of this cache, you will combine the two types, using all of the different labels.

Section II—Terminology

As with any profession, earth scientists make use of a particular vocabulary to communicate with each other and with the public at large. What follows are some definitions that you might find useful in completing the requirements for this Earthcache. All of these terms can be applied to some parts of the outcropping between its easternmost and western most sections. The definitions are taken from the Dictionary of Geological Terms, except where indicated by an asterisks(*), in which case they are my own.

1. Angular unconformity: An unconformity in which the bedding planes of the rocks above and below are not parallel.
2. Bedding: A characteristic of sedimentary rocks in which parallel planar surfaces separating different grain sizes or compositions indicate successive depositional surfaces that existed at the time of sedimentation.
3. Calcite: A carbonate mineral that forms deposits in sedimentary rock. Crystalline in appearance.(*)
4. Chemical weathering: The total set of all chemical reactions that act on rock exposed to water and atmosphere and so change it minerals to stable forms.
5. Chert: A sedimentary form of amorphous or extremely fine-grained silica, found in concretions and beds.
6. Cross-Bedding: Layers of rock that sit at non-parallel angles, in which layers intersect, or cross, with other layers. These intersections are the result of deposition, and not subsequent geological upheaval.(*)
7. Datum plane: An artificially established, well surveyed horizontal plane against which elevations, depths, tides, etc. are measured (for example mean sea-level, or a road).
8. Deposition: A general term for the accumulation of sediments by either physical or chemical sedimentation.
9. Elevation: The vertical height of one point on the Earth above a given datum plane, usually sea level.
10. Erosion: The wearing-away of rock and soil by natural processes and elements such as wind and water.(*)
11. Facies: Distinctive rock units that form during sedimentation, and which are usually distinguished by vertical layering.(*)
12. Joint: A large and relatively planar fracture in a rock across which there is no relative displacement of the two sides.
13. Lineation: Any linear arrangement of features found in a rock.
14. Outcrop: And exposed section of bedrock or underlying geologic formation.(*)
15. Sandstone: A clastic sedimentary rock in which the particles are dominantly of sand size, from 0.062 mm to 2 mm in diameter.
16. Sedimentary Rock: A rock formed by the accumulation and cementation of mineral grains transported by wind, water, or ice to the site of deposition or chemically precipitated at the depositional site.
17. Sedimentary Structure: Any structure of a sedimentary or weakly metamorphosed rock that was formed at the time of deposition; includes bedding, cross-bedding, graded bedding, ripples, scour marks, mud-cracks.
18. Shoreline: The line separating land and water. Fluctuates as water rises and falls.
19. Stratification: A structure of sedimentary rocks, which have recognizable parallel beds of considerable lateral extent.
20. Tensile Fracture: A fracture caused by tensional stress in a rock.
21. Vein: a line of crystalized mineral formed from the presence of water. In marine sandstone these usually form within cracks. geological activity causes the cracks to re-open, which in turn causes further depositon of aqueous minerals.(*)

Section III—Geology of the Cape Town Region

Cape Town sits in the Western Cape of South Africa, along what is called the Cape Fold Belt. Most of the prominent rock formations are composed of either sedimentary sandstone, metamorphic shale, or igneous material.

Geologically, the area is composed of the the Cambrian-Carboniferous Table Mountain Supergroup (mostly sandstones), Cambrian Cape Granite, and the Precambrian Malmesbury Group (dark mudstone and light sandstone). The area is geologically very old, with the rocks having formed hundreds of millions of years ago. From the Department of Geological Sciences at the University of Cape Town:

The late-Precambrian age Malmesbury Group is the oldest rock formation in the area, consisting of alternating layers of dark grey fine-grained greywacke sandstone and slate, seen along the rocky Sea Point and Bloubergstrand shorelines. These sediments were originally deposited on an ancient continental slope by submarine slumping and turbidity currents. The sequence was subsequently metamorphosed by heat and pressure and folded tightly in a NW direction so that the rock layers are now almost vertical. Many tall buildings in the Cape Town CBD are founded on these rocks which were in most places, scoured by wave action during past periods of higher sea level.

The Peninsula Granite is a huge batholith that was intruded into the Malmesbury Group about 630 million years ago as molten rock (magma) and crystallized deep in the earth, but has since then been exposed by prolonged erosion. The characteristic spheroidal shapes of granite boulders are a result of preferential weathering along intersecting fractures and are well displayed around Llandudno and Simonstown. Close up, the granite is a coarse-grained rock consisting of large (2-5cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica containing inclusions (xenoliths) of dark Malmesbury hornfels. In some places, intense weathering has altered the granite to kaolin clay soils that cause slope stability problems in road cuttings. High quality kaolin is mined near Fish Hoek and Noordhoek.

The contact zone where the Malmesbury Group was intruded by molten granite can be seen at Sea Point and was made famous by Charles Darwin during his voyage of scientific discovery on H.M.S. Beagle in 1844. Here, slivers of dark coloured Malmesbury rocks, altered by intense heat are intermingled and folded with the pale coloured intrusive granite to form a complex mixed rock (migmatite) . Large feldspar crystals occur in both the granite and dark hornfels layers

Though initially intruded at great depth, prolonged erosion eventually exposed the granite at surface and it now forms a basement upon which younger sedimentary rocks of the Table Mountain Group were deposited.

Table Mountain Group sandstones were deposited on this eroded surface of granite basement, in the stream channels and tidal flats of a coastal plain and delta environment that extended across the region about 450 million years ago. The spectacular Chapman's Peak roadway has been constructed along the contact unconformity between granite and the overlying Table Mountain sequence. The sand, silt and mud deposits were lithified by pressure and then folded in the Cape Fold Belt, extending along the southern coast.

The sandstone at GZ is a sedimentary rock which was formed by the layering of various materials on the surface of the earth or on the ocean floor. The materials can be organic or mineral, and the process of formation means that sedimentary rocks cover most of the earth's surface, but are underlayered by igneous and metamorphic rock. There are four types of sedimentary rocks: clastic, biochemical, chemical, and a broad category of "other" types. The category of clastic rock includes sandstone, which is common to South Africa. Sandstone is formed from the accumulation of small grains, most commonly quartz and feldspar. Sandstone can be any color, but typically follows the colors of sand found on beaches--white, grey, tan, brown, pink, or black. Marine sandstone is the type of sandstone most commonly found along the southern coast of South Africa.

Sandstone forms in two stages. First, sand accumulates through sedimentation, as sand settles to the bottom of rivers, lakes, the ocean, or falls to the ground, typically in deserts. In the second stage, as deposits accumulate, the underlying layers are compacted and cemented together by the minerals in the sand. Sandstone therefore takes on the grainy texture, and while it can be used in building materials, it is gritty and easy to carve. The process of formation also means that the face of sandstone formations can have a layered appearance, oftentimes with small waves or ripples, or even mineral deposits but also, just as often, as stacked chunky layers.

The appearance of sandstone can be altered by a number of factors, most commonly (for the purposes of this earthcache) erosion and fracturing.

Requirements for logging this cache. Read this carefully before beginning!

1. Create two detailed field sketches, as described in Section I above, of the rock at GZ and the one at Stage 2. 
   • The sketch at GZ should cover one of the four visible sides (the bottom and the side closest to shore are not easily visible) of the rock slab formation and must be clearly marked with natural boundaries that I and other visitors will easily recognize.
   • The sketch at Stage 2 should cover the top of the slab of rock and must be clearly marked with natural boundaries that I and other visitors will easily recognize.
2. The field sketch must be annotated with a BOLTS/SNOT diagram, again as described in Section I above. Your diagram must contain all of the components of BOLTS/SNOT, using the following guidelines:
   • Each field sketch should have at least five distinct features of the section using terminology from Section II above, and drawing on the knowledge gained from Section III.
   • Your sketch should indicate cardinal directions, as well as the rough cardinal directions of the intrusions and/or weathering.
   • The five features on each should be labeled with numbers, not with the terms.
   • The drawings should indicate the scale.
   • You must clearly sign the BOLTS/SNOT sketches with your geocaching name across some part of the drawings, without rendering the drawings unreadable.
3. Use your phone to take a photo of your diagram, or scan it using a scanner. Upload the file with your log. The photo/scan of your BOLTS/SNOT diagrams must accompany your log. Logs not accompanied by a picture or scan of the two BOLTS/SNOTs will be deleted.
4. In an separate e-mail, answer the following:
   • List what each of the numbers represents on your diagram.
   • Compare the quartz feldspar intrusions at GZ and Stage 2. Describe the differences in size at GZ and Stage 2, and speculate, based on the information provided in this description, for the differences.

A final note. First, please practice Cache In, Trash Out wherever you go. You don't have to haul out a Hefty 50-gallon trash bag for every cache you find, but please be conscientious about the areas in which you cache. It doesn't take a whole lot of effort to leave a caching area bit nicer than you found it—taking away one piece of trash will do it. That makes it better not just for the next cacher, but also for the muggles. To mangle a phrase: Take Out Some Trash, Leave Only a Signed Log.