EC101: Imagine that! EarthCache

This earthcache was especially developed for those cachers who always say “Earthcaches are not really our thing”. If you don’t like earthcaches because you think they are “too complicated” or “make you feel like being back at school”, give this one a go. It was developed to give you an easy and hopefully interesting introduction to earthcaching and should be all about your experience in the field. Don't focus so much on the questions - to enjoy an earthcache you need to think about how the information in the listing applies to the environment around GZ and use your imagination!

OVERVIEW
This is the FIRST of three earthcaches in the area that introduce geocachers to some of the most fundamental concepts of geology – “Earthcaching 101” if you would. With this particular cache we will endeavour to show you that earthcaching is not “just about rocks”, but that it also helps you to appreciate and understand the LANDSCAPES around you, teaches you about the GEOLOGICAL EVENTS & PHENOMENA that are shaping our world and allows you to TIME TRAVEL.

LANDSCAPES
Earthcaches will engage you with the landscapes around you - they will help you understand why the world around you looks the way it does. You should especially appreciate this fact with this particular cache as you’ll drive to the cache along one of the most scenic roads in South Africa; with a towering mountain range on your one side and a rugged coastline on the other. Closer examination of this landscape will reveal that it consists of various defined geological elements (see annotated photograph, taken from GZ, below), each able to tell you something about the appearance, history and/or current geology related events in the area.



Sandstone cliffs near the mountaintops:
The imposing cliffs above you consist of rocks that were formed from sandy sediments, deposited ~500-290 million years ago (MYA) in a coastal delta associated with the Agulhas Sea. Today these sedimentary rocks dominate the mountainous landscapes of the South-Western Cape from the Cederberg Mountains (Clanwilliam) in the west to the Zuurberg Mountains (Port Elizabeth) in the east. The cliffs in this specific area consist of Table Mountain Group rocks (generally known as Table Mountain sandstone), the lower most and in other words oldest (500-410 MYA), layer of what is known as the Cape Supergroup sedimentary rocks.

So how did sandy deposits at the bottom of a sea end up as mountain ranges…? Approximately 80 million years after the Table Mountain sediments were deposited and lithified (~330 MYA) a subduction zone developed along the southern margin of the then supercontinent, Pangaea. This resulted in the northerly compression of the landmass which caused the thickening of the crust and the rise of the Cape Fold Mountains, where formerly the Agulhas Sea had existed. You will also notice that the sedimentary rock layers in these formations are often inclined quite dramatically from the horizontal. This was caused by the tectonic forces that bent (folds) or broke (faults) the sedimentary layers and in doing so displaced them from their original horizontal state. The visible inclinations in the cliffs to the south of GZ, highlighted by the yellow, dashed lines in the photograph above, are therefore a clear indication of the mountains’ forceful formation.

Finally, even though the Cape Fold Mountains are very old (~330 million years) by Andean (~220 million years) and Alpine (~280 million years) standards, they remain steep and rugged due to their quarzitic sandstone geology, which makes them very resistant to weathering.

Colluvium along the base of the mountains:
What we typically refer to as “the slope of a mountain” is in fact colluvium, i.e. unsorted sediment, originating from weathering, high-lying rocks, transported downward by gravity and deposited at the base of a cliff or slope. So, it’s basically mountain rocks in the process of becoming soil. You will therefore also note that this is the part of the mountain with the most vegetation on. It is a typical feature of the Cape Fold Mountains with their sandstone “caps”.

Also, because all granular matter, including loose earth materials like rocks, sand and soil, have a maximum angle or slope at which they remain stable, i.e. the angle of repose, these colluviums have fairly consistent slopes of between ~32 & 34^o.

Sporadic dark rocks along the shoreline:
GZ was chosen specifically to introduce you to the oldest rock formation you’ll encounter in the Cape Peninsula; the Malmesbury Group that underlays the Table Mountain sandstones. The group includes sandstone and even volcanic rocks, but the most recognisable exposures are those of the finely grained, blue-grey coloured mudstones visible at GZ. The muddy sediments that formed these rocks are thought to have been deposited ~550 MYA in an ancient ocean basin called Adamastor.

We know that the first Table Mountain sandstones sediments were deposited well after the Malmesbury sediments, because there is clear evidence of an unconformity between these layers. This means that there is a gap in the geological record between them – in this case the upper surface of the Malmesbury rocks shows clear evidence of erosion before the first sandstone layers were deposited.

Sandstone boulder beaches:
The majority of the rocky beach consists of smooth sandstone boulders that originate from the slopes above and which were mechanically weathered through wave action, which drives a number of specific erosional processes such as abrasion, attrition and scouring.

GEOLOGICAL EVENTS & PHENOMENA
The point of this section is to discuss a few geological events and phenomena, of which the evidence of is observable from GZ, to illustrate how your understanding of these will not only enhance your earthcaching experiences, but also your general travelling and/or hiking excursions in the area.

As already discussed above, from GZ you can clearly see the evidence of some major geological events that shaped the landscape around you. Tectonics, for example, was responsible for the formation of the Cape Fold Mountains and also to a large extent determines where the current coast line is. Weathering and erosion are responsible for shaping the mountain peaks, colluvium slopes, valleys and rocky beaches. Remember, weathering is the actual process of breaking-up a rock while erosion refers to the transportation of the resulting fragments or clasts.

On a much smaller basis you’d also be able to see evidence of honeycomb weathering at GZ, so called because the resulting weathered rock is covered with bowl-shaped cavities resembling a honeycomb (see photograph below). It can be found in all climate types, but is most abundant in intertidal zones and semi-arid and arid deserts in granular rocks such as sandstone, mudstone and granite. Salts crystallize within minute pores in the rock and in doing so pry apart the rock, leaving them vulnerable to other forms of weathering and erosion which slowly enlarge the cavities.



Sedimentation generated the sediment layers for both the Malmesbury and Table Mountain Groups and as these were buried deeper and deeper they lithified (became rock) over great periods of time and under extreme pressures and heat to form all the rocks you see around you. Under these extreme conditions some minerals in the rocks can actually be transformed to form new minerals - this is known as metamorphism and the newly formed rocks are called metamorphic rocks. In this way quatzitic sandstone, a course, brownish and opaque sedimentary rock, can for example be transformed into quartzite, a fairly smooth, whitish and even translucent metamorphic rock as can be seen at GZ (photograph below).



Other earthcaches will introduce you to many more geological phenomena, which in combination over time will help you appreciate how Earth was formed and changes over time.

TIME TRAVEL
In general, when visiting an earthcache site you are not only visiting a place but also a specific point in time, usually VERY long ago. Rocks preserve snapshots of time, telling us more about what the conditions were when they were formed and also what happened since then. But you should realise that most geological processes occur very, very slowly, in fact so slowly that it can be mind boggling to comprehend their timelines…

The geologic time scale is a system of chronological measurement that relates stratigraphy (the study of how rocks are layered) to time (measured in millions of years) and is used by earth scientists to describe the timing and relationships between events that have occurred over the course of Earth's history, which spans approximately 4.5 billion years. It is difficult to fully appreciate the geologic time scale because it is so far removed from our everyday experiences of time which we measure in hours, days and a lifetime of almost a hundred years – at most.

One of the most useful tools we have come across that puts geologic time into perspective, is the “24 hour geological clock” where geologic time is equated to a 24 hour day. If Earth was formed at the stroke of midnight, i.e. 00:00 = 4,500 MYA, and today (early 21st century) is at midnight, 24 hours later, major geological events can be placed on this 24 hour clock to give you a relative indication of when they happened (figure below).



On this scale the oldest rocks still present on Earth (including rocks in the greenstone belt near Barberton in Mpumalanga, which are 3,800 million years old) were formed at 03:44 in the morning. The oldest fossils, those of algae that lived 3,500 MYA, were captured in rocks that were formed just a “little later” at 05:20. In contrast the Bokkeveld fossils here in South Africa were captured in the mud only 400 MYA – already at 21:52 on our clock! The first dinosaurs appeared 230 MYA at 22:46 and humans only 0.2 MYA at 23:59:56 – humans have therefore only been around for four (4) seconds of Earth’s 24h existence!

On the same scale the Table Mountain sandstone where you parked next to the road was deposited at 21:20 (500 MYA) and the Malmesbury mudstone at 21:00 – 60 million years earlier. You have therefore walked back 60 million years in time from the road to GZ to stand on the oldest rocks in the Cape Peninsula! Amazing, isn’t it?