Planetary Geology on Earth: The Stuff
Please note that the posted coordinates are approximately 50m due north of the EarthCache location. This is to avoid overlap of icons on geocaching maps
This EarthCache is based on an extraordinary looking isolated boulder located just off a popular Silvermine trail. The rock is also the location of a traditional cache, Ge0cath’s cache The Thing which provided the inspiration for the name of this EarthCache.
What is also extraordinary is that this rock provides a connection between Silvermine and Mars! Both are places where conglomerate is found. See below for information on Mars conglomerate finds.
To Reach the EarthCache Location:
- Park at the eastern limit of the jeep track just before the boom. This will save you a total of 500m additional walking distance. This should be safe enough, especially if you ensure you hide away any items of potential interest in your vehicle. But, if not comfortable with this spot, there are numerous alternatives along the track from the Silvermine Gate 2 entrance boom.
- Follow the main jeep track heading south to S 34 5.948 E 18 25.726 the junction of the path heading down and across the top of the Silvermine river waterfall, which can be used as an alternative to following the main track around a long bend. This rejoins the main track @ S 34 6.065 E 18 25.797.
- Using either route, continue to the jeep track/path junction @ S 34 6.292 E 18 25.821 and follow the main track left heading east up to the cache location.
To Claim the EarthCache
Please make the necessary observations on site and answer the following questions, submitting answers to the CO by GC message from my Profile page or email (forshaw.chris@gmail.com):
- Estimate the approximate length and width of the largest clast on the top surface of the rock and describe its appearance. Based on its size, what name would you give to the clast?
- You will see that some clasts are broken and their exposed interior is completely different. Please describe these differences.
- How would you know that a clastic rock like this is a conglomerate rather than breccia?
- Having examined the rock, what mineral would you say has precipitated to cement the clasts into conglomerate.
‘The Thing’ is a sedimentary rock consisting mainly of clasts – fragments of rocks and minerals. Such rocks are first classified according to the roundness of the clasts. If the clasts that form it are mainly well-rounded to subrounded it is a conglomerate. If the pebble chips forming it are largely angular, it is a breccia. Such breccias may be called sedimentary breccias to distinguish them from other breccia types.
Thus, conglomerate is a clastic sedimentary rock (see chart in Gallery) consisting of a significant proportion of rounded to sub-rounded gravel-sized (ie. coarser than sand or >2mm diameter) clasts.
Clastic rocks are composed of fragments (clasts) of pre-existing minerals and rock which constitute geological detritus - chunks and smaller pieces of rock broken off other rocks by physical processes of weathering and erosion.
The matrix (space between the clasts) is generally filled with finer-grained sediments, such as sand, silt or clay and the clasts and sediment are bound together by a cementing medium. This might be calcrete (calcium carbonate, ie. calcite), ferricrete (iron oxides) or silcrete (quartz, ie. silicon).
Ferricrete is a hard, erosion-resistant layer of sedimentary rock, usually conglomerate or breccia, that has been cemented into a duricrust (hard layer on or near the land surface) by iron oxides.
The cement is formed from the oxidation of percolating solutions of iron salts which gives it a distinctive orange-red colour. Ferricrete may contain non-local sediments that have been carried from outside the immediate area of the deposit.
The name is a combination of ferruginous and concrete. Synonyms include ferruginous duricrust, hardpan and ironpan. In South Africa, the Afrikaans terms ‘Koffieklip’ and ‘Ouklip’ are sometimes used.
Conglomerate Composition
As a clastic sedimentary rock, a conglomerate can contain clasts of any rock material or weathering product that is tumbled downstream, down current or by strong wave action.
The clasts can be mineral particles such as quartz or feldspar, or sedimentary, metamorphic, or igneous rock fragments. Clasts may be of quartzite, sandstone, limestone, granite, basalt, and gneiss.
All clasts start out at a ‘source area’ - an outcrop (a location where rock is exposed at Earth's surface).
In the source area, chemical and physical weathering act upon the rock, causing it to break or disintegrate into smaller pieces. These pieces are usually sub-angular to angular. If the clasts accumulate near the outcrop and are cemented into a rock, that rock will have angular pieces and be a breccia.
However, if the pieces are transported by a stream or the action of waves, the clasts will be abraded against one another and against other clasts on the bottom of the stream. That abrasion will eventually cause angular shapes to become subrounded to rounded. If the rounded clasts are deposited and formed into a rock, that rock with rounded clasts will be a conglomerate.
The difference between conglomerate and breccia is thus a result of the clasts’ transportation history.
Conglomerate Formation
Conglomerate forms by consolidation and lithification of clasts. This occurs where sediments of rounded clasts at least 2mm in diameter are deposited and accumulate. A strong water current is needed to transport and produce a rounded shape on particles of this size. Wind transport is unlikely to produce a conglomerate.
The environment might be along a swiftly flowing stream, or a beach which experiences strong waves (see photo).
Such conditions may only occur during times of extreme flow or wave action, and it is therefore during these times that much of the Earth's sediments are picked up, carried away, and deposited elsewhere.
For a conglomerate to form, there must be an up-current source of clasts whose rounded shape shows that they have been tumbled for a significant distance, or to-and-fro by running water or moving waves. Such water movement may be found in streams, lakes, and oceans.
Conglomerates typically start when sediment of mainly pebble - and cobble-size clasts is deposited. Such deposits may be on the seafloor, in lakes, depressions, or in valleys. Deposits in the sea, lakes or depressions usually form extensive sheets of conglomerate whereas valley deposits end to form narrow bands of conglomerate – as is the case at GZ. The finer sand and clay particles are deposited later, on top of the clasts sifting down between them to fill the interstitial spaces. Later compaction and precipitation of a chemical cement between the grains binds the various pieces of sediment into a rock.
Mars Conglomerates
In September 2012 a conglomerate outcrop named 'Link' (left photo below) was discovered on the surface of Mars by NASA's Mars rover Curiosity (see mission news here and Mars/Earth rock outcrop comparison here).
Its rounded clasts showed that strong current or a wave action had repeatedly moved and tumbled the rocks which eventually became eroded into rounded pebbles. This find provided convincing evidence that water once flowed on the planet’s surface.
The above image was taken on Lower Mount Sharp on 22 July 22 2016 - Martian day (sol) 1,408 of Curiosity Mars Rover’s work on the planet. It shows boulders partially comprising pebble-size (0.5-6.5cm width) and larger rock fragments whose size and shape give clues as to the origins of these rocks.
The relatively flat rock to the left is about 1.5m across. The rock in the right foreground is named ‘Balombo’ and the site where these boulders are located is ‘Bimbe’.
Curiosity’s team (remotely!) drove the rover to Bimbe to further understand check out the groups of boulders first seen from orbit and then occasionally on the rover's traverse across the planet’s surface.
The boulders comprise several rock types and some include pieces (‘clasts’) of smaller, older rock cemented together – which as we now know are called breccias or conglomerates. Whether such inclusion clasts are rounded or sharp-edged may show how far they were transported, and by what processes.
Looking at these boulders, both angular and rounded clasts can be seen leading to some uncertainty about how they formed.
Conglomerate rocks such as ‘Hottah’ were examined near Curiosity's landing site and interpreted as part of an ancient streambed. Breccias are generally formed by consolidation of fragments under pressure which on Mars might come from crater-forming impact, or by deep burial and exhumation.
YouTube videos about conglomerates: See here for a well-illustrated short article on conglomerate, here for a short video ‘Conglomerate rock - a complete guide’ and here for another on ‘Detrital & chemical sedimentary rocks’.
As to how ‘The Thing’ came to be where it is, a likely explanation is that it was bulldozed or scraped there when the adjacent jeep track was being made. This track runs right across a (mainly concealed) band of conglomerate which originated from sandstone and quartz clasts deposited in a narrow stream that ran through the peninsular formation. Many millions of years later these were consolidated as duricrust into narrow band of conglomerate that runs more or less south to north at GZ.
Sources used:
https://geology.com/rocks/conglomerate.shtml
https://en.wikipedia.org/wiki/Conglomerate_(geology)
Acknowledgement:
Thanks are due to Richard Fuggle (Richard.F), retired UCT Professor of Environmental Studies, for expert on-site observations and kind and careful review of the draft cache description.