I was so excited to see that something as simple as a geology map could get so much interest! My wife keeps asking me who exactly are these people that love a chronostratigraphic lithology map 😊

I noticed a few people ask me to add a little bit “about the map”. And so, this blog will cover two aspects, firstly how I made the map, and secondly, what it represents.

How the map was made

How do you make a map like this? As an avid DIY-er, I know that most jobs are fairly easy if you have just have the right tools. And that is much the same for making maps, or working with location data in general. To start, I downloaded 1:1,000,000 geological data available from the Council for Geoscience. This data is “GIS-ready” because it includes polygons with attribution that describe each polygon (see below for more details on what the data includes) as well as a pre-determined styling (colours) based on geological standards (see the original litho map produced by the CGS with all this detail). I threw this into a new project in ArcGIS Pro 2.7 and decided to use a local scene to better accentuate the scale of the country – or simply put, to give it a nice 3D perspective.

To make this “raw” data look better, I used a few cartographic tricks. First, I wanted to highlight and show the geology against the topography of the country. This is best visualised using a hillshade (or relief) map. Fortunately, Esri provide a pre-rendered global hillshade layer ready for you to use – one of many incredibly useful, pre-built layers available on the ArcGIS Living Atlas. Putting this under the geology obscures it, so I changed up the transparency of the geology layer, but it looks a little washed out. And here is the real trick, Pro 2.7 now supports layer blending, which allows me to combine the pixel values of each layer dynamically using one of many methods – much the same as how photographers and photoshoppers have been working for years. You can play around all day with this, but I settled on the most common method – Overlay. Finally, to exaggerate the elevation more, I changed up the vertical exaggeration of the scene to 5. This leaves me with a much richer, punchier-looking map.

Note that I also added a masking layer to show only South Africa, and reduced transparency to 35% to allow the hillshade to show through.

There was one last piece missing though, it lacks a little “realism”. To add that, I searched for and added the Esri World Imagery layer to the map. I used the layer blending technique again and voila, we have a beautiful map!

But, I couldn’t just present the map as is, so I went and did some finishing using a graphics editing programme called GIMP. I added a drop shadow, a graticule (exported from ArcGIS Pro) and some basic descriptive text. And there you have it… Nothing stopping making your own.

What does the map show us?

So it is a beautiful map, but what about the content? Although it would be great to make maps just for the sake of it, they should always be delivering some sort of information. What does chronostratigraphic lithology actually mean? I will let the experts explain, but in a practical sense, this shows you what the predominant rock that exists on or near the surface at a given time/time period.

Here is a sample description of what lithology represents, in geology-speak:

Lithologic properties are commonly influenced more strongly by local environment than by age, the boundaries of lithostratigraphic units eventually cut across synchronous surfaces, and similar lithologic features occur repeatedly in the stratigraphic sequence. Even so, a lithostratigraphic unit always has some chronostratigraphic connotation and is useful as an approximate guide to chronostratigraphic position, especially locally.

Distinctive and widespread lithologic units also may be diagnostic of chronostratigraphic position.

International Commission on Stratigraphy

(As an aside, my favourite joke about geology data is when someone asks if it’s the “latest version”… to which the reply is something along the lines of “the geology has been the same for a few hundred million years” 😊)

In this case, I used the default lithology-based symbology provided with the data that describes the rock types, so really should have just called it a Lithology map… But in any case, what the data tells us is that, for instance, 14% of Southern Africa is covered in “Mudrock, subordinate sandstone”.

Probably of most interest to the geologists, is the area in the north, which you can just make out on this map as the Bushveld Igneous Complex, an ancient geological structure that projects certain minerals in a bowl shape near the surface. This was first uncovered by Dr Hans Merensky when he stumbled upon some rocks he recognised as platinum-bearing. Since that day in 1924, people have been digging and extracting these minerals such as platinum, rhodium and palladium by the truck-load and in fact this area is understood to contain over 75% of the world’s known platinum reserves.

Here is a breakdown of the top 5 Lithostratigraphic groups (as defined in the original map produced by CGS here) summarised by total surface area of South Africa (and Lesotho and Swaziland/eSwatini).

Lithostratigraphic groups	Percentage land covered
Mudrock, subordinate sandstone	14%
Superficial deposits comprising gravels, clays, sandstone, silcrete, calcrete and aeolian sand	9%
Mudrock	6%
Network of dolerite sills, sheets and dykes, mainly intrusive into the Karoo Supergroup	6%
Red and greenish-grey mudstone, fine- to medium-grained sandstone	5%

If you want to know more about the platinum and gold mining areas, what has been mined and what is still available (resources and reserves), why not head over to the National Resource Atlas (built and maintained by the Mandela Mining Precinct) to discover more – it requires registration, but it is free and will give you access to an unprecedented wealth of historic information that you can use directly on the Atlas, or connect up to your own GIS of choice for further analysis.

– Richard