Peers' Cave - Skildergat EarthCache

This Earthcache will take you to a very cool archaeological site. Although I am an archaeologist, and would love to go on and on about the site, the rules of earthcaching say you must be here to learn a geology lesson, not an archaeology lesson. So I will do my best to stick to the rules (and sneak in a little bit of archaeology!)

This cave is located down a fairly easy hike from pull out parking on Ou Kaapse Weg. The parking is easy to see from the road, but I've given you a waypoint. Remember to not leave any valuables in your car. It is reasonably safe here, but windows do get smashed from time to time. At the trail head you will see a sign for Peers' Cave. The trail is about a kilometer long and is mostly flat, but does have some boulders on the way. GPS accuracy at GZ is bad, but you can't miss the spot. You are looking for a large rock shelter, not a little hidden container. The UCT archaeology department brings first year students here for a practical. This cache is partly an adaptation of that assignment. The dificulty rating of this cache reflects the dificulty of these questions.

Today, the cave is named after the local excavators who dug most of the cave, a father and son team named Victor and Bertie Peers. It is also known as Skildergat, or Skildegat. This name may be derived from the rock paintings in the shelter (Afrikaans Skilerye + gat or painted hole/cave). Although faint, and often covered by graffiti, these are the only old rock paintings within 100 km radius of Cape Point. The cave may also be named after a farm hand named Schilder who used the cave as shelter for cattle in the 1800s (Schilder's kop).

Geology Lesson: Deposition, Sediment Traps, and Stratigraphy:

The geology lesson I'd like to teach at this site is about deposition and stratigraphy. My father asked me a simple but important question when I first started studying archaeology. He noticed that archaeologists were always digging things up. This led him to wonder three related questions:

1. Where does all that dirt come from?
2. If it keeps burying archaeological sites, does the planet just keep getting bigger over time?
3. Why did people a long time ago spend so much time in caves, wasn't it nice outside?

I'm going to tell you the answers to two and three, the answer to question one is what you will need to figure out. It isn't from meteorites; that was his first guess.

The answer to number two, as any geologist can tell you, is no; the earth isn't getting bigger. Erosion removes sediment (dirt) from one place, either by wind or water, and leaves it behind, deposits it, somewhere else. By this method, dirt moves from one place to another, exposing some things once buried, and burying other things once exposed.

The answer to number three is related to this process as well. Archaeologists dig where we are likely to find stuff from the past. These places are not necessarily the places where people hung out the most, but are places where dirt has accumulated. Places that collect dirt (sediment traps) also bury evidence from the past. In other words, the reason archaeologists dig up caves is not because people in the past only hung out in caves. Rather we like caves, because caves can trap sediment, and keep it dry and safe.

A friend of mine published a paper about why the area where he works in Iran has caves with deposits on one side of the valley, and the opposite side of the valley all the caves were all empty (Heydari, 2007). His explanation was important for interpreting how people lived in the past. He showed that the absence of evidence was due to geological causes, and not due the behaviour of people. I bring it up here because it is helpful for understanding sediment traps. In many earthcaches you've learned about bedding planes. Sedimentary rocks (like the Table Mountain Sandstone here) form in layers. These layers typically are bedded at an angle (their dip and strike), usually due to movement of the rocks forming mountains. The following illustration comes directly from his paper.

Don't worry too much about understanding everything here. The main thing I want you to get is that because of the way 'Type A' sites are angled in the bedrock, they collect sediment like a bucket. Because of the way 'Type B' sites are angled, they don't collect sediment very well at all, much like a tipped over bucket. Peers' Cave is more or less a 'Type A' site. A good example of a nearby 'Type B' site would be Elephant's Eye Cave. For those of you who've been there, you know it slopes steeply down and out of the cave. The fact that we don't have evidence of people living there is not because they didn't, it is because the shape of the cave didn't allow any sediment to accumulate, and thereby leave behind evidence. Although this may seem simple and abundantly obvious, all good science does once it is pointed out and explained.

Sediment traps, such as caves, are great for archaeology because there are many different periods of deposition that create different identifiable layers. Much of how we are able to know the age of something is due to its position in this sequence of layers. Understanding the sequence and process of deposition by looking at these layers is called stratigraphy and is exactly the same in Geology. In fact, archaeologists stole the method from geologists. Archaeologists, like geologists, reconstruct the sequence in which sediment was deposited by looking at a cross section of layers, called a profile.

Here is a drawing of the cross-section of the cave that was published in 1931 (I added colours to make it clearer). You can see the sediment inside the 'bucket' of the rock shelter. In the above diagram, layer 3, that contained the skeleton, must be older than layer 2, because 2 is on top of it. It must also be younger than layer 5, because 5 is beneath it. This is called the "Law of Superposition" which is just a fancy way of saying that the stuff on top got there later than the stuff under it.

The surface of the sediment in the cave before it was excavated can be seen on the rock wall when you vist. The deepest parts were dug over six meters below this original level. The Peers found nine human skeletons in this shelter. One of them became famous as the 'Fish Hoek Man', as it came from a layer that contained very old stone tools. The skeleton (its position is labeled in the drawing) is often mentioned in histories of the area as being 12,000 years old. This turns out not to be true, and I will tell you more below.

The Peers did not dig in a way that today we would consider to be careful. They used shovels, picks and dynamite, and as a result, the layers they identified are very simple. To anyone who has worked in caves, this is clear from their drawing above; it is way too simple. Their column represents about 14 feet of depth (just over 4 meters), and only identifies five layers. There is another (big) problem with this drawing, but identifying it will be one of your questions. Sediment traps, such as this rock shelter, create very complicated series of layers. Once you add people living there, making fires and digging holes, things get even more complicated. Understanding these layers, and knowing what came out of each of them, is what archaeologists (and geologists) do to understand the past formation of rocks and sediment. On the left is an example of cave stratigraphy. It comes from the cave site of Hohle Fels in Germany. This is what a cross section of a cave should look like. Really, really complicated.

Zambesiboy, in his answers, pointed out another interesting aspect of sediment traps. They trap the fine sediment (silt and clay) that gets blown away in other parts of the landscape. As a result, the soil in shelters is very rich. Or as he put it: "It actually looks quite good for the garden". Many of the archaeologically rich caves in South Africa were destroyed by farmers with this exact thought before there were laws to protect them. The first of these laws was enacted in 1911, a 100 years ago. So, please don't go collecting sediment from here, you would not only destroy valuable information, you'd also be breaking the law.

Peers' Cave is not a proper cave, but a rock shelter formed in the Ordovician aged Table Mountain Sandstone that makes up this rock outcrop. The shelter was formed by the weathering of less resistant rock along the outcrop, with more resistant rock above it forming the roof. It faces south across the Fish Hoek/Noordhoek valley which is covered by shifting sand dunes.

The sea levels have changed a fair amount in the past. The last time the valley floor was under water was the late Miocene or early Pliocene, about 5 million years ago. It was during this time that a great deal of sand was deposited. Today this sand forms the dune fields you can see across the valley floor. However, for most of the time people were living here, sea level was much lower than it is today, about 40-120 meters lower. This was because much of the planet's water was in glaciers in the northern hemisphere. The lowered sea level caused the sea shore to be much farther away than it is now.

At this point, along with your keen observational skills, I've given you everything you need to answer your questions. But first let me tell you a little bit about why the site is interesting and what was found in the dirt that got trapped here.

A little information about the archaeology:

This cave contains traces of humans living here as long ago as 200,000 years ago. Much of the excavation here was done by the Peers and John Goodwin in 1925-1931. As both the Peers had full time jobs, work was limited to weekends, public holidays, and their annual leave. Field Marshal Smuts declared in 1932 that the cave "promises to be the most remarkable cave site yet found in South Africa". Keith Jolly dug here in 1946-1947, and Barbra Anthony in 1963. Recent work was done by a joint South African and American team in 2002, investigating what was left of the deposit. Unfortunately, most of the sediment was removed by the Peers.

Nonetheless, we know that the majority of the sediment in the cave contained Middle Stone Age tools of types dated to around 70,000-60,000 years ago. The Middle Stone Age is a major focus of global research today, as it was during this period that anatomically modern humans (people like us) first appeared. The upper portions of the cave have evidence of more recent people, from within the last 10,000 years. Their tools were different, as they hunted smaller game, collected shellfish and fish, and gathered plant food. By 2000 years ago the people living in this valley were herders of domesticated sheep. There were also some finds that are likely from the historic period. Jan van Riebeeck noted in his diary that indigenous people were living in this valley in the 1650s, and his interpreter 'Harry' spent time here, perhaps even in this cave.

Even at the time when the 'Fish Hoek Man' was excavated, there were doubts about its age. The preservation of this skeleton did not match the other animal bones found in the same layer. These were very fragmentary compared to the fairly intact human skeleton. Unfortunately, once something has been dug up, you can never recreate its position in the layers. Excellent notes and careful excavation are the only way to dig correctly, as there is only one chance to get it right. However, bone can be dated directly using the method of carbon 14 dating. Eventually, this question was addressed by a group of Cape Town scientists (Stynder et al, 2009). They dated the skeleton directly, and determined it to be just over 7000 years old. Even though it is much younger than previously thought, it is still one of the oldest skeletons found in the region.

Questions to answer:

1. What effect do you think the presence of the large rocks at the front of the cave had on the archaeological deposits?
2. Given the "Law of Superposition", explain why Layer 4 in the above depiction of the cave's stratigraphy is a problem. (you do not need to google the cultures mentioned or understand the archaeology, this is only a question of thinking about the deposition of these layers logically)
3. How do you think the sediment in the cave got here (by what methods)?
4. Where did it come from?
5. Look at the sediment in the cave carefully. What colour is it? What texture does it have? Is this the same as its source? If not, what do you think caused it to be this colour?
6. OPTIONAL: A photo of you and your GPS at the site in your log is always fun.

References consulted:

Deacon, J., Wilson, M., 1992. Peers Cave: the "Cave the World Forgot.". The Digging Stick 9: 2–5. (link)

Heydari, S., 2007. The impact of geology and geomorphology on cave and rockshelter archaeological site formation, preservation, and distribution in the Zagros mountains of Iran. Geoarchaeology 22: 653-669 (link)

Keith, A., 1931. New discoveries relating to the antiquity of man, Williams & Norgate LTD, London. (link)

Stynder, D.D., Brock, F., Sealy, J.C., Wurz, S., Morris, A.G., Volman, T.P., 2009. A mid-Holocene AMS 14C date for the presumed upper Pleistocene human skeleton from Peers Cave, South Africa, Journal of Human Evolution 56, 431-434.

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		FTF goes to MnCo & Zambesiboy!