When driving through the magnificent Hex River valley, two things stand out so much that they are impossible to miss. As far as the eye can see, vines effectively occupy this entire fertile valley where 65% of South Africa’s export table grapes are produced. But even more impressive are the mountains within which this beautiful valley nestles, particularly when considering their geology.
The Hex River mountains form part of a large anticline in the Cape Fold Belt mountain system which formed during the Gondwanaland rifting when South America separated from Africa. The Hex River range form a north-east, south-west trending mountain system, constituting the core of the Cape Syntaxis between the towns of Worcester and De Doorns. They are mostly composed of Table Mountain sandstone with most peaks reaching 2000m in height or more. The highest mountain is the Matroosberg at 2249m (the second tallest peak in the Western Cape province), lining the northern perimeter of the valley.
But for this EarthCache we’re turning our attention to the range along the southern boundary. As you enter the valley from the Worcester end, after passing through the deep Kanetvlei cutting and past the Sandhills turn-off, the next 8km stretch takes you over a succession of four gentle hills, each one lower then the previous as you drive eastwards, with the last one (at the De Doorns turn-off), barely noticeable. Also notice the enormous stash of round melon-sized boulders that the farmers had to clear to establish the vines.
What we have here are a series of very similar alluvial fans, also known as debris fans or debris cones, in this case. The Wikipedia definition is “a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain”, with the term Alluvium derived from the Latin, alluvius, from alluere, meaning “to wash against”, referring to soil or sediments deposited by a river or other running water. Such structures occur in nature on scales of millimetres to kilometers, the Hex River features being examples of the larger variety.
Quoting Wikipedia again: “The debris flows generally form when unconsolidated material becomes saturated and unstable. Flows move downhill as from the force of gravity and generally stay on downward slopes and mountain valleys. Flows can carry clasts ranging in size from clay particles to boulders. They can be triggered by large amounts of rainfall, or glacial melt, or a combination of the two”.
The size of the debris particles and by how much the cone flattens out depends primarily on the consistency of source material being washed down and the amount of water present at the time it formed. Judging by the average sizes of the boulders making up these mounds, and by their height, the Hex River valley structures formed very recent in geological terms - only about 10000 years ago. By this time the Earth had dried off considerably, leaving not enough time for this debris to erode down to finer soil or to flatten out into the valley below.
But so far we said nothing about the origin of the source material. Well, this is where you come in …
Please note that this cache is best suited when travelling eastbound (from Worcester to De Doorns). If however you are driving westward (i.e. towards Cape Town), take a close look at the mountain to your left for features that fit the above description when you near the Reference Waypoint below (S33 29.196 E19 39.785). Then look for similar structures en route to the designated coordinate and remember to log the two required waypoints along the way (see item 2 below).
To claim “Found it” you must email me satisfactory responses to the following:
Any logs not accompanied by an email will be deleted.
- Find safe parking near the designated coordinates. When such a huge amount of material washed out, it must have left a scar somewhere. Look for this scar and take a picture of you/your party and your navigation device with this evidence in the background.
- Between this point and the De Doorns turn-off are two more identical structures. Take rough GPS coordinates (you can do this while driving) at both these and send me the two coordinate sets.
- See if you notice any relation between the sizes of the four debris cones and their corresponding scars. Tell me what you observed and explain any such relationship.
- While driving along this stretch, spot the enormous mounds of bolders scattered amongst the vines. In your own words, explain the abundance of bolders here?
Note: Do not post your pictures, replies or hints to this page, even if encrypted.