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.