The first question we were asked was; "How many of you have heard of laterites?"  About three or four of us were honest enough to put up our hands. Des explained that they are a type of soil associated with tropical regions of high rainfall. We were shown a map of the world highlighting these regions and the relationship with present day laterite soils.

A laterite is similar to a clay or mud that is being continually leached by acidic water. In a rainforest, topsoils are thin and composed of mainly decaying vegetation. Nutrients are recycled by the plants and a true soil doesn't have the chance to develop. The leaching of the soil beneath removes all dissolvable products leaving the insoluble. These form a zoned structure of which the laterites form the main body, these are typically red/brown colour due to a high iron content.

So what's different about laterites and ordinary clay or mud? Simply, that if they dry out they set like concrete, a process that is irreversible!

Des has researched laterites (past and present) in three locations. The first location was Sierra Leone about 160 miles inland from the coast in the foot hills of a Precambrian range. (Don't ask me the name of the hills). The landscape was that of tropical forest which the local farmers clear for growing one year, and are supposed to leave for another 20 years for the bush to grow back. Were this was not done has resulted in the drying out of the laterite and the formation of a concrete surface which is impenetrable to plant roots. The locals use laterites for making bricks for building. (Makes the mud hut seem a lot more permanent!)

Now the important point about laterites is that they are residue concentrates that can have a significant economic resource, and are often associated with nearby bauxite deposits.

The typical laterite landscape is that of flat terraces, the edges being continually eroded and the products incorporated in the next terrace bellow. Near to and at the edges 'swallow holes' and cave complexes develop. (The pictures and diagrams we were shown reminded me a bit of how our own limestone caverns have developed.)

The next area Des discussed was the Deccan Traps in India where in one location there are flood plain basalts layered with red horizons of ancient (and solid) laterites. This does pose somewhat of a conundrum of how this has arisen. The important point again is that they are flat and sheet like!. Des was very noncommittal on any theories (who can blame him or perhaps time was against him to explain).

The last location was that of Gambia, we were briefly shown a few pictures and were asked to note that the apparent flood plain of the river in view (the name escapes me) was again a rainforest based by laterites and that we should note the typical sharp edge drop at the river. This being the terraced feature again.

During question time the problem of the age of laterites was raised. Des pointed out that the bed rocks in all three locations had a very wide age range, and it was muted that laterites sort of transport themselves as tectonic uplift occurs, a terraced structure develops and the erosion at the edges enables the whole lot to progress across the forming landscape to a new location! He added that they did try tacking core samples from the Deccan Traps laterites to test for palaeomagnetism but test proved inconclusive

I suppose this is how the laterites became incorporated with the Deccan Traps at that time. As for the age of present day laterites, the clue would seem to lay with the restriction to tropical regions of high rainfall but how quick these soils develop and from which original source (sedimentary?) is another question.

Des had on display a few lumps of solid laterite, he said that they seem to sometimes harden with a pisolith texture or alternatively similar to a worm tube structure. (Not that evident at first glance with that on display)

Perhaps not a talk for everyone's taste but I think Des had a hard time trying to pitch the talk for the abilities of us all, having had a look on the Internet, there is certainly a lot of very detailed information out there and the concentration of insolubles and their potential economic significance is extremely interesting, have a look for yourself!

I just hope I haven't misrepresented, missed or glossed over any salient points made by Des, these notes are my understanding of the talk!

R Coleman

Members' finds on display:

Not much new, but Bob Williams brought in these two crabs:

Dromilites bucklandi

One of the species of crabs found by Bob during his time at the Aveley Pit. A rather wonderful detailed specimen. The preservation underneath is nearly as good as the top, enough to conclude that the one featured here is a male.

Size about 40mm to 50mm across

Homolopsis sp.

As yet an undescribed species of the genus "Homolopsis" from the London Clay. Homolopsis is a Cretaceous species, the London Clay is Eocene! (Bob Williams)

This little fossil crab is no more than 20 -25 mm long, and was found by Bob during his collecting period at the Aveley Pit. However, it missed inclusion in the biota list of his paper (Note 1) as it was misidentified and grouped with others at the time. Later preparation revealed it to be quite an unusual find for the B2 Division (?) of the London Clay. (A specimen worth a little more research, so expect more news to come.).

Note 1: Williams, R.J. 2001 Observations on the London Clay excavations at Aveley, Essex.
Tertiary Research vol. 21(1-4) pp. 95-112

Jeff Saward had a box full of new crabs and lobsters, mostly from along the Kent coast from Sheppey to Seasalter. He says that January is a good time because of some advantages tides and the lack of green algae/moss that has been scoured away during winter. Judging by his results he knows what he is talking about. He also recovered half a "flint hand axe".

Lastly, a new member, Kevin North brought in a collection of gemstones that he had faceted. Most were quartz but rather nice clarity. He said that he had found the quartz whilst digging the foundations for his house. Naturally I asked where? He said Zimbabwe!

It isn't that usual to dig up quartz of facet rough quality. Thankfully another member interested in Lapidary and brave enough to give it a go without being led by the nose!

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All text & pictures © Roger Coleman 2004