Essex Rock & Mineral Society Field Trip
The Naze, Essex, August 14th 2005
Walton on Naze, is almost a pilgrimage for our Society, it is perhaps one of the easiest places to find fossils in Essex. There are natural exposures of London Clay on the foreshore and in the cliff section. Fossils from this can be found amongst the beach gravels. The London Clay is overlain, unconformably, by Red Crag deposits, (an unconsolidated shelly sand stained red by iron oxides). These were lain down in a shallow sea some 2 to 3 million years ago. It is relatively easy to find fossil shells amongst the slumped sands.
The rate of erosion of the Red Crag cliffs is frighteningly rapid. This site provides classic examples of rotational slumping, where the sands become inundated with water and slide along the slip plain at the junction with the impermeable London Clay beneath.
Our leader on this occasion, Gerald Lucy (author of Essex Rock), brought along a specimen box of fossils to give all those unfamiliar with this locality an idea of what could be commonly found. Gerald had prepared an excellent handout that explained everything (almost) that one needed to know about the geology and fossil fauna of this classic site.
As previous reports concentrate on the fossil finds of this locality (see our report archive), it would be just repetition to continue in the same manner, and perhaps that is why many of our Society members frown and groan at our repeated visits to this locality. So what follows is what one member found interesting (mostly).
The cross bedding seen in this view of the Red Crag represents approximately a 2m vertical section. These were marine deposits and the cross bedding was said to have been formed in a similar way to sand dunes, but underwater. The horizon between the lighter sands (below) which contained small shells and broken fragments was first thought to be an unconformity.
The problem is that apparent bedding planes appear contiguous across this boundary.
The darker sands above have no evidence of shell debris and seem less coarse than below. One can conclude that this has become decalcified later. (The shells have been dissolved away)
If so, the first question is how did they become decalcified post deposition to leave such a marked horizon.
Our leader's first thoughts were that this may have been a product of permafrost conditions where the subsoil had thawed to the depth of the horizon. Ground waters were responsible for the decalcification at that time whilst the water in the sands below remained frozen. A good plausible answer from our leader put on the spot.
However, there is evidence of other periglacial freeze thaw actions in the cliff section and coupled with the fact that the bedding does not seem to have been disturbed by this action (in this view) this member ponders alternative interpretations that preserved the cross bedding.
An interesting section that deserves an informed look next time (if it is still there), mostly because, excluding the shell material in the sands below, and then comparing to the sands above the horizon, they seem so textually different and not in keeping with past experience of decalcified deposits.
Photograph courtesy of D. White
This photograph was taken from the foreshore some way back from the cliff face and what is observed in this view are moderately large scale sedimentary features.
The folding of the Red Crag is an example of repeated freeze thaw of permafrost at depth, a periglacial effect.
Just above this, and not that distinct in this reduced image, is a gravel/ pebble bed which has been interpreted as fluvial deposits from the from the former route of the Thames. (A case study all of its own, the Thames is one of the best studied fluvial systems around and is well documented, you should be able to search these out for yourselves if interested).
The folding due to repeated freeze thaw action seen at this location is a classic example not usually seen outside of a text book.
Further North along the foreshore the Red Crag deposits taper down and the London Clay begins to form the major feature of the cliff face.
In this view the first notable feature is the apparent banding. Volcanic ash layers are reported to be observable at this locality, however, it would be rash to assign these to this cause.
Close-up, the pattern seems to be weathering of the cliff face and could be a product of varying salts or organic content in the clay layers. I scraped away a small area back to a fresh surface and no distinct colour changes could be seen, hence the caution in making any assumptions. (Only a small trial, I could be wrong). Plus the banding is rather broad for volcanic ash deposits considering the overall slow rate of sedimentation of these clays.
The section is about 5 to 6m high, and central in this view is a fault with a displacement in the order of 3m. The block to the North (right of picture) appears to be the uplifted side. The bedding plane of the sediments dip northwards.
On the foreshore the beach gravels contain an increasing (northwards) amount of pyritised fossil plant material. (Contemporary with London Clay flora). Very similar to the density that one encounters on the foreshore at the Isle of Sheppey (Eastchurch gap area). It may be that they have just accumulated there due to tide action and not particularly relevant to the cliff exposure..
A closer look at the fault seen in the previous photograph.
In the uplifted side to the right of the fault and near the base one can see a thin lighter line. This is believed to be the pale blue/grey clay formed as the result of volcanic ash.
Whether the ash is a direct result of a heavy ash fall deposit directly, or washed into the marine environment as the output of river systems, or the reworking of ash deposits elsewhere at sea and accumulation at this locality due to current, and tide action is a case for speculation. The current interpretation assumes settlement from a direct ash fall.
Another
small fault and the reappearance of the ash layer in the cliff face
further north along the foreshore.
As previously mentioned the beds apparently dip to the north. The ash
band being close to the bottom of the section soon disappears below
the beach level and then reappears at the next fault.
However there is one section where it unexpected disappears abruptly as one continues further north along the beach. No distinct faulting could be seen, but I was left with a sense that the next section of the cliff face to the north was the downside of another fault. If so this was against the observed trend of the faulting northwards along the beach.
This member did wonder whether the continuation of these faults in the London Clay would leave their trace in the foreshore exposure. There were no distinct features easily observable.
The age of the faults was considered to be older than the Red Crag deposits as it did not appear to extend into them.
Photograph courtesy of D White
My umbrella and me, standing on a platform of an ash band at the cliff face. (Such an unflattering picture of me that the photographer insisted it be edited, I couldn't have cared less.) It does rather beg the question whether the clay beneath is more resistant to erosion than that above or does the clay product of the ash band protect it in someway? My thoughts turned to thinking of a wave cut platform at high tide (in miniature).
This view is the first observed occurrence of the ash bed exposed on the foreshore. Looking past the umbrella which marks the edge of the ash band to the cliff in the background one can observe another fault. Again, the uplifted side appears to be to the north, (right of picture).
Gerald Lucy, or field trip leader on this occasion , observing another fault. This one slopes down left to right, the others we observed sloped in the reverse direction.
A little earlier in the day, two of us continued along the foreshore until the cliff section petered out. The foreshore was littered with pyritised plant remains and a fair bit of building rubble, it looked more like the foreshore at Sheppey, even down to one septarian nodule all on its own. Rather odd to find so similar at this location. It was probably out of context, imported or dumped, there were no signs of any others or a nodule bed.
My personal view is that there is only one ash band exposed at this location, there may be two.
The authoritative view is that the division of the London Clay at this location is probably A1. This the base of the London Clay sequences, however there may be some readjustments to this. They occasionally move boundaries a bit when they can all agree.
Because of all the other wide indistinct banding observed my curiosity into the petrology of clays has been aroused. Perhaps a DIY project in my garden shed back home, sometime in the future.
Unfortunately, time and tide took control of the rest of the day. All these faults came as a bit of a surprise to this member. So it is back to the Naze at the first opportunity to attempt some form of analysis of this sequence of faulting. Not that the academics and geology students haven't done this already and published various papers on the subject. For me it is: — JUST FOR THE FUN OF IT. There isn't any other natural exposures with these features anywhere else in Essex.
Three of us eventually decided to call it a day and found ourselves cut off by the tide. The start of steps off the beach was below water by this time. It would have meant either wading waist deep to them or climbing over big blocks of rock used to afford some protection from coastal erosion. We scrambled over the slumped Red Crag and worked our way to the top easily enough, preferring the soft landing of any slip or fall on the loose soil to that of those hard rock blocks. Gerald turned round and said, "you would think they would have more sense and have put the stairs further back on the shore line?" They probably did originally but that is erosion for you, it does happen rather fast along this shore.
Roger Coleman, August 2005
All text and photographs copyright © R Coleman 2005 unless stated otherwise.