Caesar's approaches the enemy camp to encourage them to fight on level ground of his choosing, he arrayed in three line battle order. When it looks like there is not going to be a battle, Caesar gets his 3rd line to dig a 15’ wide ditch behind his front two ranks and out of sight of the enemy . After spending a night under arms behind this ditch, he pulled off the same trick the next day finishing off the ditch; but because materials for a rampart must have been fetched from a great distance, it was day three before he constructed a rampart to fortify his position,[Caes. Civ. 1.42].
More recent work on this section by English Heritage in 1999, , also contains an excellent summary of other recent results, along with the latest reports on the pollen  and plant macrofossils, which are well worth a closer look.
The archaeology that remains at Appletree [right] is a about 1m tall at best, and what interests us here is the “Peaty” layer which is the signature archaeology, and looks like a complex piece of Russian cake with dark chocolate layers interleaved with lighter greys yellows and greys.
In the 1999 section [below] some of these dark layers are continuous for over a meter, the lowest one is continuous and is thought to represent the original ground surface, [Layer 53], sealed by 45 -50cm of Turf wall material, [Layer 54]. In essence current thinking is that this section was built of turves and with a timber breastwork to a height of 3.6m / 12” with a near vertical northern face and sloping south side.
So let’s be clear about this, turf is sods of earth held together by the roots of plants such as grass, a substantial part of which is a block of top soil, so, in theory, the archaeology of a turf structure be layers of top soil interleaved with thin bands of organic humus, depending on the nature of the groundcover and the nature of subsequent soil formation processes.
It was this complex peaty layer [L52], the layer below it thought to be the old ground surface [L53], and similar material from the ditch [L45] that was sampled with aim if establishing the origin of the turves.
The pollen analysis discuses previous work on a less specific turf wall sample [probably 52+53],
“The pollen results revealed that a relatively open, probably grazed moorland existed immediately prior to construction of the Wall with the main; taxa represented being a mix of alder, birch, hazel, oak, heather and wild grasses. Other species present in low but consistent numbers were sedges, ribwort plantain, ferns and bracken”. 
The 1999 samples also show a similar pattern with material from the “Turf” layer being dominated by tree pollen, particularly hazel, with samples of the ditch fill show a similar pattern of Hazel, Alder and later birch pollen dominating.
The report sheds little light on the source of the turf;
“The source area of the hazel-dominated turfs is probably still from very close to the site and the higher values of hazel may suggest that the turf was removed from closer to a field boundary or woodland edge.”
The study of plant macrofossils involves breaking these peaty deposits down, sieving the content in graded sieves, and examining the residues.
While the sample from the lowest layer , the probable old ground surface, contained some sand and a little gravel along with charcoal, the layer above  produced sand, with charcoal from both trunk and branch up to 10mm. The report suggests that perhaps the turves came into contact with brushwood burnt during the land clearance for the Turf Wall. The remains of Ceratodon purpureus, Fire Moss, was also present in quantity, which as the name suggests is an early coloniser after forest fires, and among other things it commonly grows on is dead wood.
Having prejudged the nature of the material nothing quite makes sense, the “turf" is the dominated by pollen from trees species, and appears to lack lacks the range of inclusions that might derive from a top soil component, being dominated by charcoal fragments. Even given that the organic materials decompose, and that soil is often composed of finer particles which can leach away, a greater range and concentration of inclusions from a boulder clay top soil component might be expected.
A 3.6m high wall made of turf, 6m wide at the base, would require eighteen 20 cm layers of turf, which would require up a 100m wide strip turf, depending on how it sloped. A 3.6m pile of turves would decay into a significant large grassy mound, which presumably would have little requirement for the aid of mosses to facilitate regeneration.
The dominance of tree species in the pollen record, the abundance of charcoal, and the presence of fire moss are all consistent with the structure being the remains of a pile of wood.
I am now fairly sure that this is the remains of a timber structure, I think the "turf" is a red herring, and illustrates both the problem of naming things and the tendency of archaeologists to find what they are looking for. It was a long time before the postholes for the timber Wall were found, and I strongly suspect that there should be postholes in the west. To understand a structure like this would require a different type of excavation.
I have advanced three lines of argument which suggest a Wall made of Turf is probably erroneous; firstly, Caesar makes no use of it, fortifications were made of timber, secondly, it is not good military engineering; finally, the environmental evidence and the nature of the deposits themselves are not really consistent with them being simply the remains of turves.
Turf is a useful building material, and is used in Northern Europe particularly when no is better material is available, [i.e. timber] . The work at the Lunt  and Vindolanda  have demonstrated that large structures can be built with earth and turves using the minimum of wood; for experimental archaeology turf is cheap and easily sourced. However, these structures are not regularly subjected to violent assault, and a structure that is, in effect, a pile of soil, would be simple to undermine, and palisade type defences are easily pulled over, especially if planted in loose earth.
The Romans Army, whose lives could depend on the efficacy of such structures, were probably less constrained by cost or environmental impact; deforestation was a feature of the Roman occupation in the North , it was was good for security, and timber was still the principle building material.
The crucial point about the postholes of the eastern sector is that this structure was removed without any mess; it was clearly only ever conceived of as a temporary measure, yet another reason timber was used rather than turf or earth. I have previously suggested, as I am sure others may have also inferred, that the Turf Wall should have required layers of timber for structural stability alone.
In the final analysis, the precise method of construction does not matter, the recognition that this is the earliest phase in the west is more important, although without the Timber Wall in the East it makes no real strategic sense. The idea dislocation/s, break/s in wall building, presumably due to war, is now widely accepted, ; it provides a context for this Western temporary wall to become rotten or damaged, and what we see is now at Appletree is probably the remains of a pile of the wood from the rampart that could not be salvaged.
Sources and further reading
 Graafstal, Erik P.: 2012, Hadrian's haste: a priority programme for the Wall. Archaeologia Aeliana, 5th Series, vol 41, 123–84
http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0001 [Accessed 25/12/2014],
C. Julius Caesar, Commentaries on the Civil War William Duncan, Ed.