Thank you for your comments. An important part of this research has been to try and eliminate all the possible natural causes that could have created the hollows, both contemporaneous with sediment deposition and in the short time following re-exposure of the surface.

Features formed by sediment erosion and deposition in estuarine environments are well documented [1, 2]. The features observed at Happisburgh do not conform to those described in the literature, which suggests that they are not related to the sedimentary processes taking place in the estuary at the time of deposition. Depositional sedimentary structures are also visible in these sediments and are similarly preserved on exposed surfaces; these are clearly distinguishable from the features observed at Happisburgh that we consider to be footprints.

The re-exposure of the surface by recent coastal erosion may have led to modification of its morphology over repeated tidal cycles. This has resulted in episodic deposition and removal of beach sand from the surface, and this indicated that the features are not related to depositional processes on the present beach, and they are unrelated to any form of directional patterning associated with current-orientated erosion on the beach. The main impact of present-day processes has been to degrade the morphology of the surface. This was observed over the two week period from their initial discovery to final eradication.

Our conclusion was therefore that these were not naturally eroded, but more likely to have been caused by compaction. The range of sizes of most of the hollows together with the mean length to width ratio of approximately 2:1 suggested that many, if not all of, the hollows were produced by humans walking or standing in the area. On the final visit to the site, a block of sediment was removed containing a heavily eroded print. This is currently undergoing CT scanning at Southampton University and will be the subject of a further paper. Initial results indicate that some form of compression of the sediments is causing the hollows, which is consistent with footprints.

One of the difficulties of recording the surface was the short length of time between initial discovery and destruction by the sea. As each high tide was eroding the surface and it was already partially eroded when recorded, it is possible that not all footprints were necessarily complete, with the likelihood that sometimes only the deeper parts of the print have survived. This could potentially leave just the front and/or heel of the print, which may explain the less elongated hollows. An additional factor is the probable overprinting of some footprints by others, potentially truncating what would have otherwise been clearer prints. In the case where toes seem to be identifiable it has been commented that only four toes are visible. This is probably due to the small toe leaving less of an impression. Only four visible toes are also a feature of the well-documented footprint FU18 from Ileret in Kenya [3].

Erosion and overprinting have also contributed to the difficulty of recognising any clear trails, although one of these might be discernible from prints 12, 9 and 8, while prints 22, 23 and 24 and prints 29, 30 and 31 cluster into two groups with similar size and orientation, possibly representing two individuals (Figure 8). The absence of linear track or trails is consistent with the behaviour of a group of people, particularly with children, who may have paused at this location. This would result in a more chaotic arrangement of footprints rather than the linear patterns created by uni-directional travel. The arrangement and post-depositional alteration to footprint surfaces is well discussed in Marty et al. [4].

The less than ideal conditions under which recording took place meant that the full impressions were not always shown to best effect, due to residual sand and water. This has made the photogrammetry images less clear than would be ideally desired, but we are currently working with new software to try and provide clearer results. The residual sand and water has also made the use of 3D morphometrics problematic, although we will be working on this aspect of the research over the next few months. However, further processing of the imaging data has now allowed the creation of improved 3D models and 3D prints of Footprints 8 and 39 and images of these are now available on the British Museum website [5]. Both footprints show the unique morphology of a human foot: a heel impression, a longitudinal arch, the ball of the foot, the angled front of the foot and in one case the impression of toes. Although in its early stages, the functional analysis of the footprints is under way, and the 3D models investigated so far are consistent with other hominin footprints recorded in Africa and Europe.

This exposure of prints was completely unexpected and came at a time when local conditions were particularly unfavourable for their recording and preservation. If and when more prints appear, we will be able to build on our experience with the first exposure to document and preserve them to the best extent possible.

1. Reineck HE, Singh IB (1973) Depositional Sedimentary Environments. New York: Springer-Verlag.

2. Chakrabarti I (2005) Sedimentary structures of tidal flats: A journey from coast to inner estuarine region of eastern India. Journal of Earth System Science 114: 353–368.

3. Bennett MR, Harris JWK, Richmond BG, Braun DR, Mbua E, et al. (2009) Early hominin foot morphology based on 1.5-million-year-old footprints from Ileret, Kenya.

4. Marty A, Strasser, A, Meyer CA (2009) Formation and Taphonomy of Human Footprints in Microbial Mats of Present-Day Tidal-flat Environments: Implications for the Study of Fossil Footprints. Ichnos 16, 127-142.

5. http://www.britishmuseum....