overshot flaking in stock

Overshot flaking has become a thing for me recently. An overshot flake is one that travels the complete width of the artefact, and considerably reduces depth in the process. It is a part of the European Solutrean, as well as the North American Clovis technologies. I currently have an abundance of large hard hammer flakes, and my understanding of platform preparation has improved. The result of these coinciding factors is that I have begun to be more consistent with overshot flakes. This is good!

And this is the ventral. I said in a previous post that I am not consistent with overshot flaking, but I was able to turn the proto-handaxe over and after some basic hard hammer platform isolation and preparation remove another one from the opposite face. Because I was working in this considered way I started to collect the removals in order, to understand better the relationship here between my theory and my practice.

The thinning process was working and so I carried on, slowly and systematically. Ultimately things didn’t go as planned and the remnant proto-handaxe is top left in the above photograph. Number 17 was the second overshot flake and 14, 12, 10 and 8 were all good removals. I still haven’t got my handaxe, but I have got a good refitting sequence.

For those of you not familiar with the Ice-Age Atlantic crossing hypothesis, basically the idea is that during the Last Glacial Maximum (at approx. 26.500-19.000 years ago) glacial ice build up in the north Atlantic allowed European populations to cross over to North America and to colonize this virgin territory. The archaeological “cultures” typically cited in relationship to this hypothesis are Clovis in North America and the Solutrean in Western Europe. In particular, apparent similarities in the manufacture of bifacial tools between these two archaeological entities, and their reported use of overshot flaking (flakes that travel across the face of a tool to remove part of the opposing margin) to thin bifaces, have been frequently used to argue for their being a connection between Western Europe and the first peopling of the Americas. A basic introduction to the debate can be found here

In recent years considerable genetic, archaeological and paleoanthropological evidence has accrued suggesting that the first human societies in North America came from Asia, not Europe. Never the less, this debate still continues in the academic arena and 2013/2014 has thus far been a particularly productive period for publications on the topic utilizing experimental research to discuss the merits of the Ice-Age Atlantic crossing hypothesis. Three recent publications are of particular interest for those interested in the role that experimental archaeology can play in major debates such as this. The first of these contains experimental data designed to test the effectiveness of overshot flaking in thinning bifaces:

Eren et al’s results suggest that overshot flakes are by products of a general biface thinning technique and in and of themselves are not very reliable or optimal at thinning bifaces. This conclusion seems to suggest that Solutrean and Clovis bifaces were produced using similar, simple, biface thinning techniques that resulted in occasional, accidental, overshot flakes. The article also contains a discussion of the existing archaeological evidence for overshot flaking in Clovis assemblages, not much, and the lack of comparable data in Solutrean assemblages.

Lohse, J.C., Collins, M.B., Bradley, B., 2014. Controlled overshot flaking: a response to Eren, Patten, O’Brien, and Meltzer. Lithic Technology 39, 46-54.

Eren, M.I., Patten, R.J., O’Brien, M.J., Meltzer, D.J., 2014. More on the rumour of “intentional overshot flaking” and the purported Ice-Age Atlantic crossing. Lithic Technology 39, 55-63.

Abstract Lohse, Collins, and Bradley ignore or misrepresent the arguments we have made concerning “controlled” overshot flaking and the purported Ice-Age Atlantic Crossing. Here, we summarize our previous work and explain again how it directly tests the explicit claims of Stanford and Bradley (2012; Bradley and Stanford 2004, 2006). We also correct the inaccuracies and false accusations of Lohse, Collins, and Bradley, and refute their belief that arguments should be rejected or accepted on the…Expand

Schematic illustration of the relationship between EPA-PD and flake size in profile view (redrawn from Dibble & Pelcin, 1995). The dotted lines represent the flaking outcome. a When the exterior platform angle (EPA) is held at a constant, increasing platform depth will result in larger flakes. b When platform depth (PD) is held at a constant, increasing exterior platform angle will result in larger flakes

It is worth noting that these earlier drop tower experiments varied striking force in different ways. Speth (1974), for instance, varied the drop height of the ball bearing, which in turn changed the travel distance/time of the hammer and hence speed at contact while holding hammer mass and morphology constant. On the other hand, Dibble and Whittaker (1981) held speed constant but changed the size of the ball bearing, which in turn altered the mass and size (i.e., diameter) of the hammer. Importantly, it is not clear whether these alternative approaches to changing force are experimentally equivalent. It is known, for instance, that hammer size impacts Hertzian cone characteristics (Fischer-Cripps, 2007; Frank & Lawn, 1967; Roesler, 1956), but how these characteristics may change flaking outcomes is still not well understood.

From an experimental perspective, one way to think about the speed component of force in stone knapping is to consider two extremes in the distinction between static and dynamic loading. The former represents a process whereby the pressure mounts slowly (slow hammer speed), such as in pressure flaking, while the latter represents a quick delivery (fast hammer speed) of pressure in direct hammer percussion. It is commonly noted in replicative flintknapping that flakes produced through direct hammer percussion versus pressure flaking share distinct differences in shape, such that pressure flaked flakes are said to be thinner, more elongated, and more evenly shaped (e.g., Mourre et al., 2010). However, direct percussion and pressure flaking are typically applied in very different contexts in flintknapping, and these actualistic experiments leave many important variables, especially ones that are related to platform preparation, uncontrolled.

Using the new mechanical flaking apparatus, the Dibble experiments revisited this issue of percussive force by using a load cell to record the exact amount of force necessary to remove a flake. Importantly, the high-resolution load cell data shows a picture in line with what was previously proposed by Dibble and Whittaker (1981) based on the drop tower experiments. Specifically, the load cell data show that force increases to the level required for flake detachment and then declines immediately once the flake is removed. Moreover, the force recorded by the load cell correlates tightly with flake mass irrespective of variation in other variables such as exterior platform angle, platform depth, angle of blow, and core surface morphology (Fig. 5). Thus, the minimum amount of force required to remove a flake is determined by the mass of the flake, which itself is a function of platform depth and exterior platform angle. Based on this observation, applying less than the minimally required force for a given combination of exterior platform angle and platform depth will not result in a successful flake removal. Likewise, exerting more force by hitting the core harder will not change the flake mass (Dibble & Pelcin, 1995). However, first, Van Peer (2021) used data from the Dibble experiments conducted with the machine Igor to argue that flake mass starts to increase at a slower rate relative to the striking force once the force surpasses a certain threshold. Second, we note that this particular conclusion, that the application of excessive force does not make a difference in the flaking outcome, is based more on the drop tower experiments than on the new experimental design because with the use of cylinder compressive loading, it was not possible to apply forces in excess of what was minimally required to remove a flake.

One of the main critiques of the controlled flaking experiments to date is the sole reliance on glass as the flaking medium. While the new experimental design introduced by Dibble and Rezek (2009) has greatly improved the resemblance of the experimental flakes to actual lithic artifacts, the use of glass cores still raises concerns regarding the applicability of the experimental results to flakes made of different raw material types, especially because glass is an amorphous solid, whereas other materials, such as chert, are primarily made of crystalline quartz. Raw material variability features prominently in explanations of archaeological stone tool variability, especially in relation to discussions of lithic technological organization in terms of the quality and supply of raw materials in a given region (Andrefsky, 1994). Thus, the sole reliance on glass among controlled experiments inhibits further considerations of cost–benefit analyses in stone flake production based on raw material variability. However, a number of replicative flintknapping studies have indicated that raw material differences, at least among relatively fine-grained stone types, have minimal to no effect on the general morphology of the detached flakes (Clarkson & Hiscock, 2011; Eren et al., 2014; Kimura, 2002). Thus, how various raw materials respond to the knapping process under controlled conditions was important to evaluate.

To address this, Dogandžić et al. (2020) compared the glass results to flakes made on three other raw material types (basalt, flint, obsidian). All cores were made to an identical form that features a central ridge and longitudinal convexity. The results show that, in nearly every comparison of flake volume and linear dimensions, the flakes show no discernable variation among the four raw material types. Instead, the experimental flakes exhibit the same EPA-PD relationship whereby increasing either platform parameter causes flake size to rise in almost identical ways (Dogandžić et al., 2020) (see also Fig. 6). The only measurable difference among the flaking outcomes of different raw material types is in the amount of force required for successful flake detachment—more force is required to remove flakes from flint and basalt cores than from the glass or obsidian cores. The results demonstrate that the general flaking patterns observed in glass can indeed be extended to some raw materials that were commonly used in the past (Dogandžić et al., 2020). Of course, there are likely many other raw material types that may respond differently to fracture than those tested in this study, especially those that are more heterogeneous, such as, for example, porphyry (Namen et al., 2022). However, given the consistency in flake formation across multiple stone types as shown in Dogandžić et al. (2020), it is more reasonable to assert that the same fundamental fracture patterns also apply to less amorphous raw materials, though the relationship may be more varied or “noisy” due to heterogeneities such as flaws and inclusions.

At the ontset of the new experimental program, Dibble already knew that the combination of EPA-PD was a strong determining factor in flaking, an observation that was later confirmed and strengthened with subsequent experimental outcomes. At the same time, however, Dibble also knew that, while the EPA-PD model worked well on plane, symmetrical, unobstructed, and unmodified platforms, platform preparation (e.g., trimming behind the platform, faceting the platform surface, isolating the platform, etc.) altered the EPA-PD model in ways that were not all understood. For instance, previous controlled experiments (Pelcin, 1996) on plate glass tested the influence of different platform bevels (material removed from behind or from the sides of the platform) and found that flakes with beveled platforms are generally longer than those made with unbeveled platforms while showing no clear difference in mass. In these earlier experiments, the bevel morphologies also changed the bulb thickness. To explain this finding, Pelcin (1996) proposed that platform beveling likely causes flake mass to be redistributed across the core morphology to produce longer flakes without changing the overall flake mass.

Given that one of the guiding principles of the Dibble experiments was to understand the ways in which knappers can control flaking outcomes (as opposed to creating a general model of fracture mechanics) and given that platform preparation is an important aspect of flake variability in the archaeological record, an experiment was designed to investigate the effect of platform beveling on the EPA-PD model’s ability to predict flake mass (Leader et al., 2017). In this experiment, the platform surface was beveled in three ways: flat exterior bevel, concave exterior bevel, and lateral bevel (Fig. 7). The flat bevel was thought to simulate trimming or thinning the core’s exploitation surface from the platform (Fig. 7a). The concave bevel represented striking a flake from a core immediately behind a previously struck flake, meaning that the platform surface curves inward from the scar of the previous flake (Fig. 7b). The lateral bevel, on the other hand, was meant to represent uneven (i.e. not flat) platform surfaces, including the faceted types where the point of percussion is intentionally raised and isolated by removing material from either side (Fig. 7c–e). For this bevel type, three different cuts were used at angles of 30, 45, and 60° (see Fig. 7c–e). Each of these bevel types is found in the archaeological record and is thought to represent techniques used to control the flaking outcome. The impact of beveling could be compared across bevel types and to the same cores without bevels.

Replicative knapping experiments have shown that hard versus soft hammers can affect various flake attributes, including flake mass, linear flake dimensions, platform attributes, flake initiation, and flake termination (Bradbury & Carr, 1995; Buchanan et al., 2016; Damlien, 2015; Driscoll & García-Rojas, 2014; Schindler & Koch, 2012). These differences in flaking outcomes are often attributed to differences in force propagation mechanisms. For hard hammer percussion, it is often said that a conchoidal fracture takes place where the force propagates directly from the point of percussion to the termination (Cotterell & Kamminga, 1987). This process results in a clear point of initiation and a well-formed bulb of percussion. On the other hand, a bending fracture is often said to take place with soft hammer percussion (Cotterell & Kamminga, 1987), where the fracture initiates some distance away from the point of hammer contact, leading to flakes having a more diffused bulb, a smaller platform surface and a pronounced “lip” on the interior platform edge. In fact, platform lipping in particular is often used as an indicator of soft hammer percussion in the analysis of archaeological stone tools (Hayden & Hutchings, 1989; Sharon & Goren-Inbar, 1999; Sullivan & Rozen, 1985). However, prior controlled experiments by Bonnichsen (1977) and Pelcin (1997b) showed no clear differences in the occurrence of platform lipping between hard and soft hammer percussion. Pelcin (1997b) postulated that this discrepancy could be because human knappers tend to change percussion techniques, either consciously or unconsciously, when switching between hard and soft hammers (Hayden & Hutchings, 1989). As such, the presence of flake features such as platform lipping may be related to knapping factors other than hammer type.