Ask Greg: 'Invisible' Archaeology




Greg Speed

Finding archaeological features isn’t as easy as they make out on TV! In Britain it’s rarely as simple as taking off the topsoil and seeing the outline of a stone building. We usually have to settle for trying to spot the patch of mud that’s a slightly different shade of brown from the surrounding brown mud – and that’s the easy bit. Sometimes the archaeological features are exactly the same colour as the surrounding ‘natural’.

Creating what’s known in the trade as a ‘cut feature’ often consisted of someone in the past digging a hole, putting something (cess, rubbish, a dead body or, occasionally, treasure) in it then putting back the same stuff that they had dug out. Sometimes some of the topsoil might end up mixed into the fill of the feature. However, organic soils are prone to decay and leaching over the millennia, particularly on well-draining subsoils like gravel, leaving only the mineral component derived from (and looking exactly like) the underlying ‘natural’.

With the gravel example, we archaeologists then come along and take away the modern topsoil with a mechanical excavator. We’re left with an expanse of gravel. Features such as ditches which filled up with water-borne or wind-blown silts different from the natural may be obvious and easy to dig. Other features which were deliberately backfilled with gravel might show up if either the buried material is visible (prehistoric pits often have lots of charcoal in them and show up as black circles), or where organic materials have decayed and subsided, allowing the resulting hollow at the surface to silt naturally. Within the constraints of a commercial excavation, these are the types of features most likely to get dug and described in reports, but what about the others?

Following the gravel theme (other subsoils present different challenges), here are a few examples from projects I’ve been involved in at Catterick and Scorton in North Yorkshire. The terraces to either side of the River Swale in this area comprise either ‘dirty’ mixed gravel and clay, or very clean periglacial sand and gravel left as the last ice sheet in the area melted.

We’ve been lucky enough over the years to excavate two fairly large cemeteries in the area, at Hollow Banks and Bainesse. On both sites, finding the burials in ‘dirty’ gravel was an absolute nightmare. At Hollow Banks, none of the graves were initially visible but since we weren’t expecting any that wasn’t a problem! The first undisturbed burial was found as a result of realising our site metal-detectorist had, on different days, found an Anglo-Saxon spearhead and part of a shield boss only 30cm apart, so I did some measuring out on site (no GPS back then) and got someone to dig a hole, which turned out to have a skeleton and the rest of the shield boss in it. Nearby, some potsherds were poking out of the surface.

After a week, a vague outline gradually darkened around them – another grave! Since there was other stuff to do, we left it alone and as the days passed more darker shapes gradually appeared in a line. A second string of features revealed themselves over the next two weeks. Unfortunately, this leave-it-and-see-what-happens approach can rarely be taken for most short-duration commercial excavations. In gaps in the distribution we dug a hole anyway and often found the ‘missing’ grave. As a last resort, we got the machine back in and stripped more off the site and started again from a lower level, and we eventually found 120 burials, a considerable advance from the ‘none’ visible on day one. This turned out to be vital experience when we came to excavate the nearby Bainesse Roman cemetery 16 years later during the recent widening of the A1 (Link here).

On the ‘clean’ gravel in the area, there is the danger that you strip off the topsoil, see nothing and walk away. Gravel-filled features that are cut into gravel generally don’t show up well on aerial photographs or geophysical survey; they can usually only be found by excavation, and then only with extreme difficulty. Here are two examples to demonstrate what can be achieved if enough time is spent on the challenge. A few years back, we were lucky enough to dig part of the Scorton Cursus in advance of quarrying. The Neolithic monument basically consisted of two parallel ditches with a bank in the middle. We dug the customary sections across the ditches (which were visible before excavation because they had silty fills), but one of my colleagues wasn’t happy with the gravel edges of their bit. I trusted them to carry on tinkering with it, pebble by pebble, for a few days. Good move! It turns out that, on one side of the cursus, its construction phase 1 appears to have been excavation of a very large ditch, which for whatever reason  the original excavators almost immediately filled in with the same gravel and then they dug the ‘normal’ cursus ditch along the top. We tested this sequence across the site with a few machine-sections and the earlier ditch seemed to be extensive. It’s quite likely that, being backfilled with clean gravel, it has been missed on earlier excavations.

On the same site there were some palisade trenches (construction cuts for rows of posts). We dug lots of sections and did lots of recording, but something never seemed quite right with them. There were no post-settings for a start. Each excavated section ended up as a shallow, steep-sided trench with uniform gravel sides and a flat gravel base. I spent a long time staring at them, recleaning bits and generally poking about. As a last resort, in one section I started tapping the bottom with the handle of my trowel. One spot sounded slightly different. Since the section had already been recorded, I used my supervisor’s prerogative and dug a hole. Digging largely by feel, the looser gravel eventually came back to a slightly more solid vertical edge. Occasional flatter stones stood vertical against the edges and there was some dark staining on the gravel sides. Twenty minutes later I had a cylindrical post-pipe! More tapping eventually found a whole row of post-pipes (the hole left when a post rots in the ground). Subsequent excavation showed that the ‘base’ of the palisade trench (which was actually twice as deep) was merely the top of gravel that had been packed around the posts. So, a palisade trench had been cut in clean gravel, backfilled with clean gravel and with post-pipes, into which clean gravel had fallen as the posts rotted; the only real difference between each fill was the varying compaction of the gravel. That’s about as challenging as excavation gets! Again, we were able to replicate this process on a different palisade trench and in a series of machine-cut sections.

Common threads between these examples are that (1) in archaeology you can’t beat experience, and (2) when you can’t see the archaeology, make a best guess and try digging a hole. Or, occasionally, just digging a hole at random can produce fantastic results (see our Aldin Grange project page). It’s surprising how often this works, but maybe I’ve just been lucky!


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