The artefact hunting group "Digging up the PASt" run by David Hutchings claims that it is rescuing evidence of the past from being lost due to redevelopment. On their "About us" page they demonstrate this with reference to the development "within the last few years" of a site in Crick in Northamptonshire on which was built the Daventry International Rail Freight Terminal. Here there was a settlement of the Bronze Age. They claim the site has been "lost forever" when the archaeological remains were built over and the site is now occupied by concreted areas and a number of warehouses which has led to the destruction of the "information available from such an important site". In such a situation, they argue, if the archaeologists are not going to rescue that information, metal detectorist hoiking is entirely justified.
What is the truth behind this tekkie horror story? About twenty-five seconds on Google reveals that, of course, the tekkies are spreading misinformation. What they mean is that they cannot get to the archaeological record of this site to plunder it for collectables, because its under concrete. But has the site been destroyed? Hardly. Almost at the top of the search results is a document the title of which made me mutter 'uh-oh', but then if you actually read a few pages into it, you realise that (if its true and representative of what actually went on during the construction of the site) that the tekkies either have not understood something, not checked something, or are deliberately telling their readers untruths.
David J. Leigh 2005, An Archaeological Watching Brief on Land at DIRFT Central Northamptonshire, March - may 2005', Northamptonshire Archaeology Report 05/086.
It turns out that over the whole site was a thick enough deposit of recent 'made ground', that the development did not penetrate the archaeological stratigraphy. The site has in effect not been destroyed, but been "Preserved in Situ" and will become available for investigation only some time in the future when the rail terminal (which will not last forever) is redeveloped. This the whole point of preservation of the archaeological resource, to keep as much as possible for future use (we have quite enough of a backlog of excavated material to be getting on with studying). Obviously the less some greedy metal detector waving archaeology-gobbling collectors have been at it before it is sealed, the better for the information recovery in the future. Let nobody kid themselves that the artefacts which detectorists are "preserving" will be available for study retaining their original findspot information by that time. Let nobody kid themselves even that the PAS database will exist as such by that time.
Its worth noting that there were also earlier excavations here:
Chapman, A, 1994, Excavation of Iron Age and Roman sites at the Daventry Rail Freight Terminal near Crick, Northamptonshire, 1994.
Birmingham University Field Archaeology Unit 1998 The excavation of an Iron Age Settlement at Covert Farm (DIRFT east), Crick, Northamptonshire: Post excavation and updated research design.
Pat Chapman 2004, 'Iron Age settlement and Romano-British enclosures at Coventry Road,
Hinckley, Leicestershire', Trans. Leicestershire Archaeol. and Hist. Soc., 78, pp. 35-82.
So basically, having metal detectorists hoover the site taking out as many of the collectable diagnostic finds as they can get their hands on is not a good thing when it damages the archaeological record, while producing nothing but a handful of random and decontextualised artefacts, for the most part lacking even the most basic record of where they came from.