Although iron bearing rock all across the northern hemisphere does produce natural deposits of iron ore at surface bogs, exactly where you might find a viable deposit is not so simple to discover. In the simplest terms, the chain of production is this:
1) The under laying bed rock needs to contain quantities of elemental iron.
2) The local geography needs to have areas flat enough to allow the formation of relatively stagnant pools of standing water.
3) The conditions need to promote the formation of bogs - with rotting vegetation producing low concentrations of tannic acid in those pools.
This allows the acidic water in the bogs to leach out the iron from the under laying water. The water will have a distinctive brownish red colour, looking like weak tea (this primarily from the tannic acid content). There will be a distinctive 'oil slick' layer that appears on the surface of the water along the margins or in smaller puddles around the larger main pools. This effect is caused by small amounts of iron, at the level of individual molecules, that has oxidized to Fe2O3 in contact with the surface air.
This all is sure sign that a viable deposit of primary bog iron ore may be nearby.
Small amounts of this surface oxidized material may precipitate out, forming a kind of 'pudding' like layer of Fe2O3. As small amounts, this material is very hard to gather in any useful quantity, as it contains a large amount of water. Left for long enough, especially as it would fall and collect into thicker mats at the bottom of pools or lakes, certainly this can form useful deposits. The 'lake ores' gathered in Sweden is just this type of deposit. This resource would be harvested using wide rakes mounted on long poles worked from boats historically.
To find a working deposit of primary bog iron ore, you need to find a small brook that tumbles downwards from the source bog. Once again a specific geography comes to play. The tumbling of the brook over stones will inject oxygen into the stagnant bog water, thus creating molecules of Fe2O3.
Now comes the strange (and not entirely understood) part. There is a specific bacteria that is anaerobic, (oxygen hating) which lives along the boundary layer between the vegetation layer on the surface and the sterile subsoil. The bacteria absorbs the water as part of its life cycle, but considers the iron oxide as a poison. So the iron oxide is deposited, grain by tiny grain, in small sacks (called vacuoles) in its body. When the bacterial dies, these grains are left behind. The bacteria lives in colonies, so over years lumps of Fe2O3 are gathered and left behind in lumps under this action.
As the stream course wanders back and forth, these lumps are exposed along the margins. There is some relationship between the location of the source bog, the amount of elevation drop and size and action of the individual stream. (And sorry, I can't give you more than a very vague gut call on this. At this point I just have not had enough experience successfully gathering bog ore in the field.)
The normal way to find a deposit of primary bog iron ore is to first examine the potential source bog for the indicators. Then you work downstream from the bog along the draining stream. Every so often (suggest about every 5 metres or so) you reach down along the side of the bank, feeling into the boundary between the top layer and the sterile gravel underneath. Primary bog iron ore can vary in size and colour. The deposit in the image above is an obvious rust red colour, the individual particles about the size of small peas. Individual pieces at other locations have ranged up to the size and shape of a thumb, even as large as fist size. Small amounts of other metallic oxides can vary the colour of the material considerably. The ore found along Black Duck Brook for example contains traces of manganese, which turns it a dark black colour.
The trick to all this is of course finding a large enough concentration and large enough volume of bog ore that it can be usefully gathered in quantities required for an actual iron smelt.
In 2001, the location where these images where taken yielded about a five gallon pail full of primary bog ore, when washed and dried roughly *** kg total. The image above was taken at almost exactly the same place as I had gathered over a decade earlier.
The bad news is that there has been a 'rune stone' 'discovered' further up the rough trail leading to the upland source bog here. The curious have torn up the surface of the bog and ripped up the natural stream course. To make matters worse into the future, the local tourism board has decided to install a permanent gravel trail bed with drainage culverts up into the area. This all will have the result of pretty much destroying the natural environment that allows for the chain of elements needed for bog iron ore creation.
Regular readers have noticed a huge gap in postings of late. After getting home from the DARC at LAM 2012 adventure, I embarked on a problem plagued 8 days to the Goderich Celtic College and Festival (where I still am as I post this). More regular postings should resume in a week or so!