PBA, just to take a shot at the iron question, iron is one of the most abundant elements in the Earth's crust (consider Earth's iron core). Although abundant, iron is usually locked up in minerals (compounds) or in ores (mixtures of compounds that form rocks). When groundwater flows through the ground in an aquifer, it will often pick up iron and carry it to emerge in a spring. When iron is reduced (the charge of the iron ion is changed), it will form a compound that is insoluble with water, making it visible to the naked eye as a precipitate. Bacteria accelerate the deposition process by basically aggregating (not using the right word, I know) the precipitate because they use the energy from the reaction to convert carbon dioxide to carbohydrates much the same way that plants do.
Now for a shot in the dark, I believe that the pine barrens iron deposition is heavier in general than other areas because when organic matter breaks down, the pH decreases (it becomes more acidic) and this increases the availability of iron or other metals. I may be totally off with this one so don't take it as gospel. Also, another guess regarding the Cedar Creek question may be that there aren't as many springs that are feeding the iron that is picked up as water moves through the aquifer (or maybe the geology of the aquifer is such that the water does not move over a large area of iron-rich sediment?). That being said, I think that there is also an element of flushing because the Cedar Creek seems to move fast, plus it is a very gravelly bottom right? Maybe the lack of mud or smaller particles for the iron to 'cling' to adds to the flushing effect.
Hopefully at least some of this is correct