Bog iron is, literally, iron from a bog. You might well have seen traces of bog ore yourself when walking near wetland environments, although it can often look like pollution.

An extrusion of red-orange mud by a stream leaks what appears to be oil or petrol into the water, producing an orange sludge in the bottom of the stream and a reflective and refractive scum on top of the water.

The way to tell whether the scum is petrol/oil pollution or iron is by taking a stick and breaking the scum. If it’s petrol/oil, the scum will flow back together and reform. If it is iron, the scum breaks but does not readily reform. It’s as if you’ve broken a thin skin of floating iron, which is exactly what has happened.