Not all the features of bogs and marshes are the same. It will depend on the amount (if any) of recent rainfall, depth, consistency etc. In this example, the surface looked totally benign, otherwise I would not have trodden where I did. It is worth using a trekking pole or similar to test ground of which you are uncertain. The incident reminded me of the tragedy which occurred to fell-runner Bill Smith on Saddle Fell, Bowland in September 2011 - he died in a peat bog.