I came across this the other day -- an erratic boulder in a field near here, broken during the process of field clearance with a large hydraulic digger. Click to enlarge.
The weathered layer penetrates several centimetres in towards the core the boulder -- first a "skin" of foxy red rock altered by oxidation -- a process that needs both oxygen and acidic rainwater or groundwater. Then there is a whitish layer which is even thicker -- I must be honest and say that I'm not sure what process is responsible for that, except to say that it will be a chemical process of some sort. Then the unaltered rock is at the core of the boulder -- I think it's a fine-grained grey dolerite, but the wavy "micro-terrain" on the boulder surface makes me a bit unsure of that diagnosis!
Note that the weathering penetrated deeper on the angles or sharp edges of the boulder, as yopu would expect since chemical penetration is coming from two directions on those corners.
This sort of weathering only occurs when a boulder is exposed at the ground surface, and this one must have been exposed to chemical weathering attack from all sides -- if one of these faces had been embedded into the ground there would be less weathering on that face.
This gives us one reason why cosmogenic dating is so difficult. Imagine a boulder like this being picked up and transported by a glacier, with some parts of it being eroded away during transport. You would then get a false age from your sample, since it would have an "inherited" age different from the apparent age found on other faces of the same rock.
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