What causes this?

[Stonehopper]

Not much going on - so . . .

Snow and ice can make some impressive sculptures and patterns, but metal?

This is a wheel bearing race from the front of the GS. It was replaced several years ago, but the fascination is in the location, and formation of the indents. It's as though they were engraved with a tool, yet they were made by the passing over of the ball bearings.

It's called brinelling, but this example shows only two indents in the entire race.

Closest I have come across is this image on the net:
https://duckduckgo.com/?q=brinelling&t= ... 1372916295

[aerodynamica]

I had the same on a CX front wheel bearing race. Gave a regular rhythmic noise that increased with speed but there was no 'play' in the wheel as would often be seen in a dead bearing.

[CitroJim]

Last time I had that was on an MG Metro rear wheel bearing.. The car was almost new and very low mileage... I put it down to a small imperfection in the race heat-treatment or a flaw in the steel that prevented heat treatment working correctly...

Of all the Citroen and Peugeot wheel bearings I've replaced across many Xantias, XMs, 205s, Saxos and AXs I've not seen this, the bearings having failed simply through the grease generally drying out and hardening into lumps... The balls and tracks looked superficially OK and likely with a clean-up and new grease they could have done another turn - had they been amenable to sensible disassembly rather than being forcibly popped apart during their pressing out of their housings...

[Stonehopper]

Thank you gents. Two replies on consecutive days!

It does seem to be a strange anomaly. No play was detectable in the wheel bearing, though it would growl on right hand turns. Long since replaced.