A close look at the edge reveals that the patch of "fur" is actually made up of the legs of thousands of harvestmen (daddy-long-legs). They appear to wedge their bodies against the rock face with their legs facing out. You can see how closely-packed in they are at the edge. Note the one at the top that has all its legs oriented in the same direction. I think that's how they pack in so closely together with just their legs sticking out. Who knows how they find each other--pheremones perhaps.

A close look at the edge reveals that the patch of "fur" is actually made up of the legs of thousands of harvestmen (daddy-long-legs). They appear to wedge their bodies against the rock face with their legs facing out. You can see how closely-packed in they are at the edge. Note the one at the top that has all its legs oriented in the same direction. I think that's how they pack in so closely together with just their legs sticking out. Who knows how they find each other--pheremones perhaps.