杏吧原创

I live near Lake Clifton in Western Australia, home of these rare round thrombolites (see photo). I have always wondered what the grey 鈥渞ocks鈥� in between the thrombolites are. The one in the centre of the photo has the appearance of a fracture face on a piece of shale. I clambered down to get one of them only to find they were soft and almost jelly-like in consistency. They were just about floating, bobbing with the water action, although some were submerged. Can someone identify them?

鈥� The grey blobs in the photograph certainly look like lumps of grey shale. Shale can be more than 20 per cent porous in some cases, and this allows it to soak up water, particularly when the rock is fractured. Extended immersion can lead to the shale becoming saturated, and eventually it crumbles to leave a pile of mud particles on the lake or seabed. Salt water appears to exacerbate the process.

This explains why muddy coastlines, like those around Lyme Regis in the UK, are not choked by lumps of shale that have fallen into the sea during landslides. Instead the chunks of shale are quickly broken down into fine particles and washed out to deeper waters, where they are deposited as mud on the seabed.

Jon Noad, Calgary, Alberta, Canada

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This article appeared in print under the headline 鈥淏lob of the gaps鈥�