December 2014

Hot feet?

This footprint was discovered in Tully, north Queensland about 52 years ago.

I am curious regarding your thoughts about this footprint and your explanation as to what you think it could be.

Answer

a foot shaped imprint on a granite rock a foot shaped imprint on a granite rock with shoe

Although the structure in your photos looks like a footprint, it is not.  The primary reason is that the surrounding rock is a form of granite. Granites are a type of igneous rock that forms underground as magma slowly cools (rocks that form in this manner are called plutonic or intrusive rocks). In other words, the structure has formed in a place that is not only inhospitable for feet, it’s also completely inaccessible. The ‘footprint’ is composed of a different type of rock, which is eroding slightly faster than the surrounding granite. This means that it has lower relief, creating the impression that it is sunk into the rock. Plutonic rocks sometimes contain chunks of other rocks which are called xenoliths. These can be fragments of the rock that the magma was breaking through, or parts of the magma that cooled earlier. In either case, xenoliths tend to be obviously different to the surrounding plutonic rock. If the xenolith is of different hardness to the enclosing granite, it will erode at a different rate. It looks to me like your footprint is a xenolith that just happens to resemble the shape of a foot.

Here’s how to recognise fossil footprints: they are generally found in sedimentary rocks, particularly those that formed on land or in shallow water. Footprints are usually part of a larger trackway, so a large exposure of rock should contain multiple footprints. Finally, footprints will be within a single layer (‘bedding plane’) in the rock; they won’t be at an angle across layers.

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