Dating Cave Art
Developing · Paraphrase rõ nhưng distractor gần nghĩa hơn.
A Some cave paintings cannot be dated directly because their pigments contain too little suitable organic material. A mineral layer that formed above or below an image can instead provide a boundary on its age. B The enlarged section shows a tiny sampling area rather than an entire cave wall. Conservators first confirm that the mineral deposit has a clear relationship to the pigment and that removing a sample will cause minimal damage. C A micro-drill approaches the surface and removes powder from a calcite crust. Because that crust lies over the image, its formation is younger than the pigment beneath it. D The pigment layer rests directly on the limestone wall. Powder from the crust is transferred into a sample vial for uranium-series measurement, while the painted layer itself remains in place. E Analysts measure uranium and its decay products in the mineral powder. Contamination, movement of uranium after formation or mixing material from several growth phases can weaken the resulting age estimate. F A date from the upper crust gives a minimum age for the art, not the exact day when it was painted. Several carefully documented samples are preferable to a single number with an uncertain physical relationship to the image.