witness mark
n.— «When recovering evidence for further observation, preservation of any dirt or grease mark on samples can be vital if critical evidence of its past history is not to be lost. Such trace evidence (witness marks) can help in reconstructing past movements of the sample, such as contact with older, perhaps mating parts.» —by Peter Rhys Lewis, Ken Reynolds, Colin Gagg Forensic Materials Engineering: Case Studies Sept. 29, 2003. (source: Double-Tongued Dictionary)
Further reading
Twice a day the River Thames recedes, revealing a muddy shoreline. Hobbyists known as mudlarks stroll the surface searching for objects that have found their way into the river over the centuries, everything from ancient Roman jewelry to modern...
She sells seashells by the seashore. Who is the she in this tongue twister? Some claim it’s the young Mary Anning, who went on to become a famous 19th-century British paleontologist. Dubious perhaps, but the story of her rise from seaside...