Answer: They are geographically widespread
Explanation: Markers of geological time refers to indicators or guide employed in the estimation and depiction of the geological time a particular rock deposit or event occurred. The markers above, the volcanic ash are particles ejected during volcanic eruption which involves the flow of very hot lava to the earth surface which cools and solidifies very rapidly. and index fossils are usually good markers of geological time because the are geographically widespread. This characteristic ensures that mapping deposits over a large geographic regions is possible due to the occurrence of these markers over a broad geographic region. Where markers do not posses wide geographic spread, it becomes very difficult to map. In addition to the above characteristics, both markers are also formed over a short period of time.