The term GEOLOGIST'S SPAN is composed of two words: 'GEOLOGIST' and 'SPAN'. A GEOLOGIST is a scientist who studies the Earth's physical structure and history. SPAN is a noun that refers to a period of time, usually a long one. Therefore, GEOLOGIST'S SPAN can be interpreted as the length of time a GEOLOGIST studies the Earth. In this case, the crossword solution for GEOLOGIST'S SPAN is ERA, which is an abbreviation for eon, epoch, or age. An eon is the longest division of geological time, while an epoch is a shorter division of geological time, and an age is the shortest division of geological time. Therefore, ERA can refer to any of these three divisions of geological time, depending on the context.