The term 'span of epoch proportions' refers to the length of time that a particular geological period or era lasts. The crossword solution for this term is EON, which is an abbreviation for an 'EON, ' which is the longest division of geological time. An EON is typically divided into eras, and each era is further divided into periods, epochs, and ages. The span of epoch proportions, then, is the length of time that a particular epoch lasts.