Which of the following is NOT one of the four major geologic time periods?

The Quaternary is indeed a geologic time period, but it is a subdivision of the Cenozoic era, making it part of the broader geological timeline. The four major geologic time periods usually referenced are the Paleozoic, Mesozoic, and Cenozoic, with each containing several smaller periods within them.

The Paleozoic era lasted from approximately 541 to 252 million years ago and is characterized by the emergence of a diverse range of life forms, including the first amphibians and reptiles. The Mesozoic era, which lasted from about 252 to 66 million years ago, is often known as the "Age of Dinosaurs," due to the dominance of these creatures during this period. The Cenozoic era, beginning around 66 million years ago and continuing to the present, includes the rise of mammals and birds, and significant changes in global climate and ecology.

Understanding this hierarchy of geological time helps to appreciate how various periods relate to one another and how the Quaternary is an essential part of the Cenozoic era, but not one of the four major periods on its own.