The longest span of time measured on the geologic time scale is the Precambrian Era (also called the Precambrian Eon). It represents the time between 4.55 billion years to about 544 million years ago, or about seven-eighths of the Earth’s history. This time period includes the beginning of the Earth’s formation, its cool-down, its crust’s formation, and, within the last billion years of the time period, the evolution of the first single-celled to multi-celled organisms. The demarcation of 544 million years ago represents a burst in the evolution of multi-celled organisms, including the first plant and animal species.

The geologic time scale is divided into epochs, ages, and other periods based on important historical events that radically changed life on Earth.

A “dip” is the angle at which a layer of rock or vein is inclined, while a “strike” is the angle made between the direction of true north and the direction of the planar feature, such as an incline or fault.