One of the fundamental tasks of historical geology is dating rocks and putting geologic events in their proper order. Geologists have been putting relative ages on rocks since the 1700’s, using basic stratigraphic principles such as superposition, cross-cutting relationships, inclusions, and original horizontality. These principles, along with fossil succession, were used to create the geologic time scale long before we knew the absolute age of the Earth. Beginning in the mid-20th century, absolute dating techniques were applied to assign actual ages in years to rocks. It is important to recall, however, that only rocks that form directly from cooling magma or recrystallized metamorphic rocks can be dated. Thus, even today we must combine relative with absolute dating techniques to reconstruct geologic history.