ABSTRACT
Geological time is an important concept. Earth is 4000M years old and has evolved continuously towards its present form. Most rocks encountered by engineers are 10-500M years old. They have been displaced and deformed over time, and some are then exposed at the surface by erosional removal of rocks that once lay above them. Underground structures and the ground surface have evolved steadily through geological time. Most surface landforms visible today have been carved out by erosion within the last few million years, while older landforms have been destroyed. This time difference is important: the origin of the rocks at the surface may bear no relationship to the present environment. The classic example is Mt Everest, whose summit is limestone, formed in a sea 300M years ago. Geological time is difficult to comprehend but it must be accepted as the time gaps account for many of the contrasts in ground conditions.
