Soil Mechanics/Engineering geology
Geology is the foundation of soil mechanics. Without a proper understanding of the underlying geologic processes that have formed the particular soil you have to deal with, a soils or geotechnical engineer will be playing with fire. This is where a well trained engineering geologist is so vitally important. They can help other people involved in the design and construction of a facility or project understand the geologic antecedents of the materials they have to use.