To be a professional in your area of specialization, one has to not only understand his/her job, but also understand the elements involved in that job, and how changes in time affect them.
Civil engineering deals with the designing, construction and maintenance of roads, among other structures such as dams, tunnels, bridges, etc.
To be properly informed about this job, a civil engineer has to understand and study the earth, the mountains, and the general topography of his work space, i.e. land.
Geology is the subject which treats all of those, and more.