From Dinwar, in the comments:

As for what it means to think like a geologist….it’s complicated. There definitely is a particular way of thinking unique to geologists. I’m convinced that it’s something you’re either born with or not; training just finishes what you started. Engineers and geologists think VERY differently, in nearly incompatible ways, which is fun because we work together all the time.

The main thing is, geologists think in terms of the context of deep time. We view everything from the perspective of millions of years, minimum. When a geologist looks at a stream they see the depositional zones, the erosional zones, the flood plane–and they are thinking both how the local geology affected it and how the stream will look in five million years. (As an aside, you get really strange looks when you discuss this with your eight-year-old son at a park.) And I do mean EVERYTHING. I remember drinking some loose-leaf tea once, adding the tea to the cup then the water, and realizing as the leaves settled that the high surface-area-to-volume ratio combined with cell damage from desiccation made them get water-logged very quickly, allowing for certain flood deposits to form. I’d always been curious about that.

Another thing to remember is that geologists by definition are polymaths. You can’t be a third-rate geologist unless you have a deep understanding of physics, chemistry, biology, anatomy, fluid dynamics, engineering, astronomy, and a host of other fields. Geology is what you get when those fields overlap. I learned as much about brachiopod anatomy from a structural geologist as I did from any paleontologist, and my minerology class started with “Here’s the nuclear physics of stellar evolution.” We’re expected to know drilling and surveying and cartography and…well, pretty much anything that could possibly affect dirt.

Ultimately, since we are dealing with historical sciences, we are detectives. We examine clues, make hypotheses, and look for evidence to support or refute them (for a fantastic discussion of this find the paper “Strong Inference”–that’s held as an ideal for geologic thinking). Like any scientist we look for subtle things, things that have a bearing on our particular field of study. I’m convinced, for example, that the soil in one area I work in has two distinct layers: a loose, fluffy depositional layer of clay, and a more firm layer of clay derived from the limestone bedrock dissolving. This is due to subtle variations in firmness, moisture content, color, whether or not limestone pieces are in the material, etc.–stuff that most people don’t notice. It’s no special ability on my part–my mother notices things about the weave of cloth that are invisible to me, because she makes the stuff. It’s all training. But the desire to look for it? That’s personality.

Field geologists are even worse–we do all that, only in conditions that would make any sane person run screaming. We’re expected to be athletes, MacGyver, scientists, managers, and Les Stroud all rolled into one. On bad days we add combat medic to the list. Hiking on a broken leg isn’t considered an unreasonable expectation (bear in mind I’m talking about the geologists–my safety manager would be VERY cranky to hear about someone doing that!). People who do this sort of thing routinely view the world in slightly different ways from most ordinary people. Most geologists go through a course called Field Camp, which is an introduction to field work. Walk into any geology department that has this and you can tell who’s gone through the class and who hasn’t.