QUESTION:
What is the exact nature of the dark markings on Mars, such as Syrtis Major? 
Why are certain areas on Mars consistently darker? It doesn't seem to be
correlated with altitude or heavy cratering. The books say that the darker
areas may represent darker-colored bedrock, while the lighter-colored dust
gets blown around and changes the apparent shapes of the dark markings on a
seasonal basis. But why are they located where they are? What's the
underlying geology here?

ANSWER from Jeff Plescia on May 26, 1999:
To date, spectral data have shown that Mars is characterized by three
distinct provinces: bright red units probably composed of aeolian dust;
intermediate albedo red areas that may be altered aeolian material; and dark
areas which may be bedrock exposures or dune material.  The red color of the
planet is attributed to ferric iron alteration products of basalts.

As you note there is really no direct correlation between either geology,
surface age or altitude.  The simplest explanation would be that the wind
patterns simply shape the albedo features.

Syrtis Major is a volcanic area.  It is characterized by a broad area of
plains with wrinkle ridges and some volcanic flows.  There are two faulted
calderas presumed to be the source of the volcanic flows.  These are named
Nili Patera and Meroe Patera.  Presumably, from the spectral data and the
morphology, the volcanic rocks are basalts of some sort.  The feature is
Hesperian in age, which puts it in the middle of Mars' geologic history.

In the case of Syrtis, presumably the wind keeps the surface largely clear
of the fine grained dust that covers other parts of the planet.  There is
dust in the area.  Many of the craters have bright wind streaks that point
to the southwest, indicating that the wind is blowing from the northeast.