Geologic Contact 1

[jafcon1]

When I look at a Geologic Map and there are areas marked as Geologic Contact, what am I looking at?

 

[Mccoy]

jafcon1,
a geologic contact is simply a boundary between different geologic formations, hence a surface where some lithological change, more or less transitional, is expected.
The intersections of the boundary surface(s)with the topographic surface are mapped as lines, which trend is a function of the geometric configuration of the intersecting planes.

For more basic concepts you may want to have a look at the following link:

http://dept.kent.edu/geology/ehlab/fundamentals/fundamentals.htm