Merging data sources And blocking out sections

[Jason R. Moore]

I would like to know the best way to merge data from different sources. 
For example I have a large over view 1:20000 map and several smaller 
areas of higher resolution what is the best way of overlying the two and 
not letting the lower resolution data show threw?
When you just stack the layers on top of each other you can see the 
lines on the lower resolution map meandering back en forth under the 
higher resolution data.

So is there a command to clear the extent of a layer or what is the best 
way to do this?

Jason