The city of Cottonmouth appears to be surrounded by several unnamed mountains. These mountains can be spotted in Downtown Cottonmouth, in the level Assassination, the studio back set in Altered State and from Blackwell Island in Best Friends. The landscape in Cottonmouth is possibly located in the middle or north part of the U.S.A. But the landscape is probably similar with landscape in the California and north-west states.
The mountains are very similarly to Liberty City hills (in the north part of Liberty City).