From Halopedia, the Halo wiki
Terminal Moraine is a snow-covered 2v2 map, with the two teamed players starting right next to each other. There are rebel bases situated to the right and the left of the starting bases, and the most notable feature of this map are the alcoves. The middle left and right sections are cut off from the rest of the map by Forerunner hard light bridges. Inside each alcove, there is a building site (defended by rebels), a Forerunner secondary reactor and two building sites. The light bridges switch between off and on automatically about every 60 seconds, and units caught on the bridge when it deactivates will fall to their deaths.
The term "Terminal Moraine" is used in glaciology for a deposit of rocks, soil, and debris at the farthest extent of the glacier.