The hills form a rolling plateaulike upland, rising sharply to the north and in the south gradually dropping to meet the plains. Fed by numerous springs that emerge along the hillsides, the slopes are covered by a mixed forest of lodgepole pine, white spruce, balsam poplar and aspen. The quick-draining open plateau supports fescue grasslands and shrubs. The area is a humid island in the semi-arid prairies, with many varieties of plants and animals representative of the ROCKY MOUNTAINS, over 200 km to the west. Over 230 bird species have been sighted here.
Capped by a layer of stream-laid gravels up to 100 m thick (the Oligocene-aged Cypress Hills formation) derived from the Rockies, the Cypress Hills are an erosional remnant of a once extensive higher-level plains surface that was largely removed by stream action during later Tertiary and early Quaternary times (see GEOLOGICAL HISTORY).
The hills were high enough to have been one of the few areas in Canada not completely ice-covered during the Late Wisconsinan Laurentide glaciation. Their higher portions projected through the ice as NUNATAKS. Wind-blown loess, in places over 2 m thick, was deposited during this period.