Some definitions include the Bonaparte Plateau to the north, which lies in the angle of the Thompson and Bonaparte Rivers, and south of the uppermost reaches of that river and a small tributary of the North Thompson, Lemieux Creek.
The dominant landscape of the Thompson Plateau is a high, almost plains-like rangeland fairly heavily forested with subalpine forest and tamarack swamp where there exists a significant cattle ranching industry, but plunging steeply to the valleys of the Thompson and Okanagan on its outer perimeter which feature more semi-arid landscapes that include rattlesnakes, a ground-creeping variety of prickly pear cactus, sagebrush, and tumbleweed, in addition to fruit growing operations that often rely on irrigation. In its core is the broad and open rangeland of the Nicola Valley, at the focus of which is the town of Merritt, British Columbia. Towards its southern edges, the plateau is fairly mountainous and includes the ski area at Apex (west of Penticton), as well as the small but rugged Kruger Range which runs south from there to the confluence of the Similkameen and Okanagan Rivers.
The rocks underlying the Thompson Plateau originated in the Pacific and were appended to the North American Plate in the Mesozoic. These rocks were pervasively intruded by magmas now forming granitic rocks. These upland surfaces are now bordered by prominent valleys over 1000 metres deep. This peneplain was overlain by thin sheets of plateau basalts formed from the Chilcotin Group lavas that flowed ten to fifteen million years ago.