The Waterton-Castle River Region includes the extreme limits of advance of both Cordilleran glaciers flowing from the Rocky Mountains eastward toward the plains, and massive Laurentide glaciers advancing upslope from the plains and the Canadian Shield toward the mountains. Its glacial record, which spans about the last third of the Quaternary Period, can be divided into four events. Each consists of a Cordilleran glacier and a corresponding Laurentide one, though during the last event Laurentide ice failed to reach the region. The record also includes the late neoglaciation found near the heads of mountain valleys. Undoubtedly, many Pleistocene glaciers are not represented in the record, because their deposits have been destroyed by later ones. Although this fact may explain the apparent absence of pre-Illinoian Cordilleran glaciers, pre-Illinoian Laurentide glaciers definitely did not reach the region. The glaciations of the region show a steady decrease in intensity and strength. The first, of Illinoian age, is here called the Great Glaciation, and its glaciers the Great Cordilleran and Great Laurentide glaciers. These were immense glaciers, far surpassing in magnitude any others before on after. The Great Cordilleran Glacier left drift in the preglacial valleys far out onto the plains, and on high areas in the foothills such as Mokowan Butte and Cloudy Ridge just east and north of Waterton Park respectively. During this time the park lay under a great ice-dome, with only its highest peaks possibly protruding. The Great Laurentide glacier dropped Shield stones high on the mountain front and spread till over the mountain drift in the preglacial valleys. Later, during Wisconsin time, Cordilleran glaciers were confined to valleys in the mountains and spread only a short distance onto the plains. Laurentide glaciers likewise were much reduced in size. The first was about 800 ft (240 m) lower than the Great Laurentide Glacier, the following one another 1,000 ft (300 m) lower, and the next one 600 ft (180 m) lower again. All the Cordilleran glaciers of the region had undergone substantial retreat before Laurentide glaciers arrived on the scene, and the amount of contact between the two types is not known. It possibly was substantial during the Great Glaciation, but none or very little during the next. During succeeding glaciations the Cordilleran and Laurentide glaciers did not meet within the region.