Mountains to the south; the Porcupine Hills to the north; and Thunder Hill to the west. To the east is Lake Winnipegosis, which is a modern remnant of glacial Lake Agassiz, a lake that filled the valley after the last ice age (the valley was then a bay on the western shore of the lake). Valley soils are formed from a mixture of glacial till and lacustrine clay. Sandy beach ridges (marking the western shore of Lake Agassiz at various times) lie throughout the area: a prominent beach ridge lies under the town's golf course, just west of town. Valley soils are typically very fertile and deep, and are the area's greatest natural resource. But they also have a high calcium carbonate content and an alkaline pH, which limits their productivity in some crops (for example, blueberries). The underlying bedrock is composed of Cretaceous shales, and, below that, sandstone, which overlie deeper deposits of Devonian limestone, which in turn overlie Precambrian granite. Ecology The Valley is near the southern limit of the boreal forest in its transition zone to aspen parkland. Valley trees include white spruce, black spruce, tamarack larch, balsam fir, jack pine, quaking aspen, balsam poplar, and paper birch. Most of the forest has been cleared from the flat valley bottom for agricultural purposes, but the surrounding hills are still heavily forested, and are designated as forest reserves. Numerous peaty bogs and wetlands are present in the area, including hundreds of oxbow lakes of the meandering Swan and Woody rivers. The surrounding hills have many lakes, as does the lower lake bottom land near the eastern edge of the valley. Climate The climate of west-central Manitoba features extreme seasonal variation, with winter temperatures below and summer temperatures above not uncommon. The town therefore has very distinct seasons. Snow usually covers the ground from early November to early April, and only two Christmases have been recorded that have not been "white" (snow-covered) since such records have been kept. The transition from winter to spring to summer is rapid, with the snow of late March turning to the full green of summer by mid-May. The onset of autumn, then winter, is equally rapid, as daytime high temperatures above are common in late September, while temperatures above freezing are rare by early November, only seven weeks later. Summer, although short, is pleasant and very green, owing to the fertility of local