More than 95% of Quebec's territory lies within the Canadian Shield. It is generally a quite flat and exposed mountainous terrain interspersed with higher points such as the Laurentian Mountains in southern Quebec, the Otish Mountains in central Quebec and the Torngat Mountains near Ungava Bay. The topography of the Shield has been shaped by glaciers from the successive ice ages, which explains the glacial deposits of boulders, gravel and sand, and by sea water and post-glacial lakes that left behind thick deposits of clay in parts of the Shield. The Canadian Shield also has a complex hydrological network of perhaps a million lakes, bogs, streams and rivers. It is rich in the forestry, mineral and hydro-electric resources that are a mainstay of the Quebec economy. Primary industries sustain small cities in regions of Abitibi-Témiscamingue, Saguenay–Lac-Saint-Jean, and Côte-Nord.