The map shows the quantity and quality of groundwater. The quality is the total dissolved solids in parts per million. The quantity is expressed in litres per second. During the Pleistocene epoch Canada was almost entirely covered by glaciers. Hence, although there are extensive and hydrologically important alluvial (river) deposits, as well as lesser areas of eolian (wind-deposited) materials, the country's unconsolidated surficial materials are predominantly of glacial derivation. These highly variable and irregular glacial materials contain many of the best local near-surface aquifers. The groundwater resource potential of the materials depends on their intergranular porosity and permeability, which result from the mode of deposition. Some glacial materials were deposited directly by ice; others, by running water and standing meltwater. In size they range from a very fine rock flour and clays through sands and gravels to large blocks. Tills and clays, however, are the predominant glacial deposits. A till is a mixture of all sizes of materials in which clays, silts, and sands dominate. Till is found in large structureless sheets extending for many hundreds of kilometres and varying in thickness from a metre or less to 25 metres or more. Other forms of glacial deposits commonly found in Canada are eskers, kames, and outwash deposits. They also have the potential to meet local water supply requirements, such as single-family domestic needs and the demands of small municipalities. Alluvial deposits, although not large enough to show on this map, can be very important locally. They have the ability to supply medium-sized municipalities and major industrial plant needs.