The New Brunswick frost line is the depth that water in the ground freezes to in winter. The line varies depending on soil type, geology, and weather conditions. Where there is soil with organic material such as topsoil, a few feet below the surface, this depth can be as much as 1 metre (3.2 ft) or more near Fredericton or Saint John. In regions of Halifax County with bedrock at or near the surface, where permafrost would have existed 20 000 years ago before climate change, it can be only a few inches deep.
Generally, however, the frost line is about 1 to 2 feet (0.3 to 0.6 m) in depth near the shore of a large body of water such as a river and it extends to depths of 4 feet (1 m) or more inland even in areas where bedrock has not been exposed at or near the surface. This is because soil typically thaws slightly each autumn and freezes again each spring, providing only slight insulation from the underlying rock as well as allowing small amounts of water to remain in it during cold weather.
A frost-free season that is long enough to kill the young sprouts of above-ground vegetation is unusual in New Brunswick; the frost-free season only lasts 120 days in Fredericton and Saint John, 135 days on southern and eastern shores of St. George’s Bay where it can be warmer than in other parts of the province, and 150 days in Kent County west of Moncton where conditions are similar to those found near Fredericton and Saint John during the Little Ice Age. The frost line is also shallower near coasts where higher daily high temperatures occur due to moderating ocean effects.