He was the first (1751) to make a geological map of northern France, and roughly traced the limits of his three bands or formations from France across the southeastern English counties.

In his work on "The degradation of mountains effected in our time by heavy rains, rivers, and the sea,"[69] he states that the sea is the most potent destroyer of the land, and that the material thus removed is deposited either on the land or along the shores of the sea.

He thought that the levels of the valleys are at present being raised, owing to the deposit of detritus in them.

He points out that the deposits laid down by the ocean do not extend far out to sea, "that consequently the elevations of new mountains in the sea, by the deposition of sediment, is a process very difficult to conceive; that the transport of the sediment as far as the equator is not less improbable; and that still more difficult to accept is the suggestion that the sediment from our continent is carried into the seas of the New World.

In short, we are still very little advanced towards the theory of the earth as it now exists." Guettard was the first to discover the volcanoes of Auvergne, but he was "hopelessly wrong" in regard to the origin of basalt, forestalling Werner in his mistakes as to its aqueous origin.