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Calcareous tufa is formed by the springs near Alwalton.

Tufa, tū′fa, n. a variety of calcium carbonate usually deposited from springs—calcareous tufa; the word was formerly used as synonymous with tuff.—adj.

Under certain conditions calcium carbonate is concentrated at or near the earth's surface by chemical agencies, as about springs where calcareous tufa, travertine, etc., are precipitated, and in caverns where stalactites and stalagmites are formed.

The accumulation of mineral matter through the action mainly of chemical agencies, occurs when the waters percolating through rocks dissolves certain substances, as calcium carbonate, for instance, and on coming to the surface as springs, or dripping from the roofs of caverns, deposit calcareous tufa, stalactites, etc.

This is the origin of the mineral called travertine or calcareous tufa, which is so commonly met with on the margins of springs, rivers, and waterfalls.