Examination of field samples shows that the alteration of the bentonite beds of Wyoming and Montana near the surface to produce high-quality, highswelling, commercial bentonites is analogous to pedocal soil development. The near surface bentonite has undergone extensive calcification, and in some zones it is so heavily altered as to diminish its swelling properties. These low-swelling bentonites can be activated by conventional soda-ash additions. The naturally high-swelling materials have undergone only partial calcification and do not require soda-ash modification. The relatively unaltered 'grey' or 'blue' bentonites have undergone little or no calcification. The latter have the highest Na to Ca ratios, are non-swelling, and do not respond markedly to soda-ash additions. Such low-swelling materials will respond to calcium-activation with markedly improved physical quality parameters of flocculation as measured by Marsh Funnel and Fann viscosimeter. The study indicates that vast reserves of low-swelling bentonites in Montana-Wyoming may be susceptible to controlled calcium activation and expanded commercial utilization.