Without a ventral (underside) view of this butterfly, we were unable to decide if this is a Western White (Pontia occidentalis) or a Checkered White (Pontia protodice). Both can be found in this general area at this time of year. Plus, they occupy similar open habitats. Appearance-wise, the Western White is more lightly marked, other than having a large, black spot near the rear corner of the forewings on the underside. Westerns and Checkereds are sexually dimorphic, with the males nearly all white on the dorsal (upperside). Caterpillars of both species feed on plants in the mustard family (Brassicaceae), among others.
Size: Western White wingspan 3.8 cm–5.3 mm; Checkered White wingspan 3.8–6.3 cm
Photo by: Bri Wentworth on 9/4/21 in Glacier National Park, MT