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A relatively large geometrid with a broad, jagged median band on the forewing and a large white discal spot. Ground colour white with heavy dark brown speckling, median band bordered with white. Hindwing pale grey-white with lighter dusting of fine brown striations or speckles. Other Alberta Caripeta have much smoother median band borders and lack the speckled forewing appearance. Epirranthis substriataria is somewhat similar, but flies in early spring usually lacks the large, conspicuous white discal spot, and the median area is not darkened to form a broad band.

The abdominal segments of the larva are thickened at the distal end, which, in combination with light yellow-brown lateral patches, give them a "bumpy" appearance much like a conifer twig (illustrated in Wagner et al. 2001 and Duncan 2003). Pupae overwinter. The grey forest looper is occasionally a destructive defoliator; outbreaks have occurred near Terrace, BC in 1961 and the west Kootenay area in 1990-91. Hemlock mortality may reach 78% in areas defoliated for two consecutive years (Duncan 2003).

Most Canadian conifer species have been recorded as larval hosts, the preferred species including hemlock (Tsuga spp.), fir (Abies spp.), Douglas-fir (Pseudotsuga menziesii) and spruce (Picea spp.) (Prentice 1963).