If the name Warren Rohrer is not a familiar one, it may be because this American painter, who died in 1995, chose to remain in rural Lancaster County, Pennsylvania, rather than join the urban Avant-garde in New York. Rohrer’s history is unusual: raised in a Mennonite community, he set out to be a Bible scholar before taking up painting. In the early 1960s, he dedicated himself to painting his native Pennsylvanian landscape. On a trip to Europe in 1972, he visited exhibitions of Mark Rothko and Barnett Newman, and subsequently began to make abstract paintings in a square format, incorporating grids. The Philadelphia Museum of Art, where Rohrer taught life drawing and still-life painting in the 1960s, is showing Rohrer’s work this summer (until 17 August), concentrating on the years 1972 to 1993, after he had moved, in the late 1960s, from landscapes to abstraction. Concurrently, Locks Gallery provides an important, complementary exhibition focusing on the crucial years 1969-71 (until 18 July). Exhibitions such as these are gratifying, as they show a painter negotiating between two approaches and reveal how the vestiges of landscape, such as the horizon line, can linger in abstract work.