Numerous wrinkle ridges similar in appearance to lunar mare ridges occur on volcanic plains units surrounding the Tharsis region of Mars, where they are oriented circumferential to the center of the plateau. By analogy with lunar mare ridges and from morphologic evidence such as the smooth slopes of arches, these features are interpreted to result primarily from compressional folding of the volcanic plains, although the uppermost ridge may represent faulting of the near surface materials. East of the Tharsis region, ridges are best developed in the Lunae Palus and Coprates quadrangles, where they can be used to indicate the degree of compressional stress occurring there. Comparison of ridge orientations in these quadrangles to a Tharsis center at 1¿N, 122¿W indicates that more than 60% of the total length of ridges is orthogonal to a normal from this center. Non-orthogonal ridges are seen to have general northerly orientations after removal of the Tharsis orthogonal population. Trends of graben in surrounding terrain indicate that non-orthogonal ridges are not greatly affected by regional scale structures. Comparison of the frequency of orthogonal and non-orthogonal ridges further suggests that loading due to the Tharsis plateau was effective in producing compressional features as far as 4000 km from the center of the load; additional sources for compressional stress are necessary to account for ridge systems at greater distances from the Tharsis plateau.