SOLAR SYSTEM/Venus 259 Figure 21 Regional plains with wrinkle ridges (pwr) The plains have a generally smooth surface with a relatively low and uniform radar albedo Numerous narrow and sinuous ridges deform the surface of the plains In the centre of the image, a narrow channel like feature that cuts the surface of the plains is visible These channels are typical features on the surface of regional plains Fragment of C1 MIDR.30N153; illumination is from the left is predominantly covered with regional wrinkleridged plains Midlands constitute the majority of the surface of Venus ($80%), occur at elevations between and km, and host the richest variety of terrains, units, and structures Highlands are above km and comprise $9% of the surface The highlands include two distinct classes of first-order features that are thousands of kilometres across The first class consists of relatively steep-sided plateaulike features, the surface of which is typically covered by tesserae (e.g., Ovda Regio and Fortuna Tessera in eastern Ishtar Terra) These features appear to be isostatically compensated at relatively shallow depth, several tens of kilometres, suggesting that they are areas of thickened crust and probably relate to ancient regimes of mantle convection This is consistent with the stratigraphic position of tesserae, which are the oldest terrain on the surface of Venus The second class of highlands includes dome-shaped rises that are typically rifted and topped by large volcanoes (e.g., Atla and Ulfrun regiones) Figure 22 Spatial distribution of regional wrinkle ridged plains (ruled pattern) The plains occupy the relatively low portion of the area (Ganiki Planitia) between elevated territories to the west and east, where the older units and structures such as tesserae and ridge belts are exposed The thick red line in the centre of the map is Baltis Vallis, which runs along the major continuous lowland Colours show the distribution of elevation relative to mean planetary radius (MPR; 6051 km) The map is in Lambert conformal projection