The Hesperian System and Period is named after Hesperia Planum, a moderately cratered highland region northeast of the Hellas basin. The type area of the Hesperian System is in the Mare Tyrrhenum quadrangle (MC-22) around . The region consists of rolling, wind-streaked plains with abundant wrinkle ridges resembling those on the lunar maria. These ridged plains are interpreted to be basaltic lava flows (flood basalts) that erupted from fissures. The number-density of large impact craters is moderate, with about 125-200 craters greater than 5 km in diameter per million km2. Hesperian-aged ridged plains cover roughly 30% of the Martian surface; they are most prominent in Hesperia Planum, Syrtis Major Planum, Lunae Planum, Malea Planum, and the Syria-Solis-Sinai Plana in southernTharsis.