Martian high-latitude zones are covered with a smooth, layered ice-rich mantle containing characteristic polygonal patterns. The mantle is especially uniform and homogeneous in the northern plains. We performed a survey of decameter-scale mantle textures and circular features, including impact craters, in the northern lowlands down to 50¿N latitude. Strongly altered and mantled craters of impact and non-impact origin form a population of subtle circular features below the mantle. Pits of non-impact origin are numerous in some regions at lower latitudes. Impact craters superposed on the mantle are small and very sparse. The inferred mean crater retention age of the mantle is ~0.1 Ma. The spatial distribution of young craters suggests an age difference between the highest latitudes (younger) and some lower-latitude regions (older). Latitudinal trends in polygon textures agree with impact crater evidence for a latitudinal age progression of mantle properties.