A novel, automated technique for delineating Martian valley networks from digital terrain data is applied to the Mare Tyrrhenum quadrangle on Mars, yielding a detailed map for the entire quadrangle. The resultant average value of drainage density for the Noachian part of the quadrangle is D ≈ 0.05 km-1, an order of magnitude higher than the value inferred from a global map based on Viking images, and comparable to the values inferred from the precision mapping of selected focus sites. Valleys are omnipresent in Noachian terrain even outside the "highly dissected" Npld unit. This suggests fluvial erosion throughout the Noachian, implying widespread precipitation. The map of continuous drainage density is constructed to study spatial variations of D. This map reveals significant variations in degree of dissection in Noachian on scale of >100 km. These variations do not correlate with any terrain parameter and their origin requires further study.