in formal stratigraphy. The age of the Early Hepserian/Late Hesperian boundary is uncertain, ranging from 3600 to 3200 million years ago based on crater counts. The average of the range is shown in the timeline below. Stratigraphic terms are typically confusing to geologists and non-geologists alike. One way to sort through the difficulty is by the following example: One could easily go to Cincinnati, Ohio and visit a rock outcrop in the Upper Ordovician Series of the Ordovician System. You could even collect a fossil trilobite there. However, you could not visit the Late Ordovician Epoch in the Ordovician Period and collect an actual trilobite. The Earth-based scheme of rigid stratigraphic nomenclature has been successfully applied to Mars for several decades now but has numerous flaws. The scheme will no doubt become refined or replaced as more and better data become available. (See mineralogical timeline below as example of alternative.) Obtaining radiometric ages on samples from identified surface units is clearly necessary for a more complete understanding of Martian chronology. Mars during the Hesperian Period The Hesperian was a time of declining rates of impact cratering, intense and widespread volcanic activity, and catastrophic flooding. Many of the major tectonic features on Mars formed at this time. The weight of the immense Tharsis Bulge stressed the crust to produce a vast network of extensional fractures (fossae) and compressive deformational features (wrinkle ridges) throughout the western hemisphere. The huge equatorial canyon system of Valles Marineris formed during the Hesperian as a result of these stresses. Sulfuric-acid weathering at the surface produced an abundance of sulfate minerals that precipitated in evaporitic environments, which became widespread as the planet grew increasingly arid. The Hesperian Period was also a time when the earliest evidence of glacial activity and ice-related processes appears in the Martian geologic record. Impact cratering As originally conceived, the Hesperian System referred to the oldest surfaces on Mars that postdate the end of heavy bombardment. The Hesperian was thus a time period of rapidly declining impact cratering rates. However, the timing and rate of the decline are uncertain. The lunar cratering record suggests that the rate of impacts in the inner Solar System during the Noachian (4000 million years ago) was 500 times higher than today. Planetary scientists still debate whether these high rates represent the tail end of planetary accretion or a late cataclysmic pulse that followed a more quiescent period