The relative rates and importance of impact cratering, volcanism, erosion, and the deposition of sediments to the early geological history of Mars are poorly known. That history is recorded in the upper crust of the pillet, which is best exposed along the 4,000-km-long canyon system called Valles Mstrincris. Previous studies of the stratfgraphy of this region have assumed that it consists of megabreccia and fractured bedrock resulting from impacts, overlain by or interbedded with relatively thin layers oflava, and tvith the layering restricted to the uppenaost level of the crust(1-6). IIere we report new high-resolution images that reveal ubiquitous horizontal layering to depths of at least 8 km in the canyons. Megabreccia should be only coarsely layered and fractured bedrock should be unlayered, so these observations indicate that volcanic or sedimentary processes were much more important in early martian history than previously believed. Morphological and compositional data suggest that the layers were formed mainly by volcanic flood lavas, Mars was therefore probably very volcanically active during at least the first billion years and after the period when the heaviest impact bombardment had ended.