<b>Abstract</b>— <p>Based on photogeological and spectral analysis of the Martian region in the upper reaches of the Nirgal Vallis between 24°−52° W and 20°−29° N, within Her Desher Vallis and in the craters of this region, outcrops of rock enriched with clay minerals were identified, and the geological and stratigraphic positions of these outcrops were determined. Based on the results obtained, it is concluded that a clay-bearing unit, representing a mixture of saponite and nontronite, was formed in the study area. At the same time, aluminum-rich clay minerals were not identified. The process of clay mineral formation occurred in the Late Noachian period. It is assumed that the clay-bearing unit was formed by the surface/subsurface weathering of basalts. The liquid water necessary for the formation of the clays could have been present on the surface due to episodic warming caused by impact activity at that time. In the Early Hesperian period, the clay-bearing unit was overlain by a lava plain and was exposed during the formation of Her Desher and Nirgal Valles and during the formation of impact craters.</p>