The Mendha Formation was named by Westgate and Knopf (1932) for exposures near the Mendha Mine on the west side of Arizona Peak in Sec. 15, T. 1 N., R. 66 E. According to Tschanz and Pampeyan (1970) the formation as defined also contains fault blocks of Ordovician rocks and a reliable composite section has not been worked out.

The uppermost portion of the Mendha Formation in the Pioche District contains fossils of Late Cambrian (Trempleauan) age. The local Lower Ordovician fauna (Merriam, 1960) appears to be the result of inclusion of faulted Ordovician strata. The top of the Mendha Formation is everywhere a fault (Tschanz and Pampeyan, 1970).

The Mendha Formation is composed of fine-grained, thick-bedded to massive limestone with algal balls, and trilobite hash, and interbedded light gray and laminated dolomite, and dark gray coarsely crystalline limestone (Westgate and Knopf, 1932). Much of the yellow-red weathering limestone is oolitic, mottled, and locally sandy. The Mendha Formation also contains chert nodules, thin shaley interbeds, and intraformational conglomerate in some beds (Westgate and Knopf, 1932; Merriam, 1964).

The Mendha is about 2,125 feet thick in a partial section exposed in the Pioche District (Wheeler and Lemmon, 1939), and is about 1,900 feet in the Dutch John Mountain area.

Within the evaluation area, the unit is only exposed in the Dutch John Mountain area. It is also present in the Pioche area and in the southern Bristol and Ely Springs Ranges.

The presence of algal balls, gastropods, brachiopods and trilobites, and intraformational limestone conglomerates suggest that the Mendha Formation represents shallow restricted shelf deposition.