Second-day road log: From Washington Ranch to Rattlesnake Spring and Black River, Carlsbad Spring and Carlsbad Flume, and Gypsum caves of the Seven Rivers Hills
— Lewis Land

Day 2 (Friday, 10/6/2023): Karst hydrology of the Black and Pecos Rivers, and gypsum karst of the Seven Rivers Hills.

The second day begins with a walking tour of the Quaternary geology of Washington Ranch, including outcrops of the Quaternary gravel conglomerate of the Black River Basin. These rocks are formed from gravels eroded from the Guadalupe Escarpment a few km to the north, locally cemented by calcium carbonate to form a well-indurated limestone cobble conglomerate. The most distinctive surface features at Washington Ranch are the extensive tufa deposits, including tufa dams, that line the Black River. The Quaternary gravel unit has been described as a locally important karstic aquifer, based on the presence of springs, sinkholes, and solutionally enlarged cavities in the carbonate-cemented gravels. Subsequent stops in the area include Rattlesnake Spring, fed by discharge from the Quaternary gravel aquifer, and the source of water supply for Washington Ranch and the visitor center at Carlsbad Caverns National Park. In the afternoon we will travel north and visit Carlsbad Spring on the Pecos River, which discharges from a karstic aquifer formed in the Capitan Reef, the principal source of drinking water for the city of Carlsbad. The afternoon will conclude with a visit to the Seven Rivers Hills, where sinkholes and caves formed in gypsum and mudstone of the Seven Rivers Formation provide evidence of past upward flow of groundwater from an underlying artesian aquifer.