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One of the principal difficulties experienced by travelers upon that portion of the Plains embraced within the limits of the Marly Clay formation, is the great scarcity of water. This arises principally from three causes: first, the great deficiency in the annual amount of rain; second, the impermeable character of the clay, which allows most of the water that falls to be conducted off, either by surface drainage or by evaporation; and, third, the general absence of extensive faults or breaks in the strata, from which cause the water, after having once entered them, is often made to flow long distances through subterranean channels before finding an outlet.

Most of the springs examined by ourselves in this formation issue from near the top of the strata, and are consequently not of a very permanent character, usually drying up soon after the close of the rainy seasons. In some few instances, however, they were found to issue from much greater depths beneath the surface, and then they are larger and yield a more constant supply of water. Wherever the gypsum prevails the water usually possesses a nauseous and bitter taste. This appears to be owing mainly to the presence of salts of lime, magnesia, and soda. It is while flowing through the gypsum formation of the Plains that the waters of many of the larger rivers of Texas, such as the Red, Brazos, and Witchita, acquire their bitter taste.