The remains of Archaic cultures, especially those dating 8,000 to 4,000 B.P., are not well documented in the archaeological record of the Central Plains of the United States. Some researchers have suggested that people abandoned the region during this period because of increased aridity and associated displacement of game. However, there is strong evidence that geomorphic processes, particularly erosion and deposition, have greatly affected the preservation and detection of Archaic and younger cultural deposits. Chronostratigraphic evidence gleaned from more than 65 localities reveals spatial and temporal patterns of Holocene erosion and deposition in river basins of the Central Plains. Both of these geomorphic processes, as well as periods of net transport and storage of alluvium, were diachronous within individual drainage basins, but were roughly synchronous in similar-sized streams of the drainage networks. Radiocarbon ages from alluvial fills in valleys of large streams (≥ fourth order) span the Holocene, whereas nearly all of those from alluvial fills associated with small streams (≤ third order) are less than 4,000 years old. The early through mid-Holocene gap in the alluvial chronology of small streams is largely filled when alluvial/colluvial fans are considered. From 9,000 to 4,000 B.P., fans typically developed where first and second-order valleys join large valleys. Although valley erosion and deposition may have several causes, major bioclimatic changes generally explain the pattern of Holocene fluvial activity detected in the alluvial stratigraphic records of the Central Plains. Reduced vegetative cover, combined with infrequent but intense rainfalls during the warm, dry Altithermal (8,000 to 5,000 B.P.), favored erosion and net transport of sediment within small valleys. As mean annual precipitation increased during the late Holocene, vegetation recovered and erosion rates decreased, promoting sediment storage in small valleys. The time-space distribution of alluvial deposits in the Central Plains explains the paucity of documented Archaic sites in the region. Within large valleys, it is likely that most of the Archaic record is deeply buried in early through late Holocene valley fills and alluvial fan deposits. Within small valleys, erosion during the early and middle Holocene probably removed most of the Early and Middle Archaic cultural deposits, and aggradation during the late Holocene favored deep burial of Late Archaic and younger cultural remains. Also, surfaces of landforms that dominate valley bottoms throughout the drainage systems are geologically quite young, often postdating 2,000 B.P. Hence, apparent gaps in the archaeological record may be a result of (1) deep burial of cultural deposits, (2) removal of alluvial deposits that contain cultural materials, and (3) young surfaces dominating valley landscapes.

Gilbert’s studies of underground water were overshadowed by his magnificent reports on geologic structure and landforms. In fact, his 1896 publication on the underground water of the Arkansas Valley in eastern Colorado was called by W. M. Davis “for the most part a straightforward geological account of the successive strata.” Gilbert’s 1896 report devoted equal space to stratigraphy and to underground water. His discussion of artesian water was based largely on Chamberlin’s excellent paper on that subject, a decade previously; he briefly acknowledged his debt to Chamberlin for the section dealing with the general occurrence of artesian water, and acknowledged more completely his debt to his U.S. Geological Survey colleague, F. H. Newell. Gilbert’s reports of 1896 and 1897 are largely practical ones on where and how to explore for artesian water, and they include maps showing areas where water can be expected in wells at different depths. Plagued by too little information, he was courageous enough to put lines on a map, although he admitted that the data “are too imperfect to fix the lines definitely except at a few points.” Thus, he said, “I confess that I have drawn this map with much reluctance.” He recommended experimental borings, to gather the additional information needed before putting down a well. Gilbert intended his text and maps to be read by local residents (nongeologists) in their search for artesian water supplies, and this is seen as a major contribution of his work on the underground water of the Arkansas Valley.