Foster draws upon the detailed diaries that each expedition kept of its route, cross-checking the journals among themselves and against previously unused eighteenth-century Spanish maps, modern detailed topographic maps, aerial photographs, and on-site inspections. From these sources emerges a clear picture of where the Spanish explorers actually passed through Texas. This information, which corrects many previous misinterpretations, will be widely valuable. Old names of rivers and landforms will be of interest to geographers. Anthropologists and archaeologists will find new information on encounters with some 139 named Indian tribes. Botanists and zoologists will see changes in the distribution of flora and fauna with increasing European habitation, and climatologists will learn more about the "Little Ice Age" along the Rio Grande.