A fascinating and visually striking geological map of the United States, showing 9 types of strata and compiled for the census of 1870. This is one of the earliest national maps to show the western United States (and in particular the Basin and Range province and the Cascadia Subduction Zone) in intricate detail. In the mid- to late-19th century, geological maps were of extreme national interest due to their ability to show where important mineral resources could be found.
This map is extremely detailed with regards to the geology of the western United States. While maps of some western regions had existed for a quarter-century, they were not incorporated into detailed national maps until much later. The Wheeler Surveys, ongoing at the time of this map's publication, would have been continuously providing new geological discoveries that allowed for such progress as shown in this map. Other surveys which would have no doubt influenced this map include the John Powell and Clarence King surveys, which had such ambitious goals as mapping the geology of the United States at 40 degrees north.
The geological interpretations represented in this map are of notable interest. The breakdown of strata are roughly recognizable today, with strata classed into the three defined Phanerozoic Eras, and all Precambrian rocks placed in the so-called "Eozoic." These classifications demonstrate the evolution of now-accepted geological divisions, for example, the placement of unconsolidated Tertiary Cenozoic strata and Cenozoic alluvium into different groups.
These classifications would have been made using fossil evidence of organisms that were only extant at particular points in Earth History. However, the Eozoic, which contains very little evidence for macroscopic life, is identified as anything below the "Paradoxides beds" which are where the earliest known large trilobites are found.
Several editions of this map were produced and continuously updated between 1872 and 1886; this example is from the earliest known edition. Later editions of this map also appeared in the Statistical Atlas of the United States. Charles Hitchcock and William Blake were important mapmakers of the late-18th century who produced the earliest detailed national geological maps of the United States. 20 years prior to the production of this map, Blake had been a part of the Railroad Surveys, and as part of this, produced the first geological map of California in 1855.