![]() ![]() Likewise, each other color corresponds to a particular period of geologic time, marked by a single type or rock or group of related strata. The dark gray units are the Carboniferous “coal measures.” The luminescent yellow stripe down the middle is a belt of honey-colored Jurassic limestones. His map elegantly summarizes years of field work, and thousands of outcrops into a single picture. Correlating the strata on the basis of these fossil “timestamps,” he was able to match up outcrops across an entire nation. In particular, he paid attention to fossil content such as ammonites, trilobites, and brachiopods. ![]() As excavation revealed rock, he noted whether the strata were sandstone, shale, limestone, conglomerate, or coal. His career occurred during the heyday of British canals, and Smith was on the front lines as new trenches were being dug. Smith gathered his data through his day job as a canal engineer. He made the decision to represent the surface occurrence of different rock units by depicting them in different colors, a practice that is still used today. In fact, Smith depicted a whole country’s worth of formations, and rendered the first ‘modern’ geologic map. The next step forward was taken in England, where geologist William Smith is renowned among geologists for his achievement of depicting more than one formation in a single map. William Smith’s 1815 geologic map of Britain, a major milestone in geologic mapping. This map is an important milestone, but it’s not what most geologists would instantly recognize as a geologic map because it emphasizes just one unit, excluding all others, and because it uses only one color. It implies that strata in the “doughnut hole” are younger than the chalk, and strata outside the ring’s periphery are therefore older than the chalk. In this case, the lumpy gray “doughnut” shape shows the area where surface outcrops of “the chalk” ( namesake of the Cretaceous) may be found in France and England. As a matter of historical significance, it shows for the first time the distribution of a single geological unit across space indeed across multiple countries. ![]() This map summarizes in a single image a large amount of geological information about a region. However, it was still an important milestone. This innovative use of landscape color was forgotten by the time French geologists Jean-Étienne Guettard and Philippe Buache published their map of chalk deposits in 1746. Jean-Étienne Guettard and Philippe Buache’s map showing the distribution of chalk around the Paris Basin (and extending across the English Channel into southern Britain), 1746. Landmarks such as gold mines, shrines, and roads are also shown: The Turin Papyrus Map is sometimes cited as the oldest example of a geological map. It shows multiple rock types by virtue of what color their mountainous outcrops appear. The oldest known use of mapping to depict the distribution of rock types on Earth’s surface was the Turin Papyrus Map, made in 1150 BCE in central eastern Egypt. The patterns that rock units (formations) make can convey key information about the geologic history of a region. To make relationships between rock units more clear, many geologic maps include cross-sections, which show a conceptual “slice” through the Earth along a straight line or multi-segment “polyline” on the map. ![]() Units (members, formations, groups, supergroups, etc.) meet at contacts, which can be of several varieties. Typically, they use different colors (or different fill patterns) to distinguish between different geologic units (or formations). Geologic maps are maps that depict the rock units that crop out at Earth’s surface. ![]()
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