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  • An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, or from about 543 to about 248 million years ago.
  • The supercontinent that existed from 300 to 200 million years ago. It combined most of the continental crust of the Earth, from which the present continents were derived by fragmentation and movement away from each other by means of plate tectonics. During an intermediate stage of the fragmentation, between the existence of Pangea and that of the present widely separated continents, Pangea was split into two large fragments, Laurasia on the north and Gondwana in the southern hemisphere.
  • Ped
    A naturally formed unit of soil structure, (for example, granule, block, crumb, or aggregate).
  • A land surface of regional scope worn down by erosion to a nearly flat or broadly undulating plain.
  • A period of the Paleozoic era (after the Mississippian and before the Permian), thought to have covered the span of time between 323 and 290 million years ago; also, the corresponding system of rocks. It is named after the state of Pennsylvania in which rocks of this age are widespread and yield much coal. It is the approximate equivalent of the Upper Carboniferous of European usage.
  • An interval of geologic time; a division of an era (for example, Cambrian, Jurassic, Tertiary).
  • The last period of the Paleozoic era (after the Pennsylvanian), thought to have covered the span of time between 290 and 248 million years ago; also, the corresponding system of rocks. The Permian is sometimes considered part to the Carboniferous, or is divided between the Carboniferous and Triassic. It is named after the province of Perm, Russia, where rocks of this age were first studied.
  • The study and classification of the surface features of Earth on the basis of similarities in geologic structure and the history of geologic changes.
  • (a) A region, all parts of which are similar in geologic structure and climate and which has consequently had a unified geologic history. (b) A region whose pattern of relief features or landforms differs significantly from that of adjacent regions.
  • An epoch of the Quaternary period, after the Pliocene of the Tertiary and before the Holocene; also, the corresponding worldwide series of rocks. It began one to two million years ago and lasted until the start of the Holocene, some 10,000 years ago. When the Quaternary is designated as an era, the Pleistocene is considered to be a period.
  • A low arcuate ridge of sand and gravel developed on the inside of a stream meander by accumulation of sediment as the stream channel migrates toward the outer bank.
  • Ordovician age group of dolomites and sandstones underlying the Glenwood-St.Peter formations, but is missing in parts of northern Illinois. It thickens considerably to the south.
  • All geologic time, and its corresponding rocks, before the beginning of the Paleozoic; it is equivalent to about 90% of geologic time. Precambrian time has been divided according to several different systems, all of which use the presence or absence of evidence of life as a criterion.
  • Of streams, deposits, and other features, being immediately in front of or just beyond the outer limits of a glacier or ice sheet, and formed by or derived from glacier ice.