Time and Rock Units
The earth’s crust is believed to be at least 3¼ billion years old. In order to deal with this vast stretch of time, geologists have divided the billions of years into various time units and have given each unit a name. The great divisions of geologic time, called [eras], are Early [Precambrian], Late Precambrian, [Paleozoic], [Mesozoic], and [Cenozoic]. These eras are divided into smaller units of time called [periods], and the periods are divided into epochs. The [xx time scale] shows the geologic time divisions. Earliest geologic time is shown at the bottom of the scale; most [recent] is shown at the top.
By examining and studying the different rocks and rock layers, geologists try to discover in which unit of geologic time these rocks formed. Those rocks that formed during a [period] of geologic time are called a [system] of rocks; those that formed during an [epoch] are called a [series]. For example, the [Cambrian] System of rocks formed during the Cambrian Period; the [Cretaceous] System of rocks formed during the Cretaceous Period; the [Tertiary] System of rocks formed during the Tertiary Period. We are now in the younger epoch (called [Recent]) of the [Quaternary] Period of the [Cenozoic] [Era]. The rocks that are forming now are the Recent Series of rocks.
Geologic time scale
[ERA] [PERIOD] [EPOCH] [CENOZOIC] [QUATERNARY] (lasted 0-1 million years) [Recent] [Pleistocene] [TERTIARY] (lasted 62 million years) [Pliocene] [Miocene] Oligocene [Eocene] Paleocene —63 million years ago— [MESOZOIC] [CRETACEOUS] (lasted 72 million years) JURASSIC (lasted 46 million years) TRIASSIC (lasted 49 million years) —230 million years ago— [PALEOZOIC] [PERMIAN] (lasted 50 million years) [PENNSYLVANIAN] (lasted 30 million years) [MISSISSIPPIAN] (lasted 35 million years) DEVONIAN (lasted 60 million years) SILURIAN (lasted 20 million years) [ORDOVICIAN] (lasted 75 million years) [CAMBRIAN] (lasted 100? million years) —600? million years ago— LATE [PRECAMBRIAN] EARLY PRECAMBRIAN
These time estimates are from the paper, Geologic Time Scale, by J. Lawrence Kulp, published in Science, Vol. 133, No. 3459, April 14, 1961. (The time divisions are not drawn to scale)
Plate 10. GENERALIZED [GEOLOGIC MAP] OF TEXAS
Modified from Geologic Map of Texas, 1933
[This map in a higher resolution]
![[This map in a higher resolution]](#7270827584188574924_map1_hr.jpg.id-1634880104388551113.wrap-0.html.html){kind=link}
EXPLANATION [CENOZOIC] 1 [Quaternary] 2 [Tertiary] (Oligocene, [Miocene], and [Pliocene]) 3 Tertiary ([Eocene]) 4 [Volcanic] ([extrusive]) [igneous rocks] [MESOZOIC] 5 Upper [Cretaceous] (Gulf [series]) 6 Lower Cretaceous (Comanche series) 7 Jurassic 8 Triassic [PALEOZOIC] 9 [Permian] 10 [Mississippian] and [Pennsylvanian] 11 [Cambrian], [Ordovician], Silurian, Devonian and undivided Paleozoic 12 Rocks ([Precambrian]) older than Paleozoic 13 [Intrusive] [igneous] rocks (Precambrian, Mesozoic or Cenozoic)
These rocks are found either at the surface or directly beneath the soils and subsoils which cover most of Texas.
Geologists also subdivide rocks into lesser units. One of these, called a group, is made up of two or more [formations]. A formation comprises rocks or strata (layers of rock) that are recognized and mapped as a unit. Some formations consist of layers of one particular type of rock, such as [limestone] or [shale]. Formations are named after a nearby geographic locality, and in some formation names, the type of rock is included. For example, three of the Texas geologic formations are called Buda Limestone, Del Rio [Clay], and Eagle Ford Shale.
[Geologic Map]
The [geologic map] (pp. [4]-5) shows the rocks that are found at the surface in Texas. Some of these are extremely old. Some, geologically speaking, are very young.