Geology of Texas Part II - Ordovician to Mississippian periods

This will mark the second in a series of writings on the Geological History of Texas. Part 1 covers the Ordovician period of 438-505 millions years ago, the Silurian period of 408-438 million years ago, the Devonian period of 360-408 million years ago and the Mississippian period 320-360 million years ago.

Ordovician Period (438-505 million years ago)
Extensive dolomite and limestone deposits, with less extensive chert deposits, were laid down in the shallow seas that covered Texas in Ordovician time. Remnant Ordovician outcrops are best seen in the Llano uplift northwest of Austin, and in the Marathon upliftand Franklin Mountains of West Texas. The Ellenburger and Maravillas formations appear along roadsides. The Ellenburger is especially noted for its gas production from great depths in basins of West Texas. Other Ordovician limestones yield brachiopods and corals on the top of the El Paso scenic drive in the Franklin Mountains. Cephalopods and gastropods are found in Ellenburger outcrops north of Cherokee in the Llano uplift region. Maravillas limestone and chert beds are seen in the Marathon uplift in West Texas.

Silurian Period (408-438 million years ago)
Limestone, dolomite and chert rocks were laid down in shallow marine waters in West Texas during the period. Silurian limestones in the Franklin Mountains bear rare brachiopods and corals. The first primitive land plants appear in the Silurian, and the corals first seen in late Ordovician time exploded in numbers and varieties.

Devonian period (360-408 million years ago)
Shale, sandstone, limestone and chert of shallow marine origin characterize the rocks of Devonian age in Texas. Amphibians first appear in the Devonian, and this was the golden age for the development of fishes. On land ferns, seedferns, and huge trees related to present-day horsetail rushes developed in Devonian but exploded in numbers in Mississippian.

Mississippian period (320-360 million years ago)
Shallow marine seas still covered Texas in the Mississippian period wherein marine shales and limestones were deposited. On land were fern-filled forests, while in the seas brachiopods, bryozoans, trilobites and corals were common. Mississippian shales and limestones are found in the Llano uplift and folded, upended roacks occur in roadcuts east of Marathon in West Texas.

Part III of the Geology of Texas will focus on the Pennsylvanian and Permian periods ranging from 245-320 million years ago.