LATER TERTIARY (MIOCENE AND PLIOCENE). 805 



The following description for northeastern Arkansas is taken from a manuscript by Crider: 



"Overlying the Tertiary in Crowleys Ridge and also on the western border, and extending 

 for a short distance over the Paleozoic rocks, there is a thin blanket of sand and gravel of the 

 Lafayette with a maximum thickness of about 30 feet. The Lafayette has been removed from 

 the lowland regions east and west of Crowleys Ridge, along with the Tertiary. 



"Where the loess is absent, as in many places on Crowleys Ridge norlh of Jonesboro, the 

 Lafayette is present on the highest divide. South of Jonesboro it appears beneath the loess in 

 the railway cuts and deep ravines which have been cut to the middle portion of the ridge. Nearly 

 aU the principal streams of Crowleys Ridge have cut through the loess and Lafayette, and their 

 materials have been carried down and spread out on the slopes and the adjacent bottoms. It is 

 probable that most of the Lafayette which was originally deposited on the eastern foothills of 

 the Paleozoic rocks from Missouri to Pulaski County, Ark., has been reworked by the streams 

 and carried farther down the slopes. 



"Where undisturbed the Lafayette is made up of chert, quartzite, and white and black 

 quartz pebbles indiscriminately mixed with a coarse, well-rounded reddish sand. The whole is 

 roughly stratified, showing the effects of ever varying currents. At the base of the formation 

 the pebbles are, in places, interbedded with materials reworked from the underlying formation. 

 The cherts constitute a large proportion of the pebbles in the formation. They are much 

 larger and more angular than the quartz pebbles, and many of them contain fossil crinoid stems. 

 They are, therefore, evidently derived from the cherty limestones of the adjacent Paleozoic 

 hills. The quartz pebbles are nauch smaller than the cherts, and are well rounded and polished. 



"Sand everywhere accompanies the gravels and in places forms the bulk of the formation. 

 The grains are well rounded and, like the pebbles, are almost universally stained with oxide of 

 iron. It is probable that some of the sand has been derived from the underlying formations, 

 but a current that would be strong enough to bring in a large amount of foreign gravel would 

 also carry a still larger amount of sand. Much of the sand, therefore, doubtless has the same 

 origin as the pebbles. 



"The Lafayette contains a large amount of ferruginous matter. Under favorable circum- 

 stances the iron oxide has cemented the sand into a rough, hard brick-red sandstone and the 

 gravels into an ironstone conglomerate which is in few places more than 5 feet thick. .These 

 hard ferruginous masses are prone to form where the percolating waters, carrying the iron oxide 

 in solution, come into contact with a compact clay or any other impervious layer. Subsequent 

 erosion exposes these cemented masses, which in places protect the hills from further erosion, 

 while large blocks break loose from the main ledge and cover the slopes." 



This description applies well enough to the other areas in Arkansas and Louisiana. 



According to Deussen (unpublished manuscript), "A wide area of the Coastal Plain in 

 Texas east of Brazos River is occupied by a series of clays, sands, and gravels, which overlap 

 the Miocene beds and in many places overlie and obscure the Fleming clay and are seen to rest 

 directly on the Catahoula formation. Originally these beds mantled the older rocks over this 

 entire area and the adjacent Black Prairie and extended in broad tongues up the valleys in the 

 region to the west. These exposed portions have been subjected to erosion for so long a time 

 that the interior margin, once very far to the west, has been gradually transferred seaward. 

 In the area of the former extent many outliers persist on the higher divides and plateaus where 

 their isolated position has protected these materials against erosion. These high-level gravels 

 are so common within this area that they generally obscure the older underlying formations. 

 The latter are exposed only in the channels and valleys cut by the streams since the deposition 

 of the Lafayette. 



"Toward the coast these sands, clays, and gravels dip beneath the overlying Pleistocene 

 gravels and are recognized in many of the wells put down in this portion of the Coastal Plain. 



"These sands and clays and gravels wherever exposed unconformably overlie the lower 

 beds, indicating an erosion epoch between their deposition and that of the underlying beds. 



"As a rule the materials carry no fossils indigenous to the strata. Waterworn fragments 

 of fossils from older rocks are sometimes found embedded in the gravels, but these give no 



