H ken B ne£y D ] GEOLOGY OF EASTERN DIVISION. 29 
Throughout the Beaumont clay region large numbers of peculiar 
mounds or hillocks occur, forming the often-described "pimpled 
plains." These mounds extend, in a few cases, within the limits of 
the coastal marshes. 
RECENT OR PORT HUDSON CLAYS. 
The recent Beaumont clays are overlain by very recent materials, the 
present river alluvium and the coast marshes. The first river bottom 
or alluvium proper fringes all the rivers in the Coastal Plain, whether 
they originate in or pass through the plain. No average width can be 
assigned to these deposits, as the low bluffs following the margins of 
the river valley may open out for several miles and again close within 
the distance of a mile so as to restrict the river to its actual water 
channel and efface the bottom land or newer alluvium altogether. If 
the lands subject to overflow be considered the representative of the 
very recent deposits, these will range from a half mile to 5 miles in 
width along each side of the river. Along the Sabine the Fayette and 
Frio clays are exposed at many places in the form of high bluffs. On 
the Trinity the overflow lands have widths of from 3 to 5 miles, and 
the Brazos bottoms at several points and at certain seasons present 
the appearance of a river from 12 to 25 miles in width. In thickness 
these deposits range from 4 to 25 feet. 
The coast marshes are restricted to the southeastern portion of 
Texas and southern Louisiana, not existing west of Galveston Bay. 
These marshes include a broad belt, but little elevated above sea 
level, extending from Galveston Bay eastward through Texas and 
southern Louisiana as far as the eastern side of Vermilion Parish. 
The northern border of this belt is extremely irregular, and in many 
places tongues of the underlying clays extend for several miles, forming 
ridges and islands, as these elevations are called. The plain is covered 
with a heavy growth of marsh grass, and with reeds and other vege- 
tation where water forms the permanent covering. Throughout the 
most southerly portion there are numerous sink holes or floating 
marshes, absolutely impassable for man or beast. 
In these coastal marshes a few sluggish bayous are found. They 
have apparently little or no current, and are more or less affected by 
tidal influences. Of these the most important in Texas are East 
Bayou and Mud Bayou in Chambers and Galveston counties, and Tay- 
lors Bayou and Hildebrands Bayou in Jefferson County. In Louisiana 
the bayous are few in number, but those which occur have expanded 
to broad sheets of water, such as Calcasieu, Grand, and Mud lakes. 
At several points along these bayous the banks form small bluffs, 
ranging from 2 or 3 feet to 20 and sometimes more feet. These bluffs 
are generally made up of shells of Gnathadon cuneatus, with a 
few species of oyster. Probably the most prominent of these is 
Grigsbys Bluff on the Neches, Shell Bluff near the mouth of the 
