FLORA OF OLDER POTOMAC FORMATION. 



387 



"■■•a <» 



The following literal extract from my notebook, written on the grovmd 

 witli the exposure before me, on the same days tliat it 

 was measured, May 6 and June 3, 1894, contains a 

 detailed description of the l)eds and will serve for all 

 the exposures in that section and in great measure 

 for the Lower Potomac beds generally: 



On the east side of Sixteenth street the exposure begins 

 about 200 feet nortli of Florida avenue. It is here IL' feet 

 liigh and consists entirely of Ra])j)ahannock sanil, loos(\ fine, 

 wliite, with yellowish-brown ferruginous streaks, stratified in 



numerous somewhat lenticular layers, but generallj' hori- ^ 



zontal, the layers themselves strongly cross-bedded, the " 



lines of bedding more or less variable for each layer, usually f 



dipping strongly northward, often at an angle of 45°. Some- S 



times they curve back and form a series of loops. p 



The exposure thickens gently northwai'd, l)ecoming 2.5 S. 



feet thick 150 feet from its point, of origin. At this maximum | 



point the clay bed begins at the top, thickens rapidly at the Z 



expense of the sand, becoming 15 feet thick where thickest, s. 



It is light ash colored, stratified, and cracks across the ?. 



.... ° 



veins. The stratification is undulating. It passes under ^ 



the roadbed 200 feet north of the point where the sand 5 



disa])])ears. It also forms the roadbed, and it is this lowest g. 



visible horizon that yielded the onl}^ plants collected fi'om S 



this locality. Near the top for most of its length this bed r 



becomes a very irregularly stratified sand, which is more or §" 



less black from the presence of limonite. S 



The Lafayette extends continuously from its point of I 



origin, 60 feet north of that of the beds last described, to the ? 



end of the section, having a nearly uniform thickness of about c 



'^ ^ fro 



20 feet. For most of the distance this is divided into three § 



clearly defined strata — a lower gravel bed of about 5 feet, a P 



middle clay-loam bed of about 10 feet, and an upper gravel ' 



bed of about 5 feet. The lower gravel bed is more irregular 



than the upper, the stones vary more in size, some of them 



V)eing large bowlders, and there is more clay and sand 



between them, the clay lumps being sometimes pink. 



The intermediate bed is clearly stratified, destitute of 



pebbles or cobbles, and varies in color from a bright [link 



to a dull yellow or diit color. It has a very definite upper 



boundary. The upper gravel bed is a little thicker than 



the lower one. The pebbles are more worn, washed clean 



and often shining, the interstices between them filled with mostly red sand. 



