FLORA OF OLDER POTOMAC FORMATION. 



387 



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

 with the exposure before me, on the same days that it 

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

 detailed description of the beds 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 Pixteenth street the exposure begins 

 about 200 feet north or Florida avenue. It is here 12 feet 

 high and consists entirely of Rappahannock sand, loose, fine, 

 white, wi+h yellowish-brown ferruginous streaks, stratified in 

 numerous somewhat lenticular layers, but generally hori- ^ 

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

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

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

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



The exposure thickens gently northward, becommg 25 | 

 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, | 

 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 pomt where the sand | 

 disappears. It also forms the roadbed, and it is this lowest | 

 visible horizon that yielded the only plants collected from i 

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

 becomes a very irregularly stratified sand, which is more or i 

 less black from the presence of limonite. j 



The Lafayette extends continuously from its point of s- 

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

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

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

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

 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 

 being 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 pink 

 to a dull yellow or dirt 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. 



