The EarthvJorJis at Fort Ancient. 279 



on the very verge of the ravines, has been raised an eml)ank?3ient of 

 unusual height and perfection. Meandering around the spurs, and 

 reentering to pass the heads of the gullies, it is so winding in its 

 course that it required one hundred and ninety-six stations to com- 

 plete its survey. The whole circuit of the work is between four and 

 five miles. The number of cubic yards of excavation may be approxi- 

 mately estimated at six hundred and twenty-eight thousand eight 

 hundred. The embankment stands in many places twenty feet in 

 perpendicular height ; and, although composed of a tough, diluvial 

 clay, without stone, except in a few places, its outward slope is from 

 thirty-five to forty-three degrees. This work presents no continuous 

 ditch ; but the earth for its construction has been dug from convenient 

 pits, which are still quite deep, or filled with mud and water. 

 Although I brought over a party of a dozen active, young engineers, 

 and we had encamped upon the ground to expedite our labors, we 

 were still two days in completing our survey, which, with good instru- 

 ments, was conducted with all possible accuracy. The work ap- 

 proaches nowhere within many feet of the river; but its embankment 

 is, in several places, carried down into ravines, from fifty to one 

 hundred feet deep, and at an angle of thirty degrees, crossing a stream- 

 let at the bottom, which, by showers, must often swell to a powerful 

 torrent. But, in all instances, the embankment may be traced to 

 within three to eight feet of the stream. Hence, it appears that 

 although these little streams have cut their channels through fifty 

 to one hundred feet of thin, horizontal layers of blue limestone, inter- 

 stratified with indurated clay-marl, not more than thi-ee feet of that 

 excavation has been done since the construction of the earthworks. 

 If the first position of the denudation was not more rapid than the last, 

 a period of at least thirty to fifty thousand years would be required for 

 the present point of its progress; But the quantity of material 

 removed from such a ravine is as the square of its depth, ^Yhich 

 ■\YOuld render the last part of its denudation much slower, in vertical 

 descent, than the first part. That our streams have uot yet reached 

 their ultimate level, a point beyond which they cease to act upon their 

 beds, is evident from the vast quantity of solid material transported 

 annually by our rivers, to be added to the great delta of the Missis- 

 sippi. Finally, I am astonished to see a work simply of earth, after 

 braving the storms of thousands of vears, still ?o entii-e and well 



*» 



years, 



marked. Several circumstances have contributed to this : the clay 

 of which it is built is not easily penetrated by water; the bank has 

 been, and is still, mostly covered by a forest of beech tx-ees, which 

 have woven a strong web of their roots over its steep sides ; and a fine 

 bed of mo?.sJ^PQhjtrichum) serves still further to afford protection." 



