i6o THE POPULAR SCIENCE MONTHLY. 



readily seen, and at the face of an excavation a slip or talus is 

 easily detected. 



" Over three years ago a sandpit was worked in this terrace 

 at its southern extremity, below Riddle's Run. While the exca- 

 vation was being made, and at a noon hour, I found a plainly 

 marked but rude flint implement imbedded in the freshly exposed 

 face of the stratified sand and gravel, under about eight feet of 

 undisturbed cross-bedded stratification, only the point of the im- 

 plement showing on the perpendicular face of the excavation. 

 The condition of the stratification in all of the superincumbent 

 eight feet, which was closely examined by me, was such as to 

 convince me that the implement was not intrusive, but had been 

 deposited with the remainder of the material of the terrace. The 

 condition of the face of the excavation above the find is fairly, 

 but not as clearly as would be desired, shown by the photograph 

 taken by Mr. Doyle of the now abandoned sandpit where the find 

 was made, where slips and talus cover the face. 



"Sam Huston." 



3. Glacial Age of the Gravel. In company with Mr. Huston, 

 Mr. Joseph B. Doyle, and Mr. Frederick C. MacClave (to whom 

 I am indebted for the photographs and many other favors) I 

 visited the abandoned pit where the implement was found, and 

 studied carefully the situation, and can add my testimony to the 

 correctness of the above description so far as it goes. But a gen- 

 eral discussion of the questions relating to these gravel terraces 

 is essential for the information of the general reader. 



As shown in the accompanying illustration, the Ohio River 

 occupies a narrow valley which might almost be called a gorge, 

 which it has eroded in the nearly parallel strata of the coal 

 measures to an average depth of about three hundred feet. This 

 gorge is continuous from Louisville, in Kentucky, to the head- 

 waters of the Alleghany and Monougahela Rivers, a distance of 

 more than twelve hundred miles. All the tributaries of the river 

 occupy gorges of similar depth. This erosion has evidently taken 

 place with considerable rapidity consequent upon an elevation of 

 the continent at the close of the Tertiary period, giving a steep 

 gradient to streams which, during the most of the Tertiary period, 

 had been very sluggish. The evidence of this is seen in the nar- 

 rowness of the gorge and in the gentleness of the slope above the 

 three-hundred-foot line. 



Along the three-hundred-foot level there is a line of rock 

 shelves which contain a shallow deposit of loam and pebbles. 

 This is very conspicuous on the Alleghany River and for some 

 distance below Pittsburg, but rather less so as far down as Steu- 

 benville. Still, those high-level deposits are clearly marked there 



