Page:Popular Science Monthly Volume 48.djvu/172

160 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 excavation 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 implement 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.

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 general 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 headwaters of the Alleghany and Monongahela 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 narrowness 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 Steubenville. Still, those high-level deposits are clearly marked there