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THE POPULAR SCIENCE MONTHLY. 



terminal moraine to Philadelphia. They are most conspicuous in 

 the great gravel terrace just south of Belvidere, gradually dimin- 

 ish in volume and height and even merge into the modern allu- 

 vium by which they are in part overlaid between Easton and 

 Trenton, become conspicuous again at Trenton (where they cover 

 an area of fully fifty square miles, and are exposed in every natu- 

 ral and artificial excavation below their maximum altitude in 

 and near the city), and finally disappear near Bristol, though the 

 cobbles are largely dredged from the channel to and beyond 

 Philadelphia. They are in part overlaid by modern alluvium, 

 into which they appear to merge midway between the moraine 

 and Trenton ; and they repose unconformably upon the greatly 

 eroded surface of the Columbia formation — the aqueo-glacial de- 

 posits of the earlier cold epoch of the Quaternary — notably at 

 Trenton, where they fill a basin lined with Columbia brick-clays 

 and gravels. 



By structure, composition, and topographic relations the de- 

 posits tell the story, as by 

 their geologic relations 

 they fix the date, of their 

 origin. At Trenton the de- 

 posits consist of stratified 

 gravels more heterogene- 

 ous than, but otherwise 

 undistinguishable from, 

 those of the terraces into 

 which the terminal mo- 

 raine merges, interspersed 

 with bowlders up to one 

 hundred cubic feet in vol- 

 ume, the whole imbedded 

 in a matrix of sand and 

 loam. The entire mass is 

 unquestionably water-laid ; 

 its continuous bedding is 

 indicative of wave-action, 

 and thus of shallow waters ; 

 and the bowlders scattered 

 throughout it are evident- 

 ly ice-borne. Its structure 

 is shown in Fig. 3, repro- 

 duced from a photograph 

 taken in the extensive gravel-pit half a mile east of the depot at 

 Trenton. The relations of these gravels to the subjacent Colum- 

 bia formation are shown in Fig. 4, also reproduced from a photo- 

 graph taken at Chambersburg — the coarse, stratified gravels. 



Fm. 3.— Abtipicial Clip 



<TON Gratels. 



