HouuEs] THE DUMBARTON HEIGHTS QUARRY 63 



raviue. Ou the plats of the new city subdivisiou.s boideiiug Massa- 

 chusetts avenue extended this locality is called Dumbarton heights. 



Although hardly beyond the city limits, this site still retains the 

 extreme wildness of a piimitive forest and is penetrated by obscure 

 trails only. The sound of the lianimer is now constantly heard, how- 

 ever, even in the wildest spots, and suburban avenues threaten it on 

 all sides. It will probably not be many years before the illustration 

 given in plate xxvi, from a jihotograph taken early in the spring of 1891, 

 will be the only memento of the primal wilderness now covering these 

 hills. A tine rivulet, tributary to Tiock creek, meanders the deep ravine, 

 overlooked on the north by the ((uarry j)romontory and on the south by 

 the observatory. 



Geology uv the Site 



In its geologic features this locality corresponds very closely with the 

 Piny branch site. A bed of Potomac bowlders caps the summit of 

 the ridge, extending to a depth of from 1 to 25 feet, and resting on the 

 somewhat uneven surface of the gueissic rocks. The main ridge, with 

 which this si>ur connects by a narrow and very slightly depressed sad- 

 dle, rises toward Teunallytown, nearly 200 feet higher, and is composed 

 of sands, gravels, and bowlder beds of more recent age. The outcrops 

 of bowlders in the gulches and slopes have bgen worked in many 

 places by the ancient quarrymen. On the spur or proiuontorj' exam- 

 ined the bowlders outcrop at a level of 280 feet above tidewater, Mhich 

 is 50 feet higher than tlie exposures on Piny branch. This difference is 

 probably to some extent an index of the slope of the ancient gneissic 

 beach or sea bed on which the Potomac bowlders were laid down. 

 The bed resting on the gneissic surface seems to ha\e contained a 

 larger percentage of workable bowlders than any of the superposed 

 deposits. This led to the almost exclusive working of this bed by 

 the ancient peoples, who must have familiarized themselves with all 

 exposed deposits of material. ' 



The beds containing ([uartzite bowlders are at this point upward of 

 20 feet in thickness, but the workable material is contined to a few feet 

 at the base, with scattering specimens in gravel deposits at higher 

 levels. The bowlders sought and worked here are almost identical in 

 every respect with those quarried on Pinj' branch. The deposits, 

 however, present some points of difference. At the latter point the 

 bowlders were pretty uniforndy bedded, and the sands and gravels 

 associated with them exhibited distinct traces of horizontal bedding; 

 but on Dumbarton heights the bowlders are distributed pretty uni- 

 formly throughout a matrix of tough argillaceous sand, iiresenting 

 the appearance of heterogeneous dumping, rather than of regular bed- 

 ding by aqueous agencies. 



Portions of the deposits were here in a most favorable condition to 

 be worked, as they occnpied the summit of the ridge and were exposed 

 to view over the surface of the entire crest. The bowlders were obtained 



