Boroughs Brooklyn and Queens Geology | Gratacap: Geology of New York City Page 203



	THE BRONX 169 174th Street westward the observer first passes over the rounded degraded knobs of rocks in Crotona Park, at a considerable elevation, surmounting a sharp rise from the West Farms section, while Crotona Park itself reveals a folded area. The exposures of gneiss in Crotona Park show in places a yery slaty and fissile rock, with corrugated beds and granite yeinings. Morainal heaps and alluvial coverings hide or bury the gneissic contours, seen somewhat markedly at Third Avenue and 175th Street. Next succeeds the Tremont gulch or channel, steeply walled by the ridge at Echo Park, where a strong development of white gneissoid granite is seen, sheathed in gray flexuous ribbons of mica schist. To the west again, as the hill slopes to Jerome Avenue, a vast hill of glacial sand occurs, through which the trolley tracks pass by a tunnel-way. Westward by Tremont Avenue another ridge is crossed made up of laminated, upturned mica gneiss, its scars and erosions molded into a smooth hill by alluvial and drift accumulations of soil and sand. In places the rock becomes granitic though retaining a gneissoid structure. On Aqueduct Avenue, at the top of this ridge, granite developments of considerable volume occur. A till with boulders is seen north of the Public School, from whose western porch the Harlem valley is commanded, the Fort George Heights, and, through the Dyckman Street intervale, the' wooded crests of the Palisades. This last ridge is heavily banked with drift on the west. South again along Sedgwick Avenue towards Washington and High Bridges, the gray gneiss, fine-grained, folded, and swerving in thin sheets, is conspicuous. Low cliffs of the gneiss have been well exposed north of 161st Street on Mott Avenue. The topographical expression is clearly north and south ridges and separating valleys. This is continued westward over Manhattan Island to the Hudson, with an accentuation in the Harlem River defile, possibly deepened by faults [(Fig. 39)- A feature of further interest in the Borough of the Bronx are the limestone beds—beds in all respects similar to those