Notes on the geology of Rock Creek Park, District Of Columbia

Abstract

Pre‐Cretaceous crystalline rocks of the Piedmont Plateau are well exposed in the valley of Rock Creek within the District of Columbia. A geologic map is presented showing the distribution of four major rock types: schist, medium‐grained granite, coarse‐grained biotite granite, and diorite. These are described and compared with the three types ‐ granite gneiss, biotite granite, and diorite and metadiorite ‐ mapped in the same area by Keith and Darton in 1901. Evidence is advanced to support a revision in the definition of the relative ages of the biotite granite and the diorite in which the author believes the diorite to be the younger rock. Several hundred Brunton readings show (1) foliation striking consistently to the northeast and dipping steeply to the northwest, (2) lineation plunging regularly and gently to the southwest, and (3) joints falling clearly into three major sets ‐ one striking normal to the foliation and dipping vertically, the other two forming cross joints striking northeasterly and northwesterly and dipping steeply eastward. The suggestion is made that aplite and quartz dikes are related genetically to the diorite with which they commonly are associated. The linear distribution of quartz blocks lying in the surficial gravels is believed to reflect the positions of buried veins that stand as walls above the erosion surface of the Pre‐Cambrian basement. Suggested projects are described to extend the scope of the present report.