GEOLOGIC EXHIBITS IN NATIONAL ZOOLOGICAL PAEK — BASSLER 267 



by erosion, mountains were formed, or, finally, with continued erosion 

 all elevated areas were again reduced nearly to base level to form 

 a peneplane. 



HARVARD STREET GEOLOGY 



Many geologic items may be studied by the actual outcrops in the 

 immediate vicinity of the Harvard Street entrance as noted in the 

 following short trip through the eastern part of the Zoo. At Six- 

 teenth and Harvard Streets, the more or less plain surface of the 

 Piedmont province, once worn down to base level or peneplaned, as 

 evidenced by its even sky line, and later elevated, is now a marked 

 feature. From here one descends Harvard Street into the stream 

 valley of Rock Creek. The artificial grassed terraces on the right 

 of the street cover former rock outcrops, one of which, on the slope 

 opposite Eighteenth Street (pi. 1, fig. 1), has been left exposed as an 

 outdoor exhibit. Here, in a circular area protected by a low hedge 

 and a row of granite blocks, is seen a small section of the earth's crust 

 illustrating a geologic section including a great geologic unconform- 

 ity of the angular type. The ancient pre-Cambrian schists of thin, 

 compressed micaceous layers representing perhaps the oldest rock 

 formation of the District are noted standing on end. Overlying 

 these are horizontal beds of stratified sediments, consisting of sand, 

 gravel, pehhles, cohhles, and houlders, these being classified by size 

 from minute sand particles to 10-inch or larger boulders, all repre- 

 senting clastic material so called because broken up from former 

 rock formations. The schists belong to the Proterozoic, the second 

 of the great eras of geologic time, and the overlying sand and boulder 

 beds to the Mesozoic, the fourth era. The line of contact separating 

 the two thus represents a great hiatus in earth history during which, 

 particularly in the Appalachian region to the west, as shown by 

 measurements, over 25 miles of sediments were laid down in the 

 oceans of the long intervening Paleozoic era. The pre-Cambrian age 

 of the scliists is determined by the occurrence above them to tlie west 

 of the earliest Paleozoic Lower Cambrian sandstones changed into 

 qaartzites, and fossil plants such as cycads and occasionally bones 

 of dinosaurs of Upper Mesozoic Cretaceous time date the boulder 

 beds. The latter, often called the dinosaur gravels, frequently start 

 with a conspicuous hasal conglomerate formed of boulders cemented 

 together and representing the initial deposit of the Cretaceous sands 

 on the schist. This same geologic section when better exposed in 

 former years (pi. 1, fig. 2) showed other geologic features character- 

 istic of initial deposits, such as the occurrence of a thin bed of bog 

 iron ore, composed of the impure hydroxide limonite, and sometimes 

 the carbonate siderite. These ore beds occur in depressions in the 

 eroded, undulating surface of the upturned schists, where they 



