A GEOLOGIST'S PARADISE BASSLER 331 



palachians, which rise west of the Piedmont plateau. These moun- 

 tains, the Catoctin, the Bhie Ridge, and ranges to the west, are com- 

 posed of sedimentary roclvs originally deposited in horizontal beds 

 under the sea. During the many millions of years since their deposi- 

 tion, they have been forced by great pressure into a series of north- 

 east-southwest folds, some extending a hundred miles in length, but 

 not more than a few miles in width. It is to historic Harpers Ferry, 

 located in the mountain region a short distance northwest of "Wash- 

 ington, near the point where Maryland, Virginia, and West Virginia 

 meet, that one may go to study the many phenomena of this region. 

 So varied are these features that a list of them reads like the table 

 of contents of a geologic textbook. Among the outstanding features 

 here are two physiographic regions, the Blue Ridge Mountains and 

 the Appalachian Valley; two base-leveled regions, or peneplains, 

 now elevated, one to form the flat summit of the Blue Ridge, the 

 other the floor of the valley proper; the greatly folded Appalachian 

 structure ; terrace formations demonstrating the former higher levels 

 of the stream beds ; and water gaps, features which give evidence that 

 the rivers were able by their erosive power to hold their original 

 courses while the slow process of mountain building was going on. 

 Of historic, as well as geologic, interest is the series of chambers 

 dissolved out of the Shenandoah limestone near Harpers Ferry 

 forming John Brown's Cave, which remains as it was when used as 

 a place of concealment for passing slaves to the North. Another 

 point of historic interest is Jefferson Rock, a mass of lower Cambrian 

 shale reposing on the high river bank overlooking the Shenandoah 

 River and surrounding country. It has been named for Thomas 

 Jefferson, our geologist-President, who frequented it for the inspira- 

 tion he received from the magnificent view — an inspiration reflected 

 in his various scientific papers. 



But we return to Washington to study more recent geologic his- 

 tory. For here are deposits of the period known as the Quaternary, 

 when the great ice sheet overspread the country to the north and 

 reached as far south as the INIissouri and Ohio Rivers. Although 

 the glaciers did not extend to Washington, their effects are exhibited 

 in the deposits brought down in the rivers swollen by the melting 

 ice. These deposits are now in the form of flat plains or terraces 

 along the river banks occurring on each side at definite altitudes, 

 thus giving evidence of several previous higher elevations of the 

 streams. The District of Columbia is the type locality for the study 

 of these terraces, hence the name " Columbia " which has been given 

 to the formation. iVn extensive development of one of these terraces 

 exists in the Mount Pleasant plain at Meridian Park and adjoining 

 upper parts of Washington, where the characteristic gravels can be 



