250 



PHYSICAL GEOLOGY 



Shales. — These are consolidated muds which were formerly de- 

 rived from the decomposition of the feldspars of igneous rocks (p. 330) 

 under the action of the water. They are usually finely laminated 

 (P- 234). 



Sandstones and shales, when traced for some distance, may become 

 more and more calcareous, gradually shading into pure limestone, 

 and vice versa. 



Deposits in Lakes and Deserts. — The deposits of lakes will not 

 be treated separately, since they consist, as in the seas, of clays, 

 sand, and occasionally of gravel, and when consolidated are dis- 

 tinguished with difficulty from marine deposits. Because of their 

 limited extent and the small chance of preservation, they are usually 

 of little importance when compared with the widespread and thick 

 marine deposits. In certain regions ancient lake sediments several 

 thousand feet deep occur. The conglomerates and sandstones of 

 desert regions have been discussed elsewhere (p. 52). 



Influence of Sedimentary Rocks upon Topography. — Firmly 

 cemented conglomerates and sandstones are important hill and 

 mountain makers. The " rock cities " of southwestern New York 

 and many of the ridges of the Appalachian Mountains stand in relief 

 because of the presence of strata of resistant sandstones or conglom- 

 erates. When pure sandstones disintegrate, they form barren soils 

 which even in populous regions are usually covered with forests, 

 since they are too poor for agriculture. The scenery of limestone 

 regions has already been described (p. 73). In such a land wide 

 joints, swallow holes, and caves are usually common, and the drainage 

 may be entirely underground. When limestone is massive it may be 

 cut down by the streams less rapidly than the neighboring strata, 

 and form high clifTs and mountains. The Helderberg escarpment 

 of eastern New York is a conspicuous line of limestone cliffs stretch- 

 ing for many miles. 



Regions underlain by shale are usually low and flat. The wide 

 Mohawk valley of New York; the level, fertile plains of Ontario; 

 the northern Middle States of the United States; and the Black 

 Belt of Alabama are examples. 



REFERENCES FOR OCEANS AND LAKES 



Akiw.K, E. A. N., — The Coast Scenery of North Devon. 

 Cornish, V., — Sea Waves. 



W. M., — The Outline of Cape Cod: Geographical Essays, pp. 690-725. 



