164. 



POPULAR SCIENCE MONTHLY. 



Fig. 6. Section showing the Floor of a Cut 

 Terrace on which rests a Beach, b and c, 

 beaches broken into ridgelets ; d, a frontal 

 sand bar ; W, old water level. 



ated by the tilting of the land, which will be noted later. The 

 closing of the old water ways ends the history of the ancient Lau- 

 rentian River. When the river began to flow again, the lacus- 

 trine epoch was established. 



Submergence and Re-elevation of the Lake District. 

 After the obstruction of the valleys with drift, the whole lake 



region was submerged; but 



this depression is best treated 

 of in the rising of the land 

 which has brought the evi- 

 dence to view. 



As the waves beat upon the 

 shores, the beaches, terraces, 

 sea cliffs, sea caves, etc., be- 

 come characteristic of coast 

 lines. When the water sinks 

 or the land rises, the various 

 stages of the deserted shore re- 

 main and record the recession of the waves. The preservation of 

 the old coast lines is often so perfect as to furnish easy identifi- 

 cation of their character, as may be seen in Figs. 6 and 7, which 

 represent sections of old beaches. Behind them lagoons often 

 occur, and the entrances of bays are often barred across with 

 beaches, as is shown in Fig. 8. 



The resemblance between modern and ancient shores is further 

 illustrated in Figs. 9 and 10, where great bowlder pavements are 

 shown, marking the modern 

 and deserted strands. In val- 

 leys, although broad, beaches 

 do not occur, but they are re- 

 placed by terraces. 



The deserted beaches and 

 terraces in the lake region oc- 

 cur at all altitudes, where such 

 could be preserved. But in 

 order to find the remains of 

 old shore lines continuous over 

 long distances, it is necessary 

 to descend to the levels where the water was more or less con- 

 fined in the western and central portions of the lake district, for 

 until a recent date there were no barriers toward the northeast 

 sufficiently high to hold the waters of the lakes above tide. In 

 the prenatal lake epoch, such an embayment called for in the last 

 sentence covered two hundred thousand square miles of the lake 

 region, and has been named Warren Gulf. As there was no land 

 barrier to this gulf in the northeastern direction, and for the 



Fig. 7. Section showing the Floor of a Cut 

 Terrace without Beach, but with Bowl- 

 der Pavement. P, bowlder pavement; 

 W, old water level. 



