Glossary 427 



Cretaceous and Tertiary age, together with the auriferous gravels and 



later lavas. 

 Syncline A fold in rocks in which the strata dip inward from both sides 



toward the axis. The opposite of anticline. 

 Terraces Floodplain terraces (the most common) are bench-like flats rising 



from a river like flights of stairs, formed by a stream whose velocity has 



been increased (generally by upward warping of the land). 

 Till Material deposited from melting ice; generally, but not always, un- 



stratified; a heterogeneous mixture of clay, sand, gravel, and boulders. 



Also called boulder-clay. 

 Unconformity The surface of contact between unconformable strata above 



and the rocks beneath them. Discordance or break due to a lapse in 



deposition, representing a "lost interval" during which the rocks beneath 



were deformed or partly eroded away, or both. 

 Vein A crack in rocks filled with mineral matter deposited from solution 



by underground waters. When metalliferous it is called by miners a 



lode; when filled by eruptive material it is called a dike. 

 Volcanic neck or plug The vent or pipe of a former volcano filled with 



solidified lava. 

 Volcano A vent in the earth's crust from which are emitted molten rock 



or lava, fragmental solid material, hot water, mud, steam and various 



gases. 



