540 



KIRK BRYAN 



I. Springs at the outcrops of a horizontal surface. — Where the 

 impervious rock has a horizontal and regular surface of large extent 

 the rock is usually a member of the sedimentary series of which the 

 overlying ; porous material is a part. Exceptions, however, occur 

 where the porous material is a surficial deposit but is so regular in 

 its thickness as to give a similar result, or where it is of volcanic 

 origin. There are three classes of these springs. 



A. Gravity springs 



Hardpan spring 



Fig. 6. — Diagram illustrating three classes of contact springs with underlying 

 bed regular and horizontal: a, gravity springs; h, mesa springs; c, hardpan springs. 



Gravity springs are those which issue at the contact of a soft 

 and previous bed with an underlying impervious bed (Fig. 6 a). 

 The term has been used by Fuller^ and others. As all springs of the 

 shallow-water type are due to gravity, this term has only the merit 

 of usage. Good examples are Camp Grounds Spring, Crab Tree 

 Spring, and others east of Austin, Arkansas. These springs occur 

 around the edges of the sand-hill area, a ridge of Tertiary rocks on 

 the border of the Mississippi embayment (Fig. 7). Here a thin 



^M. L. Fuller, "Underground Waters for Farm Use," U.S. Geol. Survey, Water- 

 Supply Paper 255 (1910), p. 22. 



