56 Lincolnshire Notes & Queries. 



of the Keuper strata, where the Trent now flows, weak as 

 these strata are, must have consumed a very long period of time. 



We have now to enquire how the change came about ; and, 

 to understand it, we must learn something of the laws relating 

 to river courses. 



Rivers may be roughly divided into two main classes, 

 "primary" and "secondary." The "primary" or, as they 

 are sometimes called, transverse rivers, from their running with 

 the dip, transverse to the strike of the beds take their rise on 

 any elevated ground, and, having a gradual slope towards the 

 sea, cut in a more or less direct line through hard ridges and 

 soft strata alike j and the valleys they form in their course, 

 with the aid of rain and atmospheric agencies, are known as 

 transverse valleys ; while the streams which flow into them 

 on the sides, and which follow the strike of the strata, cutting 

 through the softer and weaker beds between the ridges, are 

 known as " secondary " or subsequent rivers, and their valleys 

 longitudinal. These "subsequent " streams may be, and they 

 sometimes are, of greater length than the "primary" rivers j 

 and, as they deepen their beds and widen their valleys, they 

 leave the hard ridges, parallel to which they run, standing out 

 as inland cliffs or escarpments (the formation of "longitudinal " 

 valleys, and of inland escarpments, being, in fact, in each case 

 the result of one and the same process). Again, as time goes 

 on, and the " longitudinal " valleys are pushed further back, 

 these "subsequent" rivers sometimes succeed in tapping, or 

 capturing, other rivers and streams flowing at a higher level 

 than themselves, which they happen to reach. 



The Trent was a " primary " stream when it first started on 

 its course from the high district in the west a time when the 

 Derbyshire hills were hundreds of feet higher than they now 

 are and, finding a gradual slope towards the east, thither, of 

 necessity, it directed its steps, cutting through opposing ridges 

 and the more yielding strata alike till it reached the sea. 



The Humber, too, was a " primary " river when, ages ago, 

 it left its cradle in the Yorkshire hills ; and in its lower course 

 it is one still, or rather the beheaded remains of one, for its 

 upper streams which Prof. Davis thinks may have been 

 somewhere about Halifax or Huddersfield are lost. This 

 river, as it reached our land, had the same ridges to cut through 

 as the Trent the Triassic, Oolite, and Chalk and it, too, 

 found an outlet in the eastern sea. 



At that time, however, as now, it lay at a lower level than 



