38 H. E. Gregory — A Geologic Reconnaissance 



later glacial gravels rest in places on the highest lacustrine 

 strata. 



The drainage of the lake was accomplished by cutting down 

 the dam at Angostura — a process that was not continuous, as 

 is indicated by terraces cut in the gravels. Periods of vigor- 

 ous cutting alternated with periods when sinking of the outlet 

 trench was greatly retarded or even ceased. 



Although the history of Lake Morkill thus outlined is 

 believed to include the major events, two factors remain to 

 be evaluated — the extent to which the lake was obliterated by 

 filling and the effect of crustal movements expressed in the 

 distortion of the lake beds. The filling must have been con- 

 siderable, for the lake basin is less than 2 miles in width, 

 the bordering slopes are steep and thoroughly drained, and the 

 amount of debris as indicated by existing deposits is enormous. 

 The original position of the lake beds — horizontal along the 

 valley axis, gently sloping lakeward along the ancient shores — 

 has been modified along the northern shore by later movements. 

 .North of San Sebastian the dip of the lacustrine strata reaches 

 a maximum exceeding 15° ; the beds are traversed by numer- 

 ous faults with displacements measured by a few inches, bring- 

 ing the northern portion of the beds above the southern. The 

 uppermost strata as well as those below have suffered modi- 

 fication, indicating that the movements, in part at least, were 

 later than the time of maximum expansion of the water body 

 and could therefore have had little effect in producing the basin 

 itself. 



Estimates of the duration of the life of Lake Morkill 

 measured in years are of little value, because the rate of 

 accumulation of limestone, of peat, of sands, and of adobe is 

 subject to wide variations, and until the beds on the north 

 shore of the lake have been correlated with those on the south 

 shore no reliable data regarding periods of retreat and advance 

 of the waters can be obtained. That the sheet of water existed 

 while glaciers still scored the slopes of Seneca and Patchatusca 

 is indicated by the fact that early glacial gravels at least are 

 interleaved with lacustrine deposits ; gravels of a second glacial 

 epoch are piled about the ancient coast and their local accumu- 

 lation resulted in raising the lake levels or perhaps even in 

 forming the lake itself. Beginning in Pliocene time, the lake 

 is believed to have existed with many and large changes in 

 volume to the close of the last stage of ice advance. With 

 the decrease in the supply of waste following the retreat of the 



