36 H. E. Gregory — A Geologic Reconnaissance 



would have been seen to indent the shore line deeply; on the 

 right the Bay of Tticapata would have been concealed from 

 view by the blunt-headed cape of the Era Pampa. Here and 

 there the regularity of the shore line would have been further 

 modified by low wave-cut cliffs separating deltas of sand and 

 gravel brought to the lake from the bordering highlands. On 

 traversing the strand patches of swamp land and areas of 

 vegetation growing within the shallow marginal waters would 

 have been encountered, and the shells of fresh-water animals 

 could have been gathered from the beach. Mastodon and deer 

 grazing near the lake would not have been an unusual sight. 



If the hypothetical observer had been equipped with a sound- 

 ing line he would have discovered that the west end of the 

 lake was shallow with gently sloping floor extending under 

 water for a distance exceeding a mile. At the east end of the 

 lake a depth of over 200 feet would have been found half a 

 mile from the base of Sencco-Orcco. Along its southern mar- 

 gin a narrow, shallow under-water shelf would have been 

 succeeded by a cliff-like slope leading to deeper water and 

 at nearly all points along the northern shore the sounding line 

 would have indicated 50 to 100 feet of water near land. The 

 site of San Geronimo would have lain about 200 feet below 

 the surface of Lake Morkill and San Sebastian would have been 

 submerged to a depth exceeding the height of the present 

 cathedral. 



The order and significance of the various episodes in the 

 eventful life history of Lake Morkill await further study ; only 

 an outline sketch can be presented on the basis of existing 

 knowledge. 



The basin in which the waters of the lake were originally 

 collected owes its initial form and dimensions to an irregularly 

 shaped, uneven-floored depression developed by faulting during 

 one or more periods. The geologic date of the crustal move- 

 ments that produced the down-sunken block has not been 

 determined; in fact, it is probable that displacements accom- 

 panying regional uplift have occurred at intervals throughout 

 the Tertiary and later periods. The Ouzco Basin postdates 

 the development of the Inca peneplain and may be tentatively 

 assigned to late Tertiary, probably Pliocene time. The down- 

 sunken floor of this early lake basin was partly filled with 

 debris furnished by a number of swiftly flowing streams ; at 

 the same time the outlet channel below San Geronimo was 

 deepened. To what extent the lake confined within the original 



