10 



Sierra Club Bulletin. 



This, in fact, is what occurred in the Yosemite Valley. When 

 the ice front receded for the last time — there were several sepa- 

 rate glacial epochs — it made a number of minor readvances, 

 following one upon the other like so many gradually dying pul- 

 sations. Each of these readvances left a separate moraine, and 

 accordingly a number of such ridges are found spaced at inter- 

 vals across the valley floor. All of them are situated in the lower 

 half of the valley, and the moraine at the El Capitan bridge, 

 which may appropriately be called the El Capitan moraine, is 

 the uppermost, the youngest of the series. 



It is also the strongest, the most perfectly preserved of all. 

 The other moraines to-day are represented only by truncated 

 fragments, their major portions having been broken down and 

 swept away by the swollen river. Around the broken end of 

 one of these ridges, projecting from the extreme northwest 

 corner of the Cathedral Rocks, the wagon road swings as it 

 bends southward to the Bridal Veil Falls. 



The El Capitan moraine, it appears, not only escaped the 

 partial demolishment that overtook its brethren, but, by virtue 

 of its strength and peculiar situation, became a factor of im- 

 portance in the post-glacial remodeling of the valley bottom. 

 Stretching across the valley from wall to wall, like an unbroken 

 dam, it ponded the waters behind it, and, as the ice melted back, 

 transformed the upper Yosemite Valley into a lake. 



This sheet of water, — Lake Yosemite, it may aptly be called, 

 — like most lakes of a similar origin, was not destined to endure. 

 No sooner had it come into existence than the Merced River, 

 turbid with debris from the glaciers farther up, proceeded to 

 build a delta at the upper end, and this delta, slowly but inex- 

 orably advancing, in time wholly extinguished the lake. 



The manner in which the filling was accomplished one may 

 to-day watch in Mirror Lake. Already reduced from a sheet 

 of water more than a mile long, this little lake, famous for its 

 reflections, is annually being diminished in area by an appre- 

 ciable amount through the rapid forward growth of the delta 

 of Tenaya Creek. Measurements of the delta front for a few 

 consecutive years would afford a basis for an estimate of the 

 length of time that the lake is likely to continue to delight the 

 visitor with its beautiful reflections. 



