aD ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
Coastal changes of level, which though extremely slow are yet 
recorded in the uplifted terraces of wide tread and small rise 
along the Atlantic coast of the country, may account for the 
Charleston earthquake of 1886 and the many light earthquakes 
felt along the Atlantic sea board. A no less apparent cause for 
adjustment of the outer shell of the earth relates to the area of 
the Laurentian Great Lakes, and here fortunately we have a 
much greater body of evidence at our disposal. 
Epeirogenic adjustments within Great Lakes area—During the 
latest—and present—geological period, the Pleistocene, continental 
glaciers of an estimated thickness of between one and two miles 
for a portion of the time lay over northern America so as to 
cover at their culmination the greater part of the area east of the 
Rocky Mountains and north of the Missouri and Ohio rivers. 
Such a burden of ice must be conceived to have brought about a 
depression of the earth’s surface within the region, from which 
recovery would presumably be either wholly or partially obtained 
when the ice waned and finally disappeared. ‘The evidence is con- 
clusive that since the retirement northward of the latest continental 
glacier, the Laurentian drainage region has been undergoing an 
elevation which began toward the southern margin, has increased 
in amount toward the north, and is still continuing to-day at a 
somewhat rapid rate. 
There is an extensive literature of the subject ** as regards the 
nature of the evidence of uplift, but the explanation of the earth- 
quakes of the region as a consequence of this uplift and uptilt of 
the land was, so far as he is aware, first made by the writer in 
two related papers published in 1911. 
Summarized for the general reader, the available data which 
prove the uplift and uptilt of the Laurentian basin relate: (1) to 
the evidence of already accomplished movement, and (2) to the 
evidence that this upward movement still continues and so may 
be invoked to explain the earthquakes within the region. 
The evidence of the already accomplished uplift and uptilt is 
derived from the present positions and inclinations of the now 
abandoned shore lines of the system of great ice-dammed lakes 
which lay along the front of the continental glacier during its 
retreat. These shore lines, which were of course horizontal when 
first formed, are now tilted upward toward the north at angles 
which increase rather rapidly as one proceeds north. The uptilt 
has been likened to that of a trap-door in the floor rotating upon 
3 The more important series of papers are by Gilbert, Leverett, Taylor, and Goldthwait. 
» «The Later Glacial and Post Glacial Uplift of the Michigan Basin; Earthquakes in 
Michigan,” Mich. Geol. and Biol. Surv., Pub. 5 (Geol. Ser. 3), Lansing, 1911, pp. 87, pls. 
2 and 3, and figs. 48 and 5. 
