VALLEY Oi' THE OHIO. 137 



north of that river, its tliickness becomes so great, that 

 tlie ftcetz limestone is but here and there seen projecting 

 through it. 



The deposition of the geest seems to Iiave been the last 

 operation which the waters of the nortii performed upon 

 this region; and was of course sul)sequent to tlic excavation 

 of the valleys, as no deposit could liave remained upon their 

 acclivities, while the agent which formed them continued its 

 action. You are. Sir, already apprised, tiiat to tiiis forma- 

 tion belong the great blocks of foreign primitive, transition, 

 and old lloetz rocks, which have excited in travellers so 

 mucii astonishment, and which, in one point of resemblance 

 at least, approximate the region south of Erie, Huron, Mi- 

 chigan, ami the other lakes, so closely to that wliich stretches 

 tVom the southern shores of tlie Baltic sea. 



'I'hese masses, in the neighl)ourho(Hl of this place, are for 

 die most part solitary, but in the interior of the State, it is 

 not uncommon to find them grouped into heaps which are 

 slightly covered with soil ; and it is, I suspect, an aggregation 

 of this kind, on one of the islands of Lake Huron, that a 

 British officer mistook for granite iii place.* Tiie size of 

 these masses extends from that of gravel and pcbi)les to the 

 diameter of eight or ten feet. The larger blocks are fre- 

 quently found iipo7i the old alluvial plains, but never, that I 

 have understood, within them. Their geographical range is 

 over the same region with the smaller foreign debris of our 

 valleys, but more limited to tlie south west. I have never seen 

 a single block on the opposite side of the Ohio, and am not 

 informed that any have been observed lower than the thir- 

 ty-ninth degree of latitude. 



I do not entertain a doubt that these fragments were en- 

 veloped in large fields of ice in a region far beyond the 

 lakes, and floated hither by the same inundations that 

 brought down and spread over the surface of this country 

 the geest in which they are imbedded. In the southern parts 



• See Thompson's Annals of Phil, for March, 1816. 

 VOL. II. S 



