October, 1918] 



The Ottawa Naturalist 



67 



point the continuity of the plain is broken by many 

 ridges of igneous rocks isolated from the main 

 escarpment, such as the ridge north of the town of 

 Renfrew. 



The principal tributary streams to the Ottawa 

 on the Ontario side are the Rideau, Mississippi, 

 Madawaska, Bonnechere and Petewawa and on the 

 Quebec side are the Gatineau, Coulonge and Black 

 rivers. All these rivers except the Rideau have 

 their source in the Laurentian upland and descend 

 over generally steep gredients with mostly insignifi- 

 cant valleys in the general upland with the excep- 

 tion of the Bonnechere valley which is almost as 

 large as the Ottawa valley above Renfrew. 



The prevailing rocks of the region are granite 

 gneisses with which are associated some minor areas 

 of crystalline limestones, schists, gabbro, etc. These 

 rocks are of Archean origin and they form the 

 escarpments and the vast upland behind them to the 

 exclusion of all others. It is probable that the entire 

 valley bottom was at one time floored with layers 

 of later rocks, such as sandstones, shales, limestones 

 and dolomites. These rocks have been so much 

 worn down by various agencies of erosion, that only 

 patches of them remain, and the old floor of 

 Archean rocks on which they were laid down now 

 forms the bed rock over large areas of the valley. 



The bed rock is concealed over the greater (por- 

 tion of the valley plain by a sheet of unconsolidated 

 material consisting of varying thickness of sand, 

 gravel, and clay. These materials are either directly 

 glacial deposits or are the results of the glaciation of 

 the region. 



In glacial times the region appears to have been 

 covered with an ice sheet of considerable thickness 

 such as covers Greenland and the Antarctic continent 

 at present. The weight of this ice appears to have 

 depressed the land surface to such an extent that the 

 sea was able to flow into the Ottawa valley when 

 the ice finally melted. Conditions were then similar 

 to what they are now in the St. Lawrence below 

 Quebec. In other words the marine estuary reached 

 up to about the military camp at Petawawa at the 

 close of glacial times. As the land rose when 

 relieved of its immense burden of ice the sea margin 

 gradually withdrew to succeeding lower land levels 

 until it came to its present position in the St. Law- 

 rence valley. The sediments which were accumu- 

 lated in this narrow sea are now the most valuable 

 asset which the Ottawa valley possesses as they are 

 the bases of the agricultural wealth of the region. 

 Furthermore there is probably no other region in the 

 world where a recently drained sea bottom is avail- 

 able for human inspection and use on such a scale 

 as the one now in view in the St. Lawrence and 

 Ottawa valleys. It is a source of never failing profit 

 to the farmer if he uses it aright and a source of 



never failing interest to the field naturalist. This 

 belt of stoneless marine clay has had much to do 

 with the growth of towns and ''llages located on it. 



The great mounds, ridges, ^r sheets of sand, 

 gravel or stony clay were also contributed by the 

 ice sheet either during its advance or retreat. 



Some of this material is covered by the marine 

 clay but a great deal of it is exposed. Many of 

 these sand and gravel heaps are moraines, being the 

 results of drainage from the margins of the melting 

 ice. These morainal ridges or mounds are generally 

 striking topographic features in the valley plain. 

 They have their economic uses, as the sand and 

 gravel for building purposes and road making are 

 drawn from this source. A striking instance is the 

 Rideau moraine which extends from Ottawa to 

 Prescott, and if the Ottawa-Prescott road is ever 

 built, doubtless most of the materials for its con- 

 struction will be furnished by the boulders, sand 

 and gravels of this moraine. 



AGRICULTURE AND FORESTS. 



The agricultural possibilities of the region are 

 rather rigorously defined by the geology. The 

 greater part of the rugged uplands beyond the 

 escarpments that border the Ottawa valley are un- 

 derlain by granite gneiss and covered with only a 

 thin layer of stony drift or not at all. These areas 

 are absolute forest land and should never have been 

 cleared for cultivation. The lesser areas underlain 

 by crystalline limestone are generally depressions 

 on account of the relative softness of these rocks to 

 the granite gneisses which form the ridges surround- 

 ing the depressions. These limestone areas are 

 covered with sandy loam which is capable of culti- 

 vation. So closely does agriculture follow the lime- 

 stone bands in the Laurentian upland that it would 

 be almost possible for a geologist to map these bands 

 by mapping the cultivated areas, as the farmer 

 follows them up just as a miner follows ore shoots. 

 At certain points in the upland there are glacial 

 morainic ridges generally composed of sand and 

 gravel with smooth outlines which are almost sure to 

 be cleared and cultivated. This light sandy soil 

 becomes unproductive after four or five years of 

 cultivation, unless well manured, and even then does 

 not produce good crops unless during unusually wet 

 seasons, so that it is doubtful if it is economy to try 

 and cultivate these sand ridges instead of leaving 

 them in forest. 



Farms are constantly being abandoned or several 

 abandoned farms are fenced into one pasturing 

 ground or ranch for raising cattle. 



Conditions such as these tend to sparseness of 

 population and inhibit the growth of any la'-'^e 

 towns within the Laurentian plateau. 



The valley of the Ottawa below the 500 foot 

 level IS for the most part covered with a heavy 



