68 A NATURALIST IN THE GREAT LAKES REGION 



and the water, finding its way through cracks and crevices, flowed 

 along in a stream underneath the glacier. Such streams often 

 carried and then deposited material in elongated heaps of more 

 or less stratified gravel and sand. Now that the ice is gone, these 

 show as elongated hills known as eskers. 



This glacial drift is made up of soil through which are scat- 

 te^ed bowlders, small and large, or if the material is sorted by 

 water, it is laid down in layers, coarse or fine according to the 

 speed of the current that carried it to the present location. The 



FIG. 46. Crystals; at left cubes of galenite, at right quartz with a single 

 quartz crystal in center foreground, in rear of it tourmaline crystal in matrix. 



bowlders and stones are subangular and more or less scratched and 

 grooved. A large percentage of them are limestone, since the 

 bed rock for a couple of hundred miles to the north is also lime- 

 stone. But there are many samples of sandstone, quartzite, 

 granite, gneiss, schist, diorite, diabase, and greenstone that have 

 been brought into our region by the glacier from the ledges in 

 northern Michigan and Canada where are located the nearest 

 outcrops of these rocks in the direction from which the glacier 

 came. 



A rock is either a mass of some one mineral or made up of 

 grains of several minerals. A mineral is a homogeneous inorganic 



