MARYLAND WEATHER SERVICE 249 



On the road to the summit of the Blue Mountain, at Pen Mar, 

 there is exposed a similar mass of rock fragments, but these are 

 not rounded, bnt are scattered over the face of the cliff and adjacent 

 steep slope. (Plate XXVI., Fig. 2.) 



There is no high cliff for these fragments to have fallen from, 

 but the flat top of the mountain is composed of the sandstone layer 

 already mentioned, which extends southward on Catoctin, and 

 was noted at Weverton and Elk Ridge. Below this top layer, shales 

 occur which disintegrate with greater rapidity than does the sand- 

 stone, and thus allow large fragments of the latter to fall as talus. 

 The northwest face of this mountain is closely covered by these 

 groat masses. In the soil which collects in the spaces between the 

 rocks and is maintained by the fallen leaves, there is a good stand 

 of oak and chestnut forest. The soil is nearer the upper surface of 

 these talus slopes than in the case of the rock rivers; for in these 

 the action of the running water has kept the fine particles of dis- 

 integrated rock or decaying leaves from accumulating. It will be 

 a very slow process of invasion from the sides following the accumu- 

 lation of minute quantities of soil along the margins, which will 

 result in the covering of the rock rivers with tree growth. The 

 talus fragments by contrast fall upon soil already in place, and 

 even when close together allow the seeds of plants to reach the 

 ground easily, and tend to cover such with the falling leaves of each 

 season as they gather in the spaces between the rocks. The water- 

 worn rocks are more closely placed, lying in the dry bed of the 

 deserted river to a depth, apparently, of ten feet or more, and it is 

 only rarely that seeds of trees or smaller plants falling upon this 

 area can germinate and reach the upper surface. 



The forest conditions at the north end of the Blue Ridge are 

 much the same as at the points reached from Thurmont; there is 

 a considerable proportion of Hemlock, White Pine, and occasion- 

 ally a Cucumber Magnolia. Along the roads there are the same 

 forms which have been noted before, including Comptonia and 

 Rubus odoratus. In the bog pockets Veratrum viride, Tofeldtia 

 glutinosa and Aletris farinosa are added to the lists from other sta- 

 tions. These bog pockets are nearly uniform in being formed in 

 the depressions on the upper surface of the sandstone, which is 



