INCIDENTAL SURFACE CONTOURS 247 



scapes are undulating through the abundance of sink-holes — 

 shallow depressions down through which the water has percolated 

 and escaped into the underground passages. 



The writer recalls a beautiful illustration of this nature seen 

 in the limestone regions of southern Indiana, some years ago. 

 The season was that of the wheat harvest. On every side, far as 

 the eye could reach, were undulating fields of waving grain, of 

 that charming golden hue of which poets sing, with intervening 

 patches of woodland. From every farm was heard the click of 

 the reaper, and from every fence the whistle of the *^Bob 

 White." Owing to the fact that the ridges between these de- 

 pressions were drier than the bottoms, the wheat here ripened 

 earlier, and field after field showed long reaches of saucer- 

 shaped depressions green in the centre, with intervening ridges 

 of golden brown, making, with that charming hazy atmosphere, 

 a picture long to be remembered. Through accident or design, 

 the opening in the bottom of these sink-holes sometimes becomes 

 closed, giving rise thus to temporary pools, or ponds, as shown 

 in the accompanying plate. (Fig. 1, PL 23.) It is this same 

 action that has given rise to the so-called *'sandpipes" of the 

 English geologists. These are slender funnel- or tube-shaped 

 cavities found in chalk, and calcareous sandstone, sometimes filled 

 with drift gravels, sands, brick-earths, or again with fragmental 

 materials fallen into them from the overlying beds as the sup- 

 port beneath was gradually removed. In all these cases it is 

 assumed that direction was given the percolating water by pre- 

 existing fissures or lines of weakness.^ 



In regions underlaid by massive siliceous crystalline rocks, 

 and where meehanieal erosion is reduced to a minimum, land- 

 scapes are softly undulating, with few abrupt escarpments or 

 precipitous ledges, owing to the uniform rotting away of the 

 materials, and the gradual accumulation of the debris. It is to 

 this form of weathering that is due the beautiful rolling hills 

 of southwestern Maryland. The prevailing rock is granite or 

 gneiss. Decomposition follows out each line of weakness. 

 Streams erode through the softened material down to hard 

 bed-rock, while the relatively large proportion of insoluble 

 debris is left to accumulate on the gentle slopes which form 

 such an enchanting feature of these landscapes. 



^See Prestwicli, Quarterly Journal Geological Society of London, 1855, 

 p. 62. 



