17] LANDSCAPES AND VEGETATION 545 



by Streams, except that the valleys are often deeper and U-shaped 

 and their steep sides are usually scoured. The bottoms may be 

 scoured too, and retain so little soil that the vegetation suffers in 

 spite of the favourably sheltered situation. Depressions or deeper 

 troughs are often occupied by water — finger-lakes, paternoster-lakes, 

 and, especially, tarns being numerous in glaciated territory. These 

 bodies of water vary from oligotrophic and unproductive where the 

 surface material has been scoured away, to eutrophic and productive 

 where comminuted deposits were left and considerable sedimentation 

 has occurred. Here may be extensive reed-swamp and other serai 

 stages. Cirques, those rocky amphitheatres where mountain glaciers 

 started in bygone times, are usually so poorly vegetated that they 

 stand out to this day as barren and forlorn. 



Of depositional features left by glaciers there are many, such as 

 various types of moraines and glacio-fliivial deposits. Terminal 

 moraines, located at the ends of glaciers whose margins remained 

 stationary for a long time, usually take the form of hummocky belts 

 of small rounded hills and basins which are irregularly distributed 

 and often enclose lakes or swamps. Their vegetation is consequently 

 very variable from spot to spot, but is usually luxuriant in favourable 

 situations as the unassorted material includes much that is of fine 

 texture and nutritional value. Such areas are often valuable for 

 market-gardening or pasturage. Ground moraine of variable thick- 

 ness is left by continental glaciers almost everywhere when they 

 recede. Though again of unassorted material and beset by lakes 

 and swamps, and sometimes also by smooth elliptical hills called 

 drumlins, ground moraines tend to be flatter and consequently more 

 suitable for large-scale agriculture than terminal moraines. The 

 glacio-fluvial deposits of streams, which carried much of the finer 

 glacial debris out beyond the terminal moraines, are various but 

 individually assorted. Thus the coarser sand and gravel was 

 usually deposited near the terminal moraines but the finer sand and 

 clay tended to be carried and deposited much farther away, the fans 

 often coalescing to form a gently sloping outwash plain. Such 

 plains are usually well vegetated and suitable for agricultural develop- 

 ment, though they may be beset with eskers {see p. 543), with conical 

 or other deposits of assorted sand and gravel known as kames and 

 widely used for building and road-making material, or with pits 

 known as kettle-holes. These last are often filled with water and 

 were left where large blocks of ice had been buried in the moraine 

 or outwash material and subsequently mehed, giving an irregular 



