PLEISTOCENE ECOLOGY AND BIOGEOGRAPHY 381 



Pennsylvania piedmont swale marshes 50 miles beyond the ice 

 margin dates from the Full-glacial period and coincides with a 

 tundra-taiga vegetation of grasses, sedges, other non-arboreal 

 species, and scattered spruce and jack pine. 



A longer late Pleistocene pollen sequence, perhaps extending 

 back into the middle Pleistocene, has been found in the Carolina Bay 

 lakes (Frey, 1951, 1953, 1955). Possibly rebedding and truncation 

 have occurred (Wells and Boyce, 1953; Frey, 1955). If the upper 10 

 feet of Frey's sequence is continuous, his zones Ml, M2, and MS 

 should correspond to the Full-glacial period. In these pollen zones 

 oak and other deciduous trees are scarce or absent; pine predomi- 

 nates with a high frequency of small-sized grains suggesting Pinus 

 banksiana. There is a low but constant frequency of spruce. In terms 

 of structure, the M zones indicate boreal, needle-leaved forest, with 

 dominance of jack pine attributable to its well-known preference 

 for sandy situations. 



The local records of spruce pollen in Texas, Florida, and Louisiana 

 are undated and, except for Potzger and Tharp's work (1947, 1954), 

 unaccompanied by detailed stratigraphic study. If maximum dis- 

 placement is represented, the difference between Full-glacial and 

 present southern limits of marginal spruce populations would seem 

 to be the same as the corresponding past and present southern limits 

 of the boreal forest formation, that is, 800 miles. 



Some type of mixed deciduous forest occupied the extreme south, 

 with subtropical vegetation largely driven from Florida. Further 

 discussion of Florida awaits expansion of the pollen studies begun 

 by Wilson (Davis, 1946). 



Certain periglacial land forms including boulder fields, colluvial 

 soil mantles, and various types of patterned ground constitute rea- 

 sonably secure evidence of climatic change. Others, such as loess, 

 are apparently less reliable indicators of ice-margin conditions 

 (Hack, 1953; Dylik, 1954). Uncritical identification of all "peri- 

 glacial" features with a Full- or Late-glacial tundra climate is to be 

 avoided. A few of these features can form at midlatitudes today. 

 Yehle (1954) described soil tongues similar to periglacial frost cracks 

 appearing in calcareous soils under the present climate. Goodlett 

 (1954) reported patterned ground in miniature appearing on bare 

 earth in central Pennsylvania. Recent colluvial creep and earthflows 



