CLIMAX COMMUNITIES : SHIFTS WITH TIME 



303 



vegetation. These deposits, often preserved in a natural state as 

 wood, leaves, fruits, or seeds, have been uncovered by erosion, 

 excavation, and even in driving wells at considerable depth. Such 

 findings have been fortuitous largely, since the deposits do not 

 occur generally and, when stumbled upon, must be brought to the 

 attention of those interested if they are to be of any scientific 



FlG. 161. Well-preserved Pleistocene plant remains found in silt or peat 

 layers buried under 10 to 12 feet of undisturbed moraine in Minneapolis, 

 Minn. (1) Collier gon giganteum, (2) Neocalliergon integrijolium, (3) Picea 

 sp., wood structure almost perfect, (4) Picea sp., wood structure distorted by 

 pressure, (5) cone of Picea glanca, (6) cone of Picea mariana, (7) cone of 

 Larix laricina—From Cooper and Foot. 11 



value. As a result, the information they have yielded is fragmentary 

 and discontinuous both in time and space. 



A more promising approach to the problem began with the 

 study of the nature and composition of the strata of plant remains 

 and other sediments that have accumulated in lakes and ponds as 

 peat or related material. 82 These strata may give almost continuous 

 records back to glacial time, and, since deposits are distributed 

 over wide areas, their study makes possible the correlation of find- 



