106 THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



of time during the intervals of erosion which preceded and followed the 

 deposition of the Lafayette formation. 



Another important factor is found in the character of the materials. 

 It was pointed out in an earlier part of this chapter that the Sunder- 

 land, Wicomico, and Talbot all contain large quantities of gneiss, granite, 

 and gabbro boulders, while the Lafayette had few if any boulders of com- 

 plex mineralogical composition, its gravel being composed largely of sand- 

 stone, quartz, and quartzite pebbles. This variation in composition has 

 produced a confusing result. For those formations which carry gravel 

 and boulders of complex mineralogical composition appear by the rapid 

 decay of these constituents to be in a much more advanced stage of de- 

 composition than the Lafayette formation which is very much older. 

 Many of the sandstone and quartz gravels also in the younger formations 

 are in a more advanced stage of decomposition than those of the Lafay- 

 ette. If now the Lafayette formation is eliminated from the discussion, 

 and comparisons are made between the various formations of the Colum- 

 bia group, it is found that the boulders of complex mineral composition 

 in the Talbot formation are frequently as much or even more decayed 

 than similar boulders in some of the older terraces. This fact in itself is 

 sufficient to introduce no end of confusion if dependence is placed on 

 " stage of decomposition " alone as a valuable method of correlation. 



The last factor to be considered is the re-working of decayed material 

 by the waves. So much has been written about the grinding effect of 

 the waves that it is customary to conclude that all material which falls 

 nnd or their influence is ground to powder unless extremely tough and 

 obdurate. This conclusion is no doubt correct in the main, but there are 

 times when wave action is weak and unable to pulverize soft material or 

 to grind up moderately decomposed boulders if brought under its influ- 

 ence. In such cases, the material would be ultimately buried up without 

 suffering disintegration by wave action. Partially decomposed boulders 

 which had begun to decay in an older formation might then be rede- 

 ited in the wave-built terrace in certain sheltered localities and con- 

 tinue unchecked their process of decay. Such has actually been found 

 taking place at various points along the Bay shore. The presence then of 



