MARYLAND GEOLOGICAL SURVEY 105 



instance, the grinding effect of the ice is no doubt sufficient ai times to 

 pulverize all rocks which have been loosened by decay so that only the 

 toughest and freshest can survive. A ground moraine, therefore, when 

 first deposited, will be composed of hard, fresh material, while that on 

 which it rests will probably be considerably decayed through having been 

 deposited during an earlier ice advance and, therefore, exposed longer to 

 the chemical action of underground waters. The conditions under which 

 the surficial cover of the Coastal Plain was deposited were vastly differ- 

 ent from those which prevailed at the same time in the adjacent glaciated 

 region. One was chiefly the work of glacial ice, the other of river, estu- 

 arine, and marine waters aided by floating ice. In the" Coastal Plain de- 

 posits, which have been laid down by the action of water in one way or 

 another, the stage of decomposition is not entirely a function of age, but 

 depends on many other factors which vary greatly in the amount of their 

 influence with the ever-changing conditions. The possible factors, there- 

 fore, which may have combined to produce a certain stage of decomposi- 

 tion in a deposit are so many as to make the restoration of actual condi- 

 tions extremely doubtful and this element of doubt is what eliminates the 

 " stage of decomposition " as a reliable method of correlation. A few of 

 these factors will now be discussed. 



Exposure to the influence of surface waters is one of the most important 

 factors. It was shown above that the earliest formations of the Potomac 

 group were probably deposited in Jurassic time. An important stratum 

 in the Patuxent formation is arkose. This lies low in the series and is 

 of great age, and yet it is not more decayed than some of the boulders 

 of granite and gneiss which occur in the Talbot formation deposited to- 

 ward the close of the Pleistocene period. Many of the gravels in the 

 other Potomac formations are as hard and fresh to-day as when first de- 

 posited, while similar gravels in the Lafayette formation are in an ad- 

 vanced stage of decay. This discrepancy may be due to the fact that the 

 surficial deposits have been exposed to the chemical action of under- 

 ground waters for a longer time than the Potomac deposits, but on the 

 other hand it is also probable that these gravels of the Potomac which 

 appear so fresh have actually been near or at the surface for long periods 



