46 Nezv York State Museum 



ply, that is, toward the land. In continental deposits pro- 

 gressive overlap is away from the source of supply, as 

 exemplihed in lake and alluvial cone deposits. Streams 

 constantly deposit sediments in lakes and tend to fill them 

 up; the lake floor muds are gradually covered with the 

 coarser sediments brought by the streams producing a 

 lake regressive overlap, the normal lake succession. In 

 an alluvial cone each successive bed spreads out and 

 extends beyond the one previously deposited forming a 

 transgressive overlap (figure 5). 



Figure 5 Alluvial cone transgressive overlap 



The structures in sedimentary rocks may be inter- 

 preted by the trained observer. They indicate the con- 

 ditions under which the sediments composing the 

 rocks were deposited and their origin. In residual de- 

 posits, or where the sediments have been transported 

 only a short distance, the nature of the parent rock is 

 indicated. When there has been long transportation 

 and wear it is more difficult, and sometimes impos- 

 sible, to determine this. Some sandstones from their 

 composition might have been derived from igneous or 

 metamorphic rocks. Rocks show whether there has 

 been current action, as is usually the case in wind and 

 river deposits, or quiet deposition. Ripple marks, rill 

 marks, cross-bedding, lenses etc. characterize the 



