Every year in February, we (well, my advisor and his TA, which has been me the past two years) take our Historical Geology class to a fantastic barrier island system on Plum Island, MA. First of all, a barrier island is a type of landform that forms parallel to a shoreline, and is usually characterized by a beach, dunes, and a back dune area, which can be a marsh, lagoon, or other wetlands. We do this for several reasons, but mainly, so the students can see how different a beach looks during the winter, and to help them imagine how sea level will affect the main environments (beach, dune, marsh) at Plum Island.
As sea level changes, environments are pushed backward (during a sea level rise) or migrate out towards the ocean (sea level fall). Currently, as glaciers melt due to global climate change, sea level is rising at an alarming rate. This means that the environments at Plum Island will begin to be pushed backward. During times of sea level rise, we call this a transgression. This means that the beach will move to where the dunes currently are, and the dunes will migrate back to where the marsh is. The marsh will be pushed back more inland. But what will this look like in the rock record? Beach sand, when turned into rock, is called sandstone. Dunes will also turn into sandstone, but will exhibit angular bedding, called cross bedding. Marshes are characterized by very fine sediment, such as clay and silt, as well as lots of organic material (decaying grass, organisms, poop, etc.). Thus, marshes will be represented by siltstone, mudstone, and/or coal in the rock record. These environments are stacked on top of one another as they migrate landward. Thus, in the rock record, geologists would see sandstone, maybe with some shells (beach), topped with more sandstone that has abundant cross bedding (dunes), topped by siltstone/mudstone/coal (marsh).