How Do Beaches Change Over Time?

by Lily Fehler

I wanted to find out whether the location of the shoreline at Bella Beach, Oregon, has changed over time. In the past, has the shoreline been higher or lower than it is now? Has it been further inland or seaward? The present shoreline or intertidal zone can be recognized by the pebbles that are there and are not present on the beach above the shoreline. To document this, four sediment samples were collected from the modern beach and intertidal zone. Three samples were also collected from a cliff at the back of the beach that looked as if it was made of sand and pebbles. Each sample was put down a set of nine sieves, each with a different size wire mesh, with a bottom pan. The sediment from each sieve was put into a plastic bag and weighed. In total, 59 samples were weighed. Samples of modern intertidal zone and beach can be distinguished from each other based on different grain size distributions. The grain size distributions are different for the modern intertidal zone and the modern beach above the intertidal zone. The intertidal zone contains pebbles and sand, whereas the beach above the intertidal zone has no pebbles at all. The most abundant grain sizes for the beach samples from above the intertidal zone are 0.5 - 1 mm, whereas the most abundant grain sizes for the intertidal zone samples are 1-2mm and 2-4mm. Samples from the cliff can be interpreted by comparing their grain size distribution to the samples of the modern beach. The lowest and earliest cliff deposits might represent the intertidal zone a little higher and inland from the modern intertidal zone. Deposits higher in the cliff might represent a back beach with shoreline possibly seaward of the modern shoreline. All the cliff deposits represent a gradually rising shoreline. The modern shoreline is lower than the base of the cliff.