Great question! You are correct that mountains break down over time, and the rocks that break off of mountains get transported to lower areas. Mountains break down by the processes of weathering and erosion. Weathering is when the rocks break down in-place. For example, very dilute acids in rain water dissolves certain minerals in rocks and wind, rain, and ice can mechanically break off pieces of the rock. When the pieces of rock get removed from the place where they were being weathered, this is known as erosion. Transport is when pieces of rock are moved by wind, water, or gravity (rolling down a mountain).

So, you have found some rocks on the ground that have been transported from the mountains to your back yard… But when did this happen? These pieces of rock that have broken off of other rocks are called “sediments”. It can be very difficult to “date” (determine how old something is) sediments. We can use certain geologic principles to determine the relative ages of sediments and other geologic features. Let’s think of an example. Much of the rock that exposed in the Santa Ynez Mountains (the mountains above Santa Barbara) is known as the Coldwater Sandstone. We know that this sandstone formed about 40 million years ago. So, if you find a piece of Coldwater Sandstone in your back yard, you know that it was transported some time between 40 million years ago and today. 40 million years is a long time… let’s see if we can do better. The Coldwater Sandstone formed. It probably couldn’t get transported from below sea level to your back yard… It must have come from the mountains. The Santa Ynez Mountains began forming about 5 million years ago, so now we know that the piece of rock in that you found was transported to your back yard between 5 million years ago and today. That is still a long period of time. Rocks usually get beat-up and rounded if they have been rolling around for a long time. If the piece of rock that you found is angular (has sharp edges) that probably means that it couldn’t have been eroded long ago or transported very far. So maybe it was transported to your back hard over the last several hundreds or thousands of years. It is really hard to say exactly!

There are some ways that we can directly date when a big boulder broke off of a mountain, but it is very difficult to do and very expensive! I’m sorry that I cannot estimate a more exact time for when the pieces of rock that you see were transported down from the mountains. This is one of the challenges that geologists face!