Terminal moraines were formed as the glaciers began to retreat and deposited rocks of many sizes at the front or terminus of the glacier.  They mark the maximum extent of the glacier's advance.  Comparing the terminal moraine location to the current location of the glacier shows how far the glacier has melted leaving an empty valley. 

 

The photograph above shows the vegetated terminal moraine of the Findel Glacier in the lower left corner.  The empty valley lies behind the terminal moraine and beneath the gray lateral moraine on the left side.  These moraines show how much length and thickness has been lost since the glacier began its retreat.  Studies show that the glacier was at the terminal moraine around 1859 and has retreated over 2 miles to where you see it in the photograph that was taken in 2018.