Only fine-grained sediments (J) settled out of suspension when the front of the glacier was still far away. Sand and larger grains settled to the lake bottom nearer the glacier front.
In this photo, the dark layers are clay and the light layers are silt. During the summer, winds and high glacier melting rates kept the finest-grained sediments (clay) in suspension, and only the silt settled to the bottom. During the winter, when the glacier melt was less and the lake was frozen over, the clay was able to settle out. In places, over 100 feet of this sediment was deposited on the bottom of glacial Lake Watts.
The layers in this sample are distorted, either because of slumping and/or because the glacier later overrode the sediment.
As the glacier advanced into the pro-glacial lake, coarser-grained sediment (K) was deposited on top of the fine-grained sediment. The brown and gray mottled material at the base of this section is the fine-grained sediment. Above that is sand, gravel and cobble-sized sediment.
This exposure is at Nealy's Point (K), which is near Mile 6 on the bicycle trail.