South of Coupeville, Washington on Whidbey Island is a broad swath of rich farm land with thick black soil. This area has been managed as a sort of farm land for at least 3,000 years. Prior to white settlement the fields in this area were managed by frequent burning. These clear prairie lands were some of the first lands to be settled by whites and resulted in some of the understandable violence that followed.
Ebey's Landing fields south of Coupeville
But before this area was farmed it was for a short period an icy bay. As the Puget ice lobe melted out of present day Puget Sound approximately 15,000 years ago, the the southern terminus of the ice lobe lingered for a period just north of these fields. The full mass of ice from the Puget lobe had pushed the land surface downward.
Hence the area in the above photo was below sea level as a shallow bay with glacial ice just to the north in what is now the town of Coupeville. Across the valley from the picture the land is a lumpy jumble of debris left behind by the ice, but in the shallow icy bay the deposits left a smooth surface of silty soils ideal for future farming.