Looks like any number of Kettle Lakes left over from the last glacial episode some 15,000 years ago. All of which have been getting filled in with sediment over that time period.
We used to play at one as a kid. It was peat bog all around the edge and you could never get to the actual water as we would get too scared we would fall through the floating peat. We’d take a stick and when it poked all the way through the peat and into the water below we knew we were getting close.
Went back to it a few summers ago after being away for 30 years and you could walk a lot farther out on top of the lake. I suppose in another 100 years the peat will completely cover the water.
A kettle lake, kettle pond, kettle hole or pothole is a depression or hole in an outwash plain formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of dead ice left behind by retreating glaciers, which become surrounded by sediment deposited by meltwater streams as there is increased friction. The ice becomes buried in the sediment and when the ice melts, a depression is left called a kettle hole, creating a dimpled appearance on the outwash plain.
Lakes often fill these kettles; these are called kettle hole lakes. Another source is the sudden drainage of an ice-dammed lake. When the block melts, the hole it leaves behind is a kettle. As the ice melts, ramparts can form around the edge of the kettle hole. The lakes that fill these holes are seldom more than 10 m (33 ft) deep and eventually fill with sediment. In acid conditions, a kettle bog may form but in alkaline conditions, it will be kettle peatland. Kettle lakes are found in all glacial areas.