What happened to rock material on adirondack mountains that are now gone?

The rock material of the Adirondack Mountains that is now gone has eroded away over millions of years. The constant freeze-thaw cycles and the acidic nature of the water have broken down the rock into sediments, which have then been carried away by rivers and glaciers. The process is called weathering and erosion, and it is responsible for shaping the Earth's surface over geologic time.