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Our transcription: Isostasy is the process by which different thickness and different density irregularities in the outer Earth float in gravitational equilibrium with one another. When you build up a large mountain range, you're liable to have a root underneath and a lot of material piled up high on the Earth's surface, and, ultimately, if you don't have forces to keep it piled up, that is going to tend to want to equilibrate and float in gravitational equilibrium with the other areas around it. As mountain belts uplift and late in their stages, they may begin to actually undergo extensional collapse or breaking apart at the high levels due to the force of gravity. At their deeper levels, there may be plastic flow underneath them or compensation by flow in the mantle in order to let whatever root that exists to equilibrate and to come to gravitational equilibrium with the mantle and a lower crust around it. During this stage of ultimate isostatic equilibration, if there are no longer major forces uplifting the mountain range, then erosion will ultimately win out over the uplift process, and the mountain belt will be beveled to a much flatter lower relief surface. At this stage the mountain belt is well on its way to becoming part of the craton.
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