The back strap around the Achilles tendon is not adjustable.
Two problems: 1. All straps except the back strap are adjustable, so you need to choose a sandal size that fits that area of your ankle.2. The insole under the heel has an odd hump that digs into my heels. It's very uncomfortable.
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Pros
Effortlessly clear rocks and pebbles, and the durable outsole is great for rocky terrain.