This patch of sand is showing because the river was running very shallow in these parts. The fences you're seeing is to keep their goats and cows inside that space in this field. This is not an actual field before though. This was a residential area too, even had a church on it. But when Mt. Pinatubo exploded, its ash flew down this area, called Lahar; and covered this part of the town. Vegetation grew over it in time.

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