Placer gold is gold that has been deposited by rivers or other bodies of water, and is often found in the form of nuggets or flakes. The development of horizons, or layers, of placer gold occurs when gold is carried by flowing water and deposited in specific areas along the river or stream.

There are several factors that can influence the development of horizons of placer gold. One of the most important factors is the rate at which the gold is transported by the water. Faster-moving water will tend to carry larger particles, including larger nuggets of gold, and deposit them in specific areas along the river or stream. Slower-moving water, on the other hand, will tend to deposit smaller particles of gold and may not create distinct horizons.

Other factors that can influence the development of horizons of placer gold include the geology of the area, the type and size of the gold particles, and the presence of other minerals or materials that can affect the transport and deposition of the gold.

To develop a placer gold horizon, prospectors typically search for areas along rivers or streams that have a high concentration of gold and then use various techniques, such as panning or sluicing, to separate the gold from the surrounding sediment.