Plunge pool softer or fractured rock has been eroded back to a knickpoint, water continues to bombard its base. Because this rock is often less resistant than overlying strata, the water from the higher elevation continues eroding downward until an equilibrium is achieved. A somewhat similar bowl-shaped feature developed by flowing water, as opposed to falling water, is known as a scour hole. These occur both naturally and as a result of bridge building. Plunge pool A plunge pool (or plunge basin or waterfall lake) is a deep depression in a stream bed at the base of a waterfall or shut-in. It
Tullgren funnel litter descend through a filter (C) into a preservative liquid (A) in a receptacle (B). This illustration is merely a schematic, since usually the soil sample will not be crumbled and poured into the funnel (this would inevitably lead to a high amount of soil particles in the preservation fluid requiring laborious work to sort out the soil organisms). In fact, the soil sample is placed on a mesh sieve that will allow the soil animals to pass but should retain most of the soil particles. This type of extraction is commonly referred to as Berlese funnel or Tullgren funnel.