Potholes in bedrock are an interesting geologic phenomena, and can trap placer minerals and even be glory holes for nuggets.
Potholes are either mechanically or chemically formed. The mechanical ones are more familiar and include eddy holes and plunge pools, which can form in all types of rock. The chemical type, vugs, can only form in soluble rocks like limestone, marble and dolomite. Mechanical potholes are produced by the grinding of moving rocks against bedrock. However, only eddy holes are the holes that most people consider to be potholes. A pothole is usually defined as a round cavity in bedrock carved by the grinding action of stones in the stream’s currents.
Eddy holes are formed by the swirling and grinding action of rock particles caught in strong stream currents. They are circular and usually vertical columns. Plunge pools are larger holes beneath waterfalls formed by the pounding action of falling rocks against a bedrock floor. Both types can often be found in the same steeply sloping stream.
The vigorous grinding action that can scour a pothole or pound out a plunge pool would be too violent for anything but a huge nugget to settle in the pothole. If smaller gold or other placer mineral particles fall into mechanical potholes they will be ground up and flushed out. Placer minerals found in potholes are present because they landed there after the erosive action finished.
The ideal way for potholes to become filled with valuable minerals is for them to become clogged while still in the main stream. Clogging happens when a rock that is a little too big gets wedged in the hole. This will clog the hole, stop the strong jets of water, and permit gold to settle in it. If the stream had changed course instead, the pothole might have filled up with worthless sediment. A pothole must remain in line with the paystreak in order to collect the gold.
Eddy holes come in all sizes and shapes, usually it is circular or oval and and is a somewhat vertical bedrock hole. Eddy holes are not necessarily formed by the entire stream, so they are often narrower than the stream, but they can get very big. The ones that placer prospectors/miners want to find are a few inches to a few feet in diameter. The size depends on the size of the stream and the duration it had to carve a the hole.
Potholes are not always vertical, they can be inclined. The swirling action of the grinding rocks can even carve grooves that spiral the sides. Lateral grinding can cause a group of potholes to interconnect. Eddy holes are noticeably smaller than the stream width. At the bottom of eddy holes, numerous grindstones may sometimes be discovered. They can be very well rounded to perfectly spherical and can be larger than a basketball, and hopefully may contain gold as well.
A plunge pool is a large, deep hole beneath a waterfall formed by the abrasive action of waterborne rocks impacting bedrock after plunging over the falls. Plunge pools form where bedrock is soft and more easily eroded beneath a more difficult to erode cap rock layer. Since pool growth is accomplished by rock impact during floods rather than by swirling rocks, the bedrock surface of a pool is not ground smooth. Pools are often wider than the average width of the stream and are usually several times the channel width in length. Plunge pools are carved out during floods when the stream is wider and higher.
Present day plunge pools normally crush the placer minerals and they flush out into the riffles downstream. Abandoned plunge pools could act as a trap for placer minerals when the hole was backfilled. Pools usually act as heavy mineral traps around their edges. Strong currents during flood events scour the deeper portions, so the heavy minerals usually settle along the sides where the currents rise up and slow down. Boulders that are too large to be moved and washed out will often clog plunge pools. These boulders can be chunks of the cap rock that were formerly part of the waterfall before they fell into the pool. Some late traveling gold can be caught around the base of these boulders. Boil holes under waterfalls can temporarily trap gold, but, usually any gold coming over the falls will drop into this boil hole to be ground up into flakes that will soon wash out with the current.
Rapidly moving stream currents are capable of eroding oddly shaped holes in bedrock. For example, previous studies show steep grooves or chutes can form from one plunge pool to another. Such grooves are very dangerous or impossible to mine and would not make a good trap for gold anyway. Large rocks roll down these chutes, which grind off the irregularities. The only way a groove or shoot could make a decent trap is if the bedrock surface managed to keep its irregularities due to bedding or cracks. This brings the discussion back to the tried and proven principles of trapping gold. Riffles, either natural or man-made, are required to catch heavy minerals. Smooth bedrock surfaces will not trap gold, they must be rough. Hard layering in rock, often due to bedding in sedimentary rocks, is much better than smooth granite or other intrusive rocks. Monolithic granite is good for sending the gold further downstream.
Vugs are formed by water chemically dissolving holes in limestone and dolomite. This is the same process that causes limestone caverns. Vugs can range in size from microscopic to cavernous and can be essentially bottomless. The surfaces on limestones are not ground smooth; rather they are normally rough by the different rates of dissolving of the soluble constituents. Vugs are horizontal and vertical. Vugs are caused by water moving over them very slowly, often with the aid of organic acids. Auriferous vugs are likely to form in marble. Deep potholes corroded in the limestone can be exceptionally effective gold riffles.
Vugs can be excellent glory holes because they were not subjected to strong currents during their formation. Vugs in streambeds may have heavy minerals concentrated in them and even be gloryholes. On the other hand, vugs can be too deep and even connect with underground caverns, so the gold could move beyond mineable depths.
All rock types contain cracks, crevices, fractures and ledges that develop due to variations in the resistance toerosion and by the weathering action of water. Layered rocks, especially sedimentary rocks, are more prone to irregularities that form traps. If the stream gradient is low without steep reaches, then it is unlikely that mechanical potholes have developed. The chemical type, vugs, can form anywhere regardless of the steepness. Needless to say, clean out all holes in bedrock, but don’t be surprised if many potholes are dry holes. However, a glory hole is always a possibility. Happy Prospecting!