Gold in gravel or sand will migrate downward as long as the material is loose and is agitated by flowing water. As a general rule, the gold settles until it reaches bedrock, consequently the paystreak in many instances rests on bedrock. However, gold will penetrate into any crack or crevice, and this fact should be kept in mind when cleaning up bedrock. If the downward movement of the gold is stopped at a clay layer or a bed of tightly packed gravel some distance above the actual bedrock, a paystreak may be formed on what is termed “false bedrock”.
Gold is seldom uniformly distributed through the gravel, and as a consequence the prospector is usually searching for a workable paystreak. In most bars and in some bench deposits it is the upper gravel that is enriched while the lower material is low in grade or barren. The boulder clay of glacial deposits is commonly almost always devoid of gold, but in some rare instances masses of gold-bearing gravel have become incorporated in it.
The outline and position of a paystreak depends on a number of factors, of which the most important is the stream history. Many possibilities for concentration are provided by complex drainage histories, the various factors involved in placer formation, and the complicating effects of glaciation. There is no absolutely certain way of predicting the occurrence of a paystreak. A study of the stream history is helpful in formulating a working hypothesis, but test work and sampling must ultimately be done in order to ascertain the existence and define the position of a paystreak.
The richest section of a placer deposit is most likely to be close to the source of the gold or in some place where there has been a greater than ordinary amount of concentration. Stream placers in general are found to become lower in grade and contain smaller particles of gold when followed downstream. Points of greater concentration will occur where a stream crosses and erodes a higher gold-bearing deposit. Paystreaks may form on the inner sides of river bends or on bars, or in any slack water where the gold particles are dropped. A paystreak may branch, reflecting a change in course of the stream, a second run of gold, or the entrance of a tributary into the main channel. Paystreaks may terminate abruptly because of the steepening in grade of the stream or change in character of bedrock, or by having been eroded either by stream or ice action. It is only after considerable mining experience on a single placer deposit or on a single creek that the possibility of a paystreak may be predicted in advance.