It is not possible to accurately determine which rock layers in a river valley are most likely to explode without more information about the specific geologic, chemical, and physical properties of the rock layers in question. There are many factors that can affect the likelihood of a rock layer exploding, including its composition, its physical properties, and the presence of any volatile or reactive substances within it. In some cases, certain types of rock may be more prone to explosive behavior due to the presence of gases or other volatile materials trapped within them. In other cases, the physical properties of the rock, such as its porosity or permeability, may make it more or less prone to explosive behavior. It is important to carefully evaluate all of these factors when determining the potential for explosive behavior in any given rock layer.