Running water erodes vertically, eventually cutting down to the bedrock.
Soil, surface rocks, pebbles and boulders all originate from solid-rock formations. When such formations are exposed on or near the surface, the processes of weathering begins to break them up, and erosion then moves the remains from place to place. A creek bed littered with assorted rocks and sediment will often reflect the nature of the relatively solid underlying, or bedrock deposits. Finding the bedrock usually involves the removal of the loose surface material, called overburden.
Instructions
1. Examine the geology map covering the area in which the creek is situated. Establish the sequence of rock deposits present in the area and their relationship to the creek. Determine whether the same deposits underlie all of the creek or whether different rock types outcrop at different places along the course of the creek. Mark boundaries between hard rocks and softer rocks on your map as these are often the site of waterfalls or rapids and are ideal places to find exposed bedrock.
2. Walk along the side of the creek and observe the stream bed. Look for large areas of exposed solid rock on the creek bed and to either side of the stream. Consult the geological map to identify the underlying bedrock that should be present in each location, then compare it to the visible rocks. If the visible rocks are identical to the geology predicted by the map, they are exposures of bedrock.
3. Visit the "break-of-slope" locations where hard rocks give way to soft rocks. For example, the face of a waterfall will expose the bedrock underlying the upper part of the falls. The back of the plunge pool --- a deep pool formed at the base of the falls --- exposes the bedrock below the fall.
4. Dig down through the overburden when it is not possible to locate exposures of bedrock. Choose fast-flowing sections of the creek that are likely to have thinner layers of overburden as erosion is more efficient at higher-water velocities.
Tags: exposures bedrock, hard rocks, underlying bedrock, visible rocks
