Ralph A. Bagnold noticed that laminae within dry sand can be made visible using water. He describes the technique in the excerpt below, from his book "The Physics of Blown Sand and Desert Dunes" (1960):
I've refined Bagnold's method as follows:
Exposing Laminae using Line of Water
A quick way to expose laminae in dry sand is to lay down a uniform line of water, wait a minute, and then pull dry sand away from it. In the photo below, about 30cm of the crest of a 'chinese wall' (reversing dune) was removed, creating a horizontal trench with undisturbed sand at the base of the trench. A line of water was laid down on the trench floor, using a Nalgene squeeze bottle:
Fig 1. Line of moist sand in a trench through the crest of a 'chinese wall'
After about a minute to allow the water to spread within the sand, dry sand is pulled away from one side of the line of moist sand, possibly revealing protruding 'wafers' or fins where the water was conducted faster/further by laminae of finer grains. Waiting longer than a minute usually produces longer fins.
Best results are obtained water is deposited steadily and uniformly, so that variance in the seepage front is due to the sand, not to the amount of water deposited. I try for a line of wet sand about two centimeters wide. The tube on the squeeze bottle makes it possible to position the stream of water near the surface of the sand, minimizing disturbance of the surface of the sand by the impact of falling water.
(The translucent plastic sheet, at the left of the photo, was used in this instance to excavate the trench and pull sand from the line of moist sand. This was at Kelso Dunes in the Mojave Nature Preserve, California. The most recent sand-moving wind was left to right, creating the slipface on the right. You can see in the immediate background where the crest has been eroded by receding scarps, at the top of avalanche troughs. The line of water exposed several pinstripe laminae parallel to the slipface, spaced irregularly about 3 or 4 cm apart.)
Figure 2 shows the same technique being used on a slipface (Great Sand Dunes National Monument, Colorado). A masonite barricade is inserted above the area of interest, and then a horizontal area is excavated (both of these actions can be done such that the sand that is exposed is as it was prior to the excavation, preserving the laminae -- see trenching). A line of water is applied to the sand, and excavated from the side about a minute later (figure 3 below):
Fig 2. Line of moist sand exposing laminae in a slipface
Fig 3. Slipface laminae
My (untested) impression is that a larger quantity of water might reduce sensitivity. Waiting time seems to increase the size of the fins
Exposing Laminae using a Column of Water
Water injected into a column of dry sand, using a syringe and a narrow-diameter drinking straw, spreads by capillary action, making visible any layers of finer sand grains. The photos below show a syringe (obtained from Lee Valley Tools) and drinking straw, and an example of the resulting sand column (figure 5).
The goal is to inject a uniform column of water, so that variation in the resulting column of moist sand is due to characteristics of the sand, not the injection rate. My method is to insert the straw into the sand to a depth equal to the length of the syringe barrel, and then pull the syringe up the plunger while trying to keep the plunger motionless relative to the sand. This effectively transfers (by translation) the column of water in the syringe to the sand, in the same volume per unit of plunger movement.
I inject all the water steadily over a period of about 5 seconds. Injecting the water more slowly would reduce disturbance of the sand by water turbulence at the mouth of the straw, but it'd be harder to do and I think it is good that water spread more or less 'simultaneously' along the column.