Here's how they form:

1. Sand Grain Nucleus: A single grain of sand acts as the starting point for the formation.

2. Mineral Precipitation: Dissolved minerals in groundwater precipitate out around the sand grain, forming a hard shell. This happens when the water evaporates or changes its chemical composition.

3. Growth and Aggregation: The process repeats, adding more layers of minerals, causing the concretion to grow larger. Sometimes, multiple sand grains will be incorporated, forming a larger, more complex structure.

4. Erosion and Exposure: Over time, the surrounding rock can erode away, leaving the concretions exposed as rounded, often colorful, formations.

Characteristics of Sandstone Concretions:

* Rounded shape: The mineral precipitation tends to happen evenly around the sand grain, resulting in a spherical or ovoid shape.

* Hardness: They are usually harder than the surrounding rock due to the mineral cementation.

* Color: The color can vary depending on the minerals involved. Common colors include brown, yellow, red, and white.

* Size: Sandstone concretions can range from small pebbles to several feet in diameter.

Note: These formations are different from the well-known stalactites and stalagmites found in caves, which are formed by the precipitation of calcium carbonate from dripping water.