The Cow Branch Formation is a Late Triassic geologic formation in Virginia and North Carolina in the eastern United States. The formation consists of cyclical beds of black and grey lacustrine (lake) mudstone and shale. It is a konservat-lagerstätte renowned for its exceptionally preserved insect fossils, along with small reptiles, fish, and plants. Dinosaur tracks have also been reported from the formation.

The Cow Branch Formation is exposed in the Dan River-Danville Basin, a narrow half-graben which extends across the border of Virginia and North Carolina in the eastern United States. The basin has also been termed the Danville Basin (emphasizing the northern portion in Virginia) or the Dan River Basin (emphasizing the southern portion, in North Carolina). It is one of many Triassic-Jurassic rift basins stretching from northeast to southwest in eastern North America, collectively described as the Newark Supergroup.

The Cow Branch Formation was initially distinguished by Meyertons (1963), working in the Virginian portion of the basin. He considered it to be a member of the Leaksville Formation, a name which encompassed almost all Triassic sediment in the basin. Thayer (1970), working in North Carolina, split up the Leaksville Formation and raised its members to formation status within the Dan River Group. Thayer divided the Cow Branch Formation into dark-colored upper and lower members, divided by a series of red sediments. The lower member has subsequently been renamed to the Walnut Cove Formation, with the intervening red sediment named as the Dry Fork Formation. In its modern conception, the Cow Branch Formation lies above the Dry Fork Formation and below the Stoneville Formation.

The type section of the Cow Branch Formation was a former roadcut along Virginia Route 856, in Pittsylvania County southeast of Cascade. A new lectostratotype was proposed in 2015: a large stone quarry extending across the state line by the Dan River near Eden, North Carolina. This quarry, commonly known as the Solite Quarry, is technically a cluster of three quarry pits, one in Pittsylvania County, Virginia and two in Rockingham County, North Carolina. The site is home to the most extensive and fossiliferous exposures of the formation. Exceptionally-preserved fossils were first reported from the site in 1978, and collection has continued to the present.

The sediments of the Cow Branch Formation are dark grey to black in color and generally fine-grained. Blocky mudstones and thinly-laminated shale are the most common lithologies. The formation is thickest and most fine-grained at the state line, approximately in the middle of its exposed area. Here, the formation is about 1,900 metres (6,200 ft) thick. Coarser sediments such as dark grey sandstone are more prevalent to the southeast and northwest, though periodic black mudstone beds are still frequently encountered. Color is the most useful metric for distinguishing the Cow Branch Formation in the field, as red and purple sediments are practically absent, unlike the Dry Fork and Stoneville formations.

The Cow Branch Formation represents a lacustrine (lake) system in a warm tropical climate, only around 2° to 4° north of the equator. Deposition preceded at an estimated rate of around 46.3 cm/kyr. Bioturbation is almost completely absent, indicating that the lake bed was uninhabited by burrowing animals. Insect-bearing fossil layers were likely completely freshwater while the fish-bearing layers may have been somewhat saltier. As in modern rift lakes, high water levels could have initiated brine seeps along the edge of the basin, adding sodium into the lake system. Quartz is conspicuously absent even from the siliciclastic layers, having been replaced with albite (high-sodium feldspar) through diagenetic processes.

The high frequency of dolomite in the formation indicates that the lake was strongly alkaline, with its water saturated with magnesium supplied from older carbonate rocks in the area. The lack of bioturbation, mudcracks, or root casts has traditionally been taken as evidence that the waters were deep enough to be continually stratified, with the hypolimnion (deepest portion) completely lacking oxygen. An alternative hypothesis suggests that the lake was rather shallow, albeit still deep enough to have been permanent during the formation's deposition. This is supported by the abundance of dolomite, a mineral which forms most easily in salty shallow-water environments. In addition, the insect-bearing layers nearly lack organic carbon, suggesting that the lakebed was fully oxygenated even at its deepest extent. The lake sediments have a high concentration of fluorine, a fact which may help to resolve the near-absence of bioturbators. As with excessive salinity, excessive fluorine can be toxic for fully aquatic organisms (including bioturbators and freshwater plants), but air-breathing insects can persevere and thrive close to the shoreline.

Sediment cycles are readily apparent in the formation, shifting between the extremes of black microlaminated shale and massive coarse mudstone. These are identified as Van Houten cycles, a name applied to fluctuating lake depositional conditions throughout the Triassic rift basins of the Newark Supergroup. Each cycle probably corresponds to variations in precipitation tied to the earth's precession, a type of Milankovitch cycle which oscillates on a scale exceeding 21,000 years. 17 or 18 cycles are generally acknowledged in a continuous section at the Solite Quarry. Some sources estimate that up to 30 cycles were preserved at the site, factoring in all three quarry pits combined. The semi-precessional (10,000 to 15,000 year) astronomical cycle is another strong influence on sedimentation.