The mechanics of large-scale (~10-km diameter) asteroidal, cometary, and meteoroid swarm impact onto a silicate Earth covered by water and a gas layer (atmosphere) demonstrate that only ~ 15% to ~ 5% of the energy of 15 to 45 km/s bolides is taken up directly during the passage through the ocean and atmosphere, respectively. Upon impact with the Earth, ~ 10 to 10² times the bolide mass of water or rock can be ejected to the stratosphere: however, only ~0.1 bolide masses is in < 1 μm particles. The vaporized, melted, and (< 1 mm) solid ejecta transfer up to ~40% of their energy to the atmosphere and possibly oceanic surface water, giving rise to a short, possibly lethal (to large animals) heating pulse. The initial high-speed ejecta that lofted to and above the stratosphere early in the cratering flow is enriched in bolide material and has concentrations of extraterrestrial material in the range of those measured (0.01 to 0.2) in the Cretaceous/Tertiary (C/T) boundary layer. We suggest that the origin of the C/T boundary layer is this ejecta, which is heavily shocked and in the < l-μm range and, hence, once entrained in the stratosphere may be spread worldwide. Penetration of the atmosphere by the bolide creates a temporary hole in the atmosphere surrounded by strongly shocked air. The resultant inward and upward flow of the shocked atmosphere backward along the bolide trajectory lofts the vapor, fine-melted and solid ejecta to heights greater than 10 km. The larger, millimeter- to centimetersize, melt droplets that are lofted by this mechanism reenter the atmosphere and may represent microtektites and tektites. Sufficient impact-induced vapor, melted and comminuted silicate is ejected to stratospheric heights to markedly reduce the light levels at the Earth’s surface. The short-term effects of heating, followed by dust and possibly water-cloud deck induced worldwide cooling, provide several mechanisms to cause severe environmental stress to biota and possibly give rise to the varied and massive extinctions that occurred at the C/T boundary.