The cardoon and milk thistle, both European plants, cover tracts of country in South America with impenetrable thickets in which both man and beast may be hopelessly lost.
The explorations landward were, however, not so successful, and for many years the Blue Mountains, which rise a few miles back from Sydney, formed an impenetrable barrier to the progress of colonization.
The moist soil encourages luxuriant thickets of willows (Salicineae), surrounded by dense chevaux-de-frise of wormwood and thornbearing Compositae, and interspersed with rich but not extensive prairies, harbouring a great variety of herbaceous plants; while in the deltas of the Black Sea rivers impenetrable beds of reeds (Arundo phragmites) shelter a forest fauna.
Through the almost impenetrable darkness and confusion we only discern this much, that Italy was powerless to constitute herself a nation.
In the case of this one force we know far more than the interdependence supposed by Mach and Kirchhoff; we know bodies with impenetrable force causing one another to keep apart.
There's nothing quite like technology with its impenetrable jargon for frightening off your average human being.
Owing to the almost impenetrable character of the country there are scarcely any roads accessible to wheeled carriages, and, the great causeway of Shah Abbas along the coast has in many places even disappeared under the jungle.
For generations the obstinately heathen Saxons had lain, a compact and impenetrable mass, between Scandinavia and the Frank empire, nor were the measures adopted by Charles the Great for the conversion of the Saxons to the true faith very much to the liking of their warlike Danish neighbours on the other side.
In the north the Caspian is encircled by the level and swampy lowlands, varying in breadth from io to 30 m., partly under impenetrable jungle, partly under rice, cotton, sugar and other crops.
The whole is clothed with impenetrable forest.