How did they end up in a mass? Herded by extreme weather?
EARTH IN UPHEAVAL:
Extinction - Page 226,
In numerous places of the world the bone content of caves indicates that they served as hide-outs in times of supreme danger. Lions and tigers, wolves and hyenas, gazelles and hares shared the refuge and there found their common grave. But not all places where such assemblages of bones are discovered were sought for refuge.
In many cases the animals were swept from large areas by a tidal wave and thrown against rocks. And the water rushing through the fissures left behind the animals with all their bones broken within their torn bodies.
From as far as China, to England and France and the islands of the Mediterranean, examples of fissures of fissures with bones, splintered and mingled together, have been presented in the book.
Not only fissures in the rocks but caverns in the hills may have been filled with bones, though the caverns might not have been sought for shelter. An irrupting sea or great lake, lifted from its bed and carrying its own detritus and land debris, swept heterogeneous herds of animals and carried then to the farthest reaches and threw over them hills of gravel, rock and earth...
(There’s really no substitute for reading the book)