Hey /his/, I was reading about the Testudo formation and read

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Hey /his/, I was reading about the Testudo formation and read this, and didn't know what they meant by it.
>Cassius Dio - "This testudo and the way in which it is formed are as follows. The Baggage animals, the light-armed troops, and the cavalry are placed in the center of the army. The heavy-armed troops who use the oblong, curved, and cylindrical shields are drawn up around the outside, making a rectangular figure, and, facing outward and holding their arms at the ready, they enclose the rest. The others who have flat shields, form a compact body in the center and raise their shields over the heads of all the others, so that nothing but shields can be seen in every part of the phalanx alike and all the men by the density of the formation are under shelter from missiles. Indeed, it is so marvelously strong that men can walk upon it and whenever they come to a narrow ravine, even horses and vehicles can be driven over it."

What does he mean by that last part? Were there literal instances of Roman soldiers letting horses step over them while they had their shields up?