The Norman hedgerows dated back to Roman times

From Citizen Soldiers by Stephen E. Ambrose, page 35.

Just as the sunken lanes hobbled Napoleon at Waterloo, the Allies in Normandy were initially flummoxed and bogged down by the hedgerows which criss-crossed the countryside.

These were the men who had to be rooted out of the hedgerows. One by one. There were, on average, fourteen hedgerows to the kilometer in Normandy. The enervating, costly process of gearing up for an attack, making the attack, carrying the attack home, mopping up after the attack, took half a day or more. And at the end of the action, there was the next hedgerow, fifty to a hundred meters or so away. All through the Cotentin Peninsula, from June 7 on, GIs labored at the task. They heaved and pushed and punched and died doing it, for two hedgerows a day.

No terrain in the world was better suited for defensive action with the weapons of the fourth decade of the twentieth century than the Norman hedgerows, and only the lava and coral, caves and tunnels of Iwo Jima and Okinawa were as favorable.

The Norman hedgerows dated back to Roman times. They were mounds of earth to keep cattle in and to mark boundaries. Typically there was only one entry into the small field enclosed by the hedgerows, which were irregular in length as well as height and set at odd angles. On the sunken roads the brush often met overhead, giving the GIs a feeling of being trapped in a leafy tunnel. Wherever they looked the view was blocked by walls of vegetation.

Undertaking an offensive in the hedgerows was risky, costly, time-consuming, fraught with frustration. It was like fighting in a maze. Platoons found themselves completely lost a few minutes after launching an attack. Squads got separated. Just as often, two platoons from the same company could occupy adjacent fields for hours before discovering each other's presence. The small fields limited deployment possibilities; seldom during the first week of battle did a unit as large as a company go into an attack intact.

Where the Americans got lost, the Germans were at home. The 352nd Division had been in Normandy for months, training for this battle. Further, the Germans were geniuses at utilizing the fortification possibilities of the hedgerows. In the early days of the battle, many GIs were killed or wounded because they dashed through the opening into a field, just the kind of aggressive tactics they had been taught, only to be cut down by pre-sited machine-gun fire or mortars (mortars caused three quarters of American casualties in Normandy).