They’re not even that, they look to be staurolite mica schists, likely weathered smooth in a stream bed.

Schists are rocks produced by fairly high grade metamorphism (heating and pressure) of fine-grained sedimentary rocks until the clays recrystalize to mica grains and iron and manganese silicates grow into staurolite crystals. Staurolites often, but not always, grow in the shape of little crosses, hence the name.

Here’s a photo.

Not sure where Mr. Martin found his rocks in Maine, but my guess would be somewhere in the southern half to two-thirds of the state. This portion is pretty much all within the Peidmont physiogeogrpahic province of the Atlantic Coast and consists mostly of metamorphic rock and igneous intrusions.

Simplified Bedrock Geologic Map of Maine.

Dr. Tachyon

But fossilized footprints??